An asteroid is a
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
of the
inner Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
Of the roughly one million known asteroids the greatest number are located between the orbits of Mars and Jupiter, approximately 2 to 4
AU from the Sun, in the main
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
. Asteroids are generally classified to be of three types:
C-type,
M-type, and
S-type. These were named after and are generally identified with
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
aceous,
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
lic, and
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
ceous compositions, respectively. The size of asteroids varies greatly; the largest,
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, is almost across and qualifies as a
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.
Asteroids have been historically observed from Earth; the ''
Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
'' spacecraft provided the first close observation of an asteroid. Several dedicated missions to asteroids were subsequently launched by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
and
JAXA
The is the Japanese national air and space agency. Through the merger of three previously independent organizations, JAXA was formed on 1 October 2003. JAXA is responsible for research, technology development and launch of satellites into orb ...
, with plans for other missions in progress. NASA's ''
NEAR Shoemaker
''Near Earth Asteroid Rendezvous – Shoemaker'' (''NEAR Shoemaker''), renamed after its 1996 launch in honor of planetary scientist Eugene Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laborator ...
'' studied
Eros
In Greek mythology, Eros (, ; grc, Ἔρως, Érōs, Love, Desire) is the Greek god of love and sex. His Roman counterpart was Cupid ("desire").''Larousse Desk Reference Encyclopedia'', The Book People, Haydock, 1995, p. 215. In the earli ...
, and ''
Dawn
Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in Earth's atmosphere, when the centre of the Sun's disc has reached 18° below the observer's horizo ...
'' observed
Vesta and
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
. JAXA's missions ''
Hayabusa
was a robotic spacecraft developed by the Japan Aerospace Exploration Agency (JAXA) to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis.
''Hayabusa'', formerly known as MUSES-C ...
'' and ''
Hayabusa2
is an asteroid sample-return mission operated by the Japanese state space agency JAXA. It is a successor to the ''Hayabusa'' mission, which returned asteroid samples for the first time in June 2010. ''Hayabusa2'' was launched on 3 December 2 ...
'' studied and returned samples of
Itokawa and
Ryugu
162173 Ryugu, provisional designation , is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately in diameter and is a dark object of the rare spectral type Cb, with qualities of both a C-type a ...
, respectively.
OSIRIS-REx
OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a NASA asteroid-study and sample-return mission. The mission's primary goal is to obtain a sample of at least from 101955 Bennu, a carbonac ...
studied
Bennu
Bennu is an ancient Egyptian deity linked with the Sun, creation, and rebirth. He may have been the original inspiration for the phoenix legends that developed in Greek mythology.
Roles
According to Egyptian mythology, Bennu was a self-create ...
, collecting a sample in 2020 to be delivered back to Earth in 2023. ''
Lucy
Lucy is an English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings are Luci, Luce, Luci ...
'', launched in 2021, has an itinerary including eight different asteroids, one from the
main belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
and seven
Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s. ''
Psyche
Psyche (''Psyché'' in French) is the Greek term for "soul" (ψυχή).
Psyche may also refer to:
Psychology
* Psyche (psychology), the totality of the human mind, conscious and unconscious
* ''Psyche'', an 1846 book about the unconscious by Car ...
'', to be launched in 2023 or 2024, will study a metallic
asteroid of the same name.
Near-Earth asteroids can threaten all life on the planet; an asteroid
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
resulted in the
Cretaceous–Paleogene extinction. Different asteroid deflection strategies have been proposed; the
Double Asteroid Redirection Test
Double Asteroid Redirection Test (DART) is a NASA space mission aimed at testing a method of planetary defense against near-Earth objects (NEOs). It was designed to assess how much a spacecraft impact deflects an asteroid through its transfer ...
spacecraft, or DART, was launched in 2021 and intentionally impacted
Dimorphos
(65803) Didymos I Dimorphos (provisional designation S/2003 (65803) 1) is a minor-planet moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. It has a diameter of and has been characterised as a low-density rubb ...
in September 2022, successfully altering its orbit by crashing into it.
History of observations
Only one asteroid,
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
, which has a relatively
reflective surface, is normally visible to the naked eye. When favorably positioned, 4 Vesta can be seen in dark skies. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.
[ , the ]Minor Planet Center
The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory.
Function
...
had data on 1,199,224 minor planets in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations.[
]
Discovery of Ceres
In 1772, German astronomer Johann Elert Bode
Johann Elert Bode (; 19 January 1747 – 23 November 1826) was a German astronomer known for his reformulation and popularisation of the Titius–Bode law. Bode determined the orbit of Uranus and suggested the planet's name.
Life and career
Bo ...
, citing Johann Daniel Titius
Johann Daniel Titius (born Johann Daniel Tietz(e), 2 January 1729 – 16 December 1796) was a German astronomer and a professor at Wittenberg. , published a numerical procession known as the Titius–Bode law
The Titius–Bode law (sometimes termed just Bode's law) is a formulaic prediction of spacing between planets in any given solar system. The formula suggests that, extending outward, each planet should be approximately twice as far from the Sun as ...
(now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets. He wrote the following explanation for the existence of a "missing planet":
This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4 + 3 = 7. The Earth 4 + 6 = 10. Mars 4 + 12 = 16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4 + 24 = 28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4 + 48 = 52 parts, and finally to that of Saturn by 4 + 96 = 100 parts.
Bode's formula predicted another planet would be found with an orbital radius near 2.8 astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
s (AU), or 420 million km, from the Sun. The Titius–Bode law got a boost with William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
's discovery of Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
near the predicted distance for a planet beyond Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
. In 1800, a group headed by Franz Xaver von Zach
Baron Franz Xaver von Zach (''Franz Xaver Freiherr von Zach''; 4 June 1754 – 2 September 1832) was a Hungarian astronomer born at Pest, Hungary (now Budapest in Hungary).
Biography
Zach studied physics at the Royal University of Pest, and se ...
, editor of the German astronomical journal ''Monatliche Correspondenz'' (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police
The celestial police (german: Himmelspolizey), officially the United Astronomical Society (german: Vereinigte Astronomische Gesellschaft, VAG), were an informal group of astronomers working in the early 19th century with the express purpose of f ...
"), asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they later found the asteroids 2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...
, 3 Juno
)
, mp_category=Main belt (Juno clump)
, orbit_ref =
, epoch= JD 2457000.5 (9 December 2014)
, semimajor=2.67070 AU
, perihelion=1.98847 AU
, aphelion=3.35293 AU
, eccentricity=0.25545
, period=4.36463 yr
, inclination=12.9817°
, asc ...
and 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
.
One of the astronomers selected for the search was Giuseppe Piazzi
Giuseppe Piazzi ( , ; 16 July 1746 – 22 July 1826) was an Italian Catholic priest of the Theatine order, mathematician, and astronomer. He established an observatory at Palermo, now the '' Osservatorio Astronomico di Palermo – Giuseppe ...
, a Catholic priest at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th tar
Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bit ...
of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet:
The light was a little faint, and of the colour of Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, but similar to many others which generally are reckoned of the eighth magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
. Therefore I had no doubt of its being any other than a fixed star. ..The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.
Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani
Barnaba Oriani FRS FRSE (17 July 1752 – 12 November 1832) was an Italian priest, geodesist, astronomer and scientist.
Life
Oriani was born in Garegnano (now part of Milan), the son of a mason, and died in Milan.
After getting his elemen ...
of Milan and Bode in Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande
Joseph Jérôme Lefrançois de Lalande (; 11 July 1732 – 4 April 1807) was a French astronomer, freemason and writer.
Biography
Lalande was born at Bourg-en-Bresse (now in the département of Ain) to Pierre Lefrançois and Marie‐Anne‐Gab ...
. The information was published in the September 1801 issue of the ''Monatliche Correspondenz''.
By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician Carl Friedrich Gauss
Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
, then 24 years old, developed an efficient method of orbit determination
Orbit determination is the estimation of orbits of objects such as moons, planets, and spacecraft. One major application is to allow tracking newly observed asteroids and verify that they have not been previously discovered. The basic methods wer ...
. In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it. At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; however, Neptune, once discovered in 1846, was 8 AU closer than predicted, leading most astronomers to conclude that the law was a coincidence. Piazzi named the newly discovered object ''Ceres Ferdinandea,'' "in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon".[
]
Further search
Three other asteroids (2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...
, 3 Juno
)
, mp_category=Main belt (Juno clump)
, orbit_ref =
, epoch= JD 2457000.5 (9 December 2014)
, semimajor=2.67070 AU
, perihelion=1.98847 AU
, aphelion=3.35293 AU
, eccentricity=0.25545
, period=4.36463 yr
, inclination=12.9817°
, asc ...
, and 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
) were discovered by von Zach's group over the next few years, with Vesta found in 1807. No new asteroids were discovered until 1845. Amateur astronomer Karl Ludwig Hencke
Karl Ludwig Hencke (8 April 1793 – 21 September 1866) was a German amateur astronomer and discoverer of minor planets. He is sometimes confused with Johann Franz Encke, another German astronomer.
Biography
Hencke was born in Driesen, Branden ...
started his searches of new asteroids in 1830, and fifteen years later, while looking for Vesta, he found the asteroid later named 5 Astraea
Astraea () (minor planet designation: 5 Astraea) is an asteroid in the asteroid belt. Its surface is highly reflective and its composition is probably a mixture of nickel–iron with silicates of magnesium and iron. It is an S-type asteroid in t ...
. It was the first new asteroid discovery in 38 years. Carl Friedrich Gauss
Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
was given the honour of naming the asteroid. After this, other astronomers joined; 15 asteroids were found by the end of 1851. In 1868, when James Craig Watson
James Craig Watson (January 28, 1838 – November 22, 1880) was a Canadian-American astronomer, discoverer of comets and minor planets, director of the University of Michigan's Detroit Observatory in Ann Arbor, and awarded with the Lalande Priz ...
discovered the 100th asteroid, the French Academy of Sciences
The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV of France, Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific me ...
engraved the faces of Karl Theodor Robert Luther
Karl Theodor Robert Luther (April 16, 1822, Świdnica – February 15, 1900 Düsseldorf), normally published as Robert Luther, was a German astronomer. While working at the Bilk Observatory in Düsseldorf, Germany, he searched for asteroids and ...
, John Russell Hind
John Russell Hind FRS FRSE LLD (12 May 1823 – 23 December 1895) was an English astronomer.
Life and work
John Russell Hind was born in 1823 in Nottingham, the son of lace manufacturer John Hind and Elizabeth Russell, and was educated at ...
, and Hermann Goldschmidt
Hermann Mayer Salomon Goldschmidt (June 17, 1802 – August 30 or September 10 1866) was a German-French astronomer and painter who spent much of his life in France. He started out as a painter, but after attending a lecture by the famous Fren ...
, the three most successful asteroid-hunters at that time, on a commemorative medallion marking the event.
In 1891, Max Wolf
Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K ...
pioneered the use of astrophotography
Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1840, but it was no ...
to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia
Brucia (minor planet designation: 323 Brucia) is a stony Phocaea asteroid and former Mars-crosser from the inner regions of the asteroid belt, approximately in diameter. It was the first asteroid to be discovered by the use of astrophotography.
...
, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", a phrase variously attributed to Eduard Suess
Eduard Suess (; 20 August 1831 - 26 April 1914) was an Austrian geologist and an expert on the geography of the Alps. He is responsible for hypothesising two major former geographical features, the supercontinent Gondwana (proposed in 1861) and t ...
and Edmund Weiss
Edmund Weiss (26 August 1837 – 21 June 1917) was an Austrian astronomer. He was born in Frývaldov, Austrian Silesia, now Jeseník, Czech Silesia. His father, Josef Weiss (1795–1847), was a pioneer of hydrotherapy. His twin brother, Adolf Gu ...
. Even a century later, only a few thousand asteroids were identified, numbered and named.
19th and 20th centuries
In the past, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
, or astrograph
An astrograph (or astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, a ...
. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope
A stereoscope is a device for viewing a stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image.
A typical stereoscope provides each eye with a lens that makes the ima ...
. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.
These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: ). The last step is sending the locations and time of observations to the Minor Planet Center
The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory.
Function
...
, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
.
Naming
By 1851, the Royal Astronomical Society
(Whatever shines should be observed)
, predecessor =
, successor =
, formation =
, founder =
, extinction =
, merger =
, merged =
, type = NGO ...
decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz
Jean Elias Benjamin Valz (May 27, 1787 – April 22, 1867) was a French astronomer.
He was born in Nîmes and trained as an engineer. He was the son of politician Jean Valz and the grandson of the doctor, meteorologist and naturalist Pierre ...
gave it a name and a number designating its rank among asteroid discoveries, 20 Massalia
Massalia, minor planet designation 20 Massalia, is a stony asteroid and the parent body of the Massalia family located in the inner region of the asteroid belt, approximately in diameter. Discovered by Italian astronomer Annibale de Gasparis on ...
. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.
Currently all newly discovered asteroids receive a provisional designation
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
(such as ) consisting of the year of discovery and an alphanumeric code indicating the half-month The half-month is a calendar subdivision used in astronomy. Each calendar month is separated into two parts:
*days 1 to 15
*day 16 until the end of the month.
Newly identified small Solar System bodies, such as comets and asteroids, are given syste ...
of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. ). The formal naming convention uses parentheses around the number – e.g. (433) Eros – but dropping the parentheses is quite common. Informally, it is also common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.[ In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.][
]
Symbols
The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.[
In 1851, after the fifteenth asteroid, ]Eunomia
In Greek mythology, Eunomia ( grc, Εὐνομία) was a minor goddess of law and legislation (her name can be translated as "good order", "governance according to good laws"), as well as the spring-time goddess of green pastures (''eû'' means ...
, had been discovered, Johann Franz Encke
Johann Franz Encke (; 23 September 179126 August 1865) was a German astronomer. Among his activities, he worked on the calculation of the periods of comets and asteroids, measured the distance from the Earth to the Sun, and made observations ...
made a major change in the upcoming 1854 edition of the ''Berliner Astronomisches Jahrbuch
The ''Berliner Astronomisches Jahrbuch'' (abbrev. B.A.J.) is an astronomical ephemeris almanac and one of the longest publication series in astronomy. It was a compendium of ephemerides of all large Solar System bodies and of fundamental stars whi ...
'' (BAJ, ''Berlin Astronomical Yearbook''). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid. The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche
16 Psyche () is a large M-type asteroid discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 and named after the Greek goddess Psyche.
The prefix "16" signifies that it was the sixteenth minor planet in order of dis ...
, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. 20 Massalia
Massalia, minor planet designation 20 Massalia, is a stony asteroid and the parent body of the Massalia family located in the inner region of the asteroid belt, approximately in diameter. Discovered by Italian astronomer Annibale de Gasparis on ...
was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides
Fides ( minor planet designation: 37 Fides) is a large main-belt asteroid. It was discovered by German astronomer Karl Theodor Robert Luther on October 5, 1855, and named after Fides, the Roman goddess of loyalty. Fides was the last of the mai ...
.[
]
Terminology
The first discovered asteroid, Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, was originally considered a new planet. It was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
to propose the term "asteroid", coined in Greek as ἀστεροειδής, or ''asteroeidēs'', meaning 'star-like, star-shaped', and derived from the Ancient Greek ''astēr'' 'star, planet'. In the early second half of the 19th century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably.
Traditionally, small bodies orbiting the Sun were classified as comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s, asteroids, or meteoroid
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...
s, with anything smaller than one meter across being called a meteoroid. The term "asteroid" never had a formal definition, with the broader term "small Solar System bodies
A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as fol ...
" being preferred by the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU). As no IAU definition exists, asteroid can be defined as "an irregularly shaped rocky body orbiting the Sun that does not qualify as a planet or a dwarf planet under the IAU definitions of those terms".
When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation
Sublimation or sublimate may refer to:
* ''Sublimation'' (album), by Canvas Solaris, 2004
* Sublimation (phase transition), directly from the solid to the gas phase
* Sublimation (psychology), a mature type of defense mechanism
* Sublimate of mer ...
of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; "asteroids" with notably eccentric orbits are probably dormant or extinct comets.
For almost two centuries, from the discovery of Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
in 1801 until the discovery of the first centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
, 2060 Chiron
2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orb ...
in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo
944 Hidalgo is a centaur and unusual object on an eccentric, cometary-like orbit between the asteroid belt and the outer Solar System, approximately in diameter. Discovered by German astronomer Walter Baade in 1920, it is the first member of t ...
ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
, they numbered them among the traditional asteroids. There was debate over whether these objects should be considered asteroids or given a new classification. Then, when the first trans-Neptunian object
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
Typically, ...
(other than Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
), 15760 Albion
15760 Albion, provisional designation , was the first trans-Neptunian object to be discovered after Pluto and Charon. Measuring about 108–167 kilometres in diameter, it was discovered in 1992 by David C. Jewitt and Jane X. Luu at the Mauna Kea ...
, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
, trans-Neptunian object
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
Typically, ...
, scattered-disc object
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
, and so on. They inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.
The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei
The nucleus is the solid, central part of a comet, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphere sur ...
. (The much more distant Oort cloud
The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2 ...
is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the ''Stardust'' probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.
The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.[ Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the ]asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s, and near-Earth object
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). ...
s.
When the IAU introduced the class small Solar System bodies
A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as fol ...
in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
s for the largest minor planets – those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally, the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt, Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, at about across, has been placed in the dwarf planet category.
Formation
Many asteroids are the shattered remnants of planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s, bodies within the young Sun's solar nebula
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
that never grew large enough to become planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s.[ It is thought that planetesimals in the asteroid belt evolved much like the rest of objects in the solar nebula until Jupiter neared its current mass, at which point excitation from ]orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
s with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.
In the Nice model
The Nice () model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France.
It proposes the migration of the giant p ...
, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroid
D-type asteroids have a very low albedo and a featureless reddish Asteroid spectral types, spectrum. It has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their inte ...
s, and possibly include Ceres.
Distribution within the Solar System
Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km i ...
. Significant populations include:
Asteroid belt
The majority of known asteroids orbit within the asteroid belt between the orbits of Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
and Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, generally in relatively low-eccentricity
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off-center, in geometry
* Eccentricity (graph theory) of a v ...
(i.e. not very elongated) orbits. This belt is estimated to contain between 1.1 and 1.9 million asteroids larger than in diameter, and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
, and in this region the accretion
Accretion may refer to:
Science
* Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity
* Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
of planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
.
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100 km, and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. The absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
s of most of the known asteroids are between 11 and 19, with the median at about 16.
The total mass of the asteroid belt is estimated to be kg, which is just 3% of the mass of the Moon; the mass of the Kuiper Belt and Scattered Disk is over 100 times as large. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, account for maybe 62% of the belt's total mass, with 39% accounted for by Ceres alone.
Trojans
Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian point
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...
s of stability, and , which lie 60° ahead of and behind the larger body.
In the Solar System, most known trojans share the orbit of Jupiter. They are divided into the Greek camp
This is a list of Jupiter trojans that lie in the Greek camp, an elongated curved region around the leading Lagrangian point (), 60 ° ahead of Jupiter in its orbit.
All the asteroids at Jupiter's point have names corresponding to participants ...
at (ahead of Jupiter) and the Trojan camp
This is a list of Jupiter trojans that lie in the Trojan camp, an elongated curved region around the trailing Lagrangian point, 60° behind Jupiter.
All the asteroids at the trailing point have names corresponding to participants on the Troj ...
at (trailing Jupiter). More than a million Jupiter trojans larger than one kilometer are thought to exist, of which more than 7,000 are currently catalogued. In other planetary orbits only nine Mars trojan
The Mars trojans are a group of trojan objects that share the orbit of the planet Mars around the Sun. They can be found around the two Lagrangian points 60° ahead of and behind Mars. The origin of the Mars trojans is not well understood. One t ...
s, 28 Neptune trojan
Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, similar to the Trojan (astronomy), trojans of other planets. They therefore have approximately the same orbital period as Neptune and follow rough ...
s, two Uranus trojan There are two known Uranus trojans, or minor planets orbiting in the Lagrangian points of Uranus. Both are in the region:
*
*
See also
* Trojan (astronomy)
In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the ...
s, and two Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name " ...
s, have been found to date. A temporary Venus trojan is also known. Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans.
Near-Earth asteroids
Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as ''Earth-crossers''. , a total of 28,772 near-Earth asteroids were known; 878 have a diameter of one kilometer or larger.
A small number of NEAs are extinct comets
An extinct comet is a comet that has expelled most of its volatile ice and has little left to form a tail and coma. In a dormant comet, rather than being depleted, any remaining volatile components have been sealed beneath an inactive surface la ...
that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
.
Many asteroids have natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s (minor-planet moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
s). , there were 85 NEAs known to have at least one moon, including three known to have two moons. The asteroid 3122 Florence, one of the largest potentially hazardous asteroids with a diameter of , has two moons measuring across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.
Near-Earth asteroids are divided into groups based on their semi-major axis
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
(a), perihelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
distance (q), and aphelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
distance (Q):
* The '' Atiras'' or ''Apoheles'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, , which implies that the asteroid's semi-major axis is also less than 0.983 AU.
* The ''Atens
The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids . The group is named after 2062 Aten, the first of its kind, discovered on 7 Januar ...
'' have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, and . (0.983 AU is Earth's perihelion distance.)
* The ''Apollos
Apollos ( grc-gre, Ἀπολλώς) was a 1st-century Alexandrian Jewish Christian mentioned several times in the New Testament. A contemporary and colleague of Paul the Apostle, he played an important role in the early development of the c ...
'' have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, and . (1.017 AU is Earth's aphelion distance.)
* The '' Amors'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so . In summary, . (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.
Martian moons
It is unclear whether Martian moons Phobos and Deimos are captured asteroids or were formed due to impact event on Mars.[Burns, Joseph A. (1992). "Contradictory Clues as to the Origin of the Martian Moons" in ''Mars'', H. H. Kieffer et al., eds., Tucson: University of Arizona Press, Tucson ] Phobos and Deimos both have much in common with carbonaceous C-type asteroid
C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...
s, with spectra, albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
, and density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
.[Landis, Geoffrey A.; "Origin of Martian Moons from Binary Asteroid Dissociation", ''American Association for the Advancement of Science Annual Meeting''; Boston, MA, 2001]
abstract
/ref> Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane
The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projectio ...
, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s, although it is not clear whether sufficient time was available for this to occur for Deimos. Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey A. Landis
Geoffrey Alan Landis (; born May 28, 1955) is an American aerospace engineer and author, working for the National Aeronautics and Space Administration (NASA) on planetary exploration, interstellar propulsion, solar power and photovoltaics. He ...
has pointed out that the capture could have occurred if the original body was a binary asteroid
A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple ast ...
that separated under tidal forces.
Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.
Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
.[Craddock, Robert A.; (1994); "The Origin of Phobos and Deimos", ''Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14–18 March 1994'', p. 293] The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
suggest a composition containing mainly phyllosilicates
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite
A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primi ...
meteorites, again pointing away from an asteroidal origin. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's moon.
Characteristics
Size distribution
Asteroids vary greatly in size, from almost for the largest down to rocks just 1 meter across, below which an object is classified as a meteoroid
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...
. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationa ...
s. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s or fragments of larger bodies.
The dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
is by far the largest asteroid, with a diameter of . The next largest are 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
and 2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...
, both with diameters of just over . Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis
99942 Apophis is a near-Earth asteroid and potentially hazardous asteroid with a diameter of that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth on April&nb ...
.
The mass of all the objects of the asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, lying between the orbits of Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
and Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, is estimated to be , ≈ 3.25% of the mass of the Moon. Of this, Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
comprises , about 40% of the total. Adding in the next three most massive objects, Vesta (11%), Pallas
Pallas may refer to:
Astronomy
* 2 Pallas asteroid
** Pallas family, a group of asteroids that includes 2 Pallas
* Pallas (crater), a crater on Earth's moon
Mythology
* Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena
* Pall ...
(8.5%), and Hygiea (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%. The number of asteroids increases rapidly as their individual masses decrease.
The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a power law
In statistics, a power law is a Function (mathematics), functional relationship between two quantities, where a Relative change and difference, relative change in one quantity results in a proportional relative change in the other quantity, inde ...
, there are 'bumps' at about and , where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120 km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.
Largest asteroids
Three largest objects in the asteroid belt, Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, Vesta, and Pallas
Pallas may refer to:
Astronomy
* 2 Pallas asteroid
** Pallas family, a group of asteroids that includes 2 Pallas
* Pallas (crater), a crater on Earth's moon
Mythology
* Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena
* Pall ...
, are intact protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationa ...
s that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.
Ceres is the only asteroid that appears to have a plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
shape under its own gravity and hence the only one that is a dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
.[ It has a much higher ]absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
than the other asteroids, of around 3.32,[ and may possess a surface layer of ice.] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth.[
Vesta, too, has a differentiated interior, though it formed inside the Solar System's ]frost line
The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...
, and so is devoid of water; its composition is mainly of basaltic rock with minerals such as olivine. Aside from the large crater at its southern pole, Rheasilvia
Rheasilvia is the most prominent surface feature on the asteroid Vesta and is thought to be an impact crater. It is in diameter, which is 90% the diameter of Vesta itself, and is 95% the mean diameter of Vesta, . However, the mean is affect ...
, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroid A V-type asteroid or Vestoid is an asteroid whose spectral type is that of 4 Vesta. Approximately 6% of main-belt asteroids are vestoids, with Vesta being by far the largest of them. They are relatively bright, and rather similar to the more common ...
s, and is the source of the HED meteorite
HED meteorites are a clan (subgroup) of achondrite meteorites. HED stands for "howardite–eucrite–diogenite".
These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the magm ...
s, which constitute 5% of all meteorites on Earth.
Pallas is unusual in that, like Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.[ Pallas is the parent body of the ]Palladian family The Pallas family (''adj. Palladian''; ) is a small asteroid family of B-type asteroids at very high inclinations in the intermediate asteroid belt.
The namesake of the family is 2 Pallas, an extremely large asteroid with a mean diameter of about 5 ...
of asteroids.
Hygiea is the largest carbonaceous asteroid[ and, unlike the other largest asteroids, lies relatively close to the ]plane of the ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...
. It is the largest member and presumed parent body of the Hygiean family The Hygiea or Hygiean family is a grouping of dark, carbonaceous C-type and B-type asteroids in outer asteroid belt, the largest member of which is 10 Hygiea. About 1% of all known asteroids in the asteroid belt belong to this family.
Characteris ...
of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope
The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across, ...
's SPHERE
A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
imager in 2017 and 2018, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...
, or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.[
Internal differentiation of large asteroids is possibly related to their lack of ]natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating a rubble pile
In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...
structure.
Rotation
Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period less than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble pile
In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...
s formed through the accumulation of debris after collisions between asteroids.[
]
Color
Asteroids become darker and redder with age due to space weathering
Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take on m ...
. However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.
Surface features
Except for the "big four Big Four or Big 4 may refer to:
Groups of companies
* Big Four accounting firms: Deloitte, Ernst & Young, KPMG, PwC
* Big Four (airlines) in the U.S. in the 20th century: American, Eastern, TWA, United
* Big Four (banking), several groupings ...
" (Ceres, Pallas, Vesta, and Hygiea), asteroids are likely to be broadly similar in appearance, if irregular in shape. 50 km (31 mi) 253 Mathilde
Mathilde (minor planet designation: 253 Mathilde) is an asteroid in the intermediate asteroid belt, approximately 50 kilometers in diameter, that was discovered by Austrian astronomer Johann Palisa at Vienna Observatory on 12 November 1885. It h ...
is a rubble pile saturated with craters with diameters the size of the asteroid's radius. Earth-based observations of 300 km (186 mi) 511 Davida
Davida (minor planet designation: 511 Davida) is a large C-type asteroid. It is the one of the largest asteroids; approximately tied for 7th place, to within measurement uncertainties, and the 5th or 6th most massive. It was discovered by R. S. ...
, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida
Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. ...
, that have been observed up close, also reveal a deep regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid
A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has cir ...
.
''Dawn spacecraft
''Dawn'' is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres. In the fulfillment of that mission—the ninth in NASA's Dis ...
'' revealed that Ceres has a heavily cratered surface, but with fewer large craters than expected. Models based on the formation of the current asteroid belt had suggested Ceres should possess 10 to 15 craters larger than in diameter. The largest confirmed crater on Ceres, Kerwan Basin, is across. The most likely reason for this is viscous relaxation
The viscosity of a fluid is a measure of its drag (physics), resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quant ...
of the crust slowly flattening out larger impacts.
Composition
Asteroids are classified by their characteristic emission spectra
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...
, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbonaceous ( carbon-rich), metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
lic, and silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
ceous (stony) compositions, respectively. The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
mantle, and basaltic crust. Thought to be the largest undifferentiated asteroid, 10 Hygiea
Hygiea (minor-planet designation: 10 Hygiea) is a major asteroid and possible dwarf planet located in the main asteroid belt. With a diameter of and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in ...
seems to have a uniformly primitive composition of carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...
, but it may actually be a differentiated asteroid that was globally disrupted by an impact and then reassembled. Other asteroids appear to be the remnant cores or mantles of proto-planets, high in rock and metal. Most small asteroids are believed to be piles of rubble held together loosely by gravity, although the largest are probably solid. Some asteroids have moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
or are co-orbiting binaries
A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary file formats contain parts that can be interpreted as text; for example, some computer document file ...
: rubble piles, moons, binaries, and scattered asteroid families
An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...
are thought to be the results of collisions that disrupted a parent asteroid, or possibly a planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
.
In the main asteroid belt, there appear to be two primary populations of asteroid: a dark, volatile-rich population, consisting of the C-type and P-type asteroids, with albedos less that 0.10 and densities under , and a dense, volatile-poor population, consisting of the S-type and M-type asteroids, with albedos over 0.15 and densities greater than 2.7. Within these populations, larger asteroids are denser, presumably due to compression. There appears to be minimal macro-porosity (interstitial vacuum) in the score of asteroids with masses greater than .[P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56]
Composition is calculated from three primary sources: albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione
Hermione ( minor planet designation: 121 Hermione) is a very large binary asteroid discovered in 1872. It orbits in the Cybele group in the far outer asteroid belt. As an asteroid of the dark C spectral type, it is probably composed of carbonace ...
(268×186×183 km), and 87 Sylvia
Sylvia (minor planet designation: 87 Sylvia) is the one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of Cybele group located beyond the m ...
(384×262×232 km). Few asteroids are larger than 87 Sylvia, none of them have moons. The fact that such large asteroids as Sylvia may be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.
Water
Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. In 2009, the presence of water ice Water ice could refer to:
*Ice formed by water (as opposed to other substances)
*The alternate term for various similar frozen fruit-flavoured desserts:
**Italian ice primarily in Philadelphia and the Delaware Valley
**Sorbet
Sorbet (), also c ...
was confirmed on the surface of 24 Themis
Themis ( minor planet designation: 24 Themis) is one of the largest asteroids in the asteroid belt. It is also the largest member of the Themistian family. It was discovered by Annibale de Gasparis on 5 April 1853. It is named after Themis, th ...
using NASA's Infrared Telescope Facility
The NASA Infrared Telescope Facility (NASA IRTF) is a telescope optimized for use in infrared astronomy and located at the Mauna Kea Observatory in Hawaii. It was first built to support the Voyager missions and is now the US national facility ...
. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. The presence of ice on 24 Themis makes the initial theory plausible.
In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
GD 61
GD 61 is a white dwarf with a planetary system located 150 light-years from Earth in the constellation Perseus. It is thought to have been a main-sequence star of spectral type A0V with around three times the mass of the Sun that has aged ...
. On 22 January 2014, European Space Agency
, owners =
, headquarters = Paris, Île-de-France, France
, coordinates =
, spaceport = Guiana Space Centre
, seal = File:ESA emblem seal.png
, seal_size = 130px
, image = Views in the Main Control Room (1205 ...
(ESA) scientists reported the detection, for the first definitive time, of water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
on Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory
The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched until the launch of the James Webb Space Telesc ...
. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."
Findings have shown that solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
s can react with the oxygen in the upper layer of the asteroids and create water. It has been estimated that "every cubic metre of irradiated rock could contain up to 20 litres"; study was conducted using an atom probe tomography, numbers are given for the Itokawa S-type asteroid.
Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have an ultraporous lithology (UPL): porous texture that could be formed by removal of ice that filled these pores, this suggests that UPL "represent fossils of primordial ice".
Organic compounds
Asteroids contain traces of amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (an event called "panspermia
Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraf ...
"). In August 2011, a report, based on NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
studies with meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s found on Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, was published suggesting DNA and RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
components (adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...
, guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...
and related organic molecules
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
) may have been formed on asteroids and comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s in outer space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
.
In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose
Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compo ...
, in meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life
Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energ ...
, and supporting the notion of an RNA world
The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existence ...
prior to a DNA-based origin of life
In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
on Earth, and possibly, as well, the notion of panspermia
Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraf ...
.
Classification
Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
.
Orbital classification
Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.[ Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the ]Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s. Main belt families were first recognized by Kiyotsugu Hirayama
was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called " Hirayama families" in his honour.
Biography
...
in 1918 and are often called Hirayama families A Hirayama family of asteroids is a group of minor planets that share similar orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions.
Str ...
in his honor.
About 30–35% of the bodies in the asteroid belt belong to dynamical families, each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
.
Some asteroids have unusual horseshoe orbit
In celestial mechanics, a horseshoe orbit is a type of co-orbital configuration, co-orbital motion of a small orbiting body relative to a larger orbiting body. The osculating orbit, osculating (instantaneous) orbital period of the smaller body re ...
s that are co-orbital with Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
or another planet. Examples are 3753 Cruithne
3753 Cruithne is a Q-type, Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is an asteroid that, relative to Earth, orbits the Sun in a bean-shaped orbit that effectively describes a ...
and . The first instance of this type of orbital arrangement was discovered between Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moons Epimetheus
In Greek mythology, Epimetheus (; grc-gre, Ἐπιμηθεύς, , afterthought) was the brother of Prometheus (traditionally interpreted as "foresight", literally "fore-thinker"), a pair of Titans who "acted as representatives of mankind". The ...
and Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
. Sometimes these horseshoe objects temporarily become quasi-satellite
A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet (or dwarf planet) where the object stays close to that planet over many orbital periods.
A quasi-satellite's orbit around the Sun t ...
s for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
are known to have quasi-satellites.
Such objects, if associated with Earth or Venus or even hypothetically Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, are a special class of Aten asteroid
The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids . The group is named after 2062 Aten, the first of its kind, discovered on 7 Januar ...
s. However, such objects could be associated with the outer planets as well.
Spectral classification
In 1975, an asteroid taxonomic
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
system based on color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
, albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
, and spectral shape was developed by Chapman
Chapman may refer to:
Businesses
* Chapman Entertainment, a former British television production company
* Chapman Guitars, a guitar company established in 2009 by Rob Chapman
* Chapman's, a Canadian ice cream and ice water products manufacturer ...
, Morrison Morrison may refer to:
People
* Morrison (surname), people with the Scottish surname Morrison
* Morrison Heady (1829–1915), American poet
* Morrison Mann MacBride (1877–1938), Canadian merchant
Places in the United States
* Morrison, Colorad ...
, and Zellner.[ These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.
The two most widely used taxonomies now used are the ]Tholen classification
An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated ...
and SMASS classification
An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiat ...
. The former was proposed in 1984 by David J. Tholen
David James Tholen (born 1955) is an American astronomer at the Institute for Astronomy of the University of Hawaii. He holds a 1984 PhD from the University of Arizona, and specializes in planetary and Solar System astronomy. He is a discovere ...
, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.[ In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.][
The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.
]
Problems
Originally, spectral designations were based on inferences of an asteroid's composition.[ However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials.
]
Active asteroids
Active asteroids are objects that have asteroid-like orbits but show comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
's orbit (like an asteroid). These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt
David Clifford Jewitt (born 1958) is a British-American astronomer who studies the Solar System, especially its minor bodies. He is based at the University of California, Los Angeles, where he is a Member of the Institute for Geophysics and Pl ...
and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, whereas the growing population of active asteroids shows that this is not always the case.
The first active asteroid discovered is 7968 Elst–Pizarro
Comet Elst–Pizarro is a body that displays characteristics of both asteroids and comets, and is the prototype of active asteroids. Its orbit keeps it within the asteroid belt, yet it displayed a dust tail like a comet while near perihelion in 19 ...
. It was discovered (as an asteroid) in 1979 but then was found to have a tail by Eric Elst
Eric Walter Elst (30 November 1936 – 2 January 2022) was a Belgian astronomer at the Royal Observatory of Belgium in Uccle and a prolific discoverer of asteroids. The Minor Planet Center ranks him among the top 10 discoverers of minor planets ...
and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Another notable object is 311P/PanSTARRS: observations made by the Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
revealed that it had six comet-like tails. The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile
In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...
asteroid spinning fast enough to remove material from it.
Exploration
Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
can only resolve a small amount of detail on the surfaces of the largest asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
s (variation in brightness during rotation) and their spectral properties. Sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). Radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. Spacecraft flybys can provide much more data than any ground or space-based observations; sample-return missions gives insights about regolith composition.
Ground-based observations
As asteroids are rather small and faint objects, the data that can be obtained from ground-based observations (GBO) are limited. By means of ground-based optical telescopes the visual magnitude can be obtained; when converted into the absolute magnitude it gives a rough estimate of the asteroid's size. Light-curve measurements can also be made by GBO; when collected over a long period of time it allows an estimate of the rotational period, the pole orientation (sometimes), and a rough estimate of the asteroid's shape. Spectral data (both visible-light and near-infrared spectroscopy) gives information about the object's composition, used to classify the observed asteroids. Such observations are limited as they provide information about only the thin layer on the surface (up to several micrometers). As planetologist Patrick Michel
Patrick Michel (born 25 February 1970 in Saint-Tropez, France) is a French planetary scientist, Senior Researcher at CNRS (Centre National de la Recherche Scientifique), leader of the team TOP (Theories and Observations in Planetology) of the Lag ...
writes:
Mid- to thermal-infrared observations, along with polarimetry measurements, are probably the only data that give some indication of actual physical properties. Measuring the heat flux of an asteroid at a single wavelength gives an estimate of the dimensions of the object; these measurements have lower uncertainty than measurements of the reflected sunlight in the visible-light spectral region. If the two measurements can be combined, both the effective diameter and the geometric albedo—the latter being a measure of the brightness at zero phase angle, that is, when illumination comes from directly behind the observer—can be derived. In addition, thermal measurements at two or more wavelengths, plus the brightness in the visible-light region, give information on the thermal properties. The thermal inertia, which is a measure of how fast a material heats up or cools off, of most observed asteroids is lower than the bare-rock reference value but greater than that of the lunar regolith; this observation indicates the presence of an insulating layer of granular material on their surface. Moreover, there seems to be a trend, perhaps related to the gravitational environment, that smaller objects (with lower gravity) have a small regolith layer consisting of coarse grains, while larger objects have a thicker regolith layer consisting of fine grains. However, the detailed properties of this regolith layer are poorly known from remote observations. Moreover, the relation between thermal inertia and surface roughness is not straightforward, so one needs to interpret the thermal inertia with caution.
Near-Earth asteroids that come into close vicinity of the planet can be studied in more details with radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
; it provides information about the surface of the asteroid (for example can show the presence of craters and boulders). Such observations were conducted by the Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
in Puerto Rico (305 meter dish) and Goldstone Observatory
The Goldstone Deep Space Communications Complex (GDSCC), commonly called the Goldstone Observatory, is a satellite ground station located in Fort Irwin in the U.S. state of California. Operated by NASA's Jet Propulsion Laboratory (JPL), its main ...
in California (70 meter dish). Radar observations can also be used for accurate determination of the orbital and rotational dynamics of observed objects.[
]
Space-based observations
Both space and ground-based observatories conducted asteroid search programs; the space-based searches are expected to detect more objects because there is no atmosphere to interfere and because they can observe larger portions of the sky. NEOWISE
Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
observed more than 100,000 asteroids of the main belt, Spitzer Space Telescope
The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...
observed more than 700 near-Earth asteroids. These observations determined rough sizes of the majority of observed objects, but provided limited detail about surface properties (such as regolith depth and composition, angle of repose, cohesion, and porosity).[
Asteroids were also studied by the ]Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
, such as tracking the colliding asteroids in the main belt, break-up of an asteroid, observing an active asteroid
Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jup ...
with six comet-like tails, and observing asteroids that were chosen as targets of dedicated missions.
Space probe missions
According to Patrick Michel
Patrick Michel (born 25 February 1970 in Saint-Tropez, France) is a French planetary scientist, Senior Researcher at CNRS (Centre National de la Recherche Scientifique), leader of the team TOP (Theories and Observations in Planetology) of the Lag ...
,
The internal structure of asteroids is inferred only from indirect evidence: bulk densities measured by spacecraft, the orbits of natural satellites in the case of asteroid binaries, and the drift of an asteroid's orbit due to the Yarkovsky thermal effect. A spacecraft near an asteroid is perturbed enough by the asteroid's gravity to allow an estimate of the asteroid's mass. The volume is then estimated using a model of the asteroid's shape. Mass and volume allow the derivation of the bulk density, whose uncertainty is usually dominated by the errors made on the volume estimate. The internal porosity of asteroids can be inferred by comparing their bulk density with that of their assumed meteorite analogues, dark asteroids seem to be more porous (>40%) than bright ones. The nature of this porosity is unclear.
Dedicated missions
The first asteroid to be photographed in close-up was 951 Gaspra
951 Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visited by his contemp ...
in 1991, followed in 1993 by 243 Ida
Ida, minor planet designation 243 Ida, is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after a nymph from Greek mythology. ...
and its moon Dactyl
Dactyl may refer to:
* Dactyl (mythology), a legendary being
* Dactyl (poetry), a metrical unit of verse
* Dactyl Foundation, an arts organization
* Finger, a part of the hand
* Dactylus, part of a decapod crustacean
* "-dactyl", a suffix used ...
, all of which were imaged by the ''Galileo'' probe en route to Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille
9969 Braille, provisional designation , is an eccentric, rare-type and elongated asteroid from the innermost regions of the asteroid belt, classified as Mars-crosser and slow rotator, approximately 1–2 kilometers in diameter. It was discovere ...
(by ''Deep Space 1
''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies.
Launched on 24 October 1998, the ''Deep Space 1 ...
'' in 1999), 5535 Annefrank
5535 Annefrank (), provisional designation , is a stony Florian asteroid and suspected contact binary from the inner asteroid belt, approximately 4.5 kilometers in diameter. It was used as a target to practice the flyby technique that the Stard ...
(by '' Stardust'' in 2002), 2867 Šteins
2867 Šteins (provisional designation ) is an irregular, diamond-shaped background asteroid from the inner regions of the asteroid belt, approximately in diameter. It was discovered on 4 November 1969 by Soviet astronomer Nikolai Chernykh at the ...
and 21 Lutetia
)
, mp_category=Main belt
, mpc_name=(21) Lutetia
, orbit_ref =
, epoch=May 31, 2020 ( JD 2459000.5)
, semimajor=2.435 AU
, perihelion=2.037 AU
, aphelion=2.833 AU
, eccentricity=0.16339
, period=3.80 yr (1388.1 d)
, inclination=3.064 ...
(by the ''Rosetta'' probe in 2008), and 4179 Toutatis
4179 Toutatis, provisional designation , is an elongated, stony asteroid and slow rotator, classified as a near-Earth object and potentially hazardous asteroid of the Apollo asteroid and Alinda asteroid groups, approximately 2.5 kilometers in d ...
(China's lunar orbiter ''Chang'e 2
Chang'e 2 (; ) is a Chinese unmanned lunar probe that was launched on 1 October 2010. It was a follow-up to the Chang'e 1 lunar probe, which was launched in 2007. Chang'e 2 was part of the first phase of the Chinese Lunar Exploration Program, and ...
'', which flew within in 2012).
The first dedicated asteroid probe was NASA's ''NEAR Shoemaker
''Near Earth Asteroid Rendezvous – Shoemaker'' (''NEAR Shoemaker''), renamed after its 1996 launch in honor of planetary scientist Eugene Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laborator ...
'', which photographed 253 Mathilde
Mathilde (minor planet designation: 253 Mathilde) is an asteroid in the intermediate asteroid belt, approximately 50 kilometers in diameter, that was discovered by Austrian astronomer Johann Palisa at Vienna Observatory on 12 November 1885. It h ...
in 1997, before entering into orbit around 433 Eros
Eros (minor planet designation: (433) Eros), provisional designation is a stony asteroid of the Amor group and the first discovered and second-largest near-Earth object with an elongated shape and a mean diameter of approximately . Visi ...
, finally landing on its surface in 2001. It was the first spacecraft to successfully orbit and land on an asteroid. From September to November 2005, the Japanese ''Hayabusa
was a robotic spacecraft developed by the Japan Aerospace Exploration Agency (JAXA) to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis.
''Hayabusa'', formerly known as MUSES-C ...
'' probe studied 25143 Itokawa
25143 Itokawa (provisional designation ) is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. It was discovered by the LINEAR program in 1998 and later named after Japanese rocket engineer Hideo Itokawa ...
in detail and returned samples of its surface to Earth on 13 June 2010, the first asteroid sample-return mission. In 2007, NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
launched the ''Dawn'' spacecraft, which orbited 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
for a year, and observed the dwarf planet Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
for three years.
''Hayabusa2
is an asteroid sample-return mission operated by the Japanese state space agency JAXA. It is a successor to the ''Hayabusa'' mission, which returned asteroid samples for the first time in June 2010. ''Hayabusa2'' was launched on 3 December 2 ...
'', a probe launched by JAXA
The is the Japanese national air and space agency. Through the merger of three previously independent organizations, JAXA was formed on 1 October 2003. JAXA is responsible for research, technology development and launch of satellites into orb ...
2014, orbited its target asteroid 162173 Ryugu
162173 Ryugu, provisional designation , is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately in diameter and is a dark object of the rare spectral type Cb, with qualities of both a C-type ...
for more than a year and took samples that were delivered to Earth in 2020. The spacecraft is now on an extended mission and expected to arrive at a new target in 2031.
NASA launched the OSIRIS-REx
OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a NASA asteroid-study and sample-return mission. The mission's primary goal is to obtain a sample of at least from 101955 Bennu, a carbonac ...
in 2016, a sample return mission to asteroid 101955 Bennu
101955 Bennu (provisional designation ) is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table and has the highest cumula ...
. In 2021, the probe departed the asteroid with a sample from its surface. Sample delivery to Earth is expected on September 24, 2023. The spacecraft will continue on an extended mission, designated OSIRIS-APEX, to explore near-Earth asteroid Apophis in 2029.
Hayabusa2 Ion thruster.jpg, ''Hayabusa2''
File:Dawn_-_PIA12033.jpg, ''Dawn''
Lucy-PatroclusMenoetius-art.png, ''Lucy''
PSYCHE.jpg, ''Psyche''
Planned missions
Currently, several asteroid-dedicated missions are planned by NASA, JAXA, ESA, and CNSA.
NASA's ''Lucy
Lucy is an English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings are Luci, Luce, Luci ...
'', launched in 2021, would visit eight asteroids, one from the main belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
and seven Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange poin ...
s; it is the first mission to trojans. The main mission would start in 2027.
In November 2021, NASA launched its Double Asteroid Redirection Test
Double Asteroid Redirection Test (DART) is a NASA space mission aimed at testing a method of planetary defense against near-Earth objects (NEOs). It was designed to assess how much a spacecraft impact deflects an asteroid through its transfer ...
(DART), a mission to test technology for defending Earth against potential hazardous objects. DART will deliberately crash into the minor-planet moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
Dimorphos
(65803) Didymos I Dimorphos (provisional designation S/2003 (65803) 1) is a minor-planet moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. It has a diameter of and has been characterised as a low-density rubb ...
of the double asteroid Didymos in September 2022 to assess the future potential of a spacecraft impact to deflect an asteroid from a collision course with Earth through a transference of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
. ESA's ''Hera
In ancient Greek religion, Hera (; grc-gre, Ἥρα, Hḗrā; grc, Ἥρη, Hḗrē, label=none in Ionic and Homeric Greek) is the goddess of marriage, women and family, and the protector of women during childbirth. In Greek mythology, she ...
'', planned for launch in 2024, will study the results of the DART impact. It will measure the size and morphology of the crater, and momentum transmitted by the impact, to determine the efficiency of the deflection produced by DART.
NASA's ''Psyche
Psyche (''Psyché'' in French) is the Greek term for "soul" (ψυχή).
Psyche may also refer to:
Psychology
* Psyche (psychology), the totality of the human mind, conscious and unconscious
* ''Psyche'', an 1846 book about the unconscious by Car ...
'' would be launched in 2023 or 2024 to study the large metallic asteroid of the same name. ''Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
'' is a planned dual space probe to be launched as a secondary payload on the ''Psyche'' launch.
JAXA's DESTINY+
Destiny, sometimes referred to as fate (from Latin ''fatum'' "decree, prediction, destiny, fate"), is a predetermined course of events. It may be conceived as a predetermined future, whether in general or of an individual.
Fate
Although often ...
is a mission for a flyby of the Geminids
The Geminids are a prolific meteor shower caused by the object 3200 Phaethon, which is thought to be a Palladian asteroid with a " rock comet" orbit. This would make the Geminids, together with the Quadrantids, the only major meteor showers ...
meteor shower parent body 3200 Phaethon
3200 Phaethon (previously sometimes spelled Phaeton), provisional designation , is an active Apollo asteroid with an orbit that brings it closer to the Sun than any other named asteroid (though there are numerous unnamed asteroids with smaller ...
, as well as various minor bodies. Its launch is planned for 2024.
CNSA's ''Tianwen-2
''Tianwen-2'', formerly known as ''ZhengHe'', is a planned Chinese asteroid sample-return and comet exploration mission that is currently under development.
Overview
''Tianwen-2'' is planned to be launched by a Long March 3B rocket around 20 ...
'' is planned to launch in 2025. It will use solar electric propulsion
Solar electric propulsion (SEP) refers to the combination of solar cells and electric thrusters to propel a spacecraft through outer space. This technology has been exploited in a variety of spacecraft by the European Space Agency (ESA), th ...
to explore the co-orbital
In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonan ...
near-Earth asteroid 469219 Kamoʻoalewa
469219 Kamoʻoalewa (), provisionally designated , is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately in diameter. At present it is a quasi-satellite of Earth, and currently the smallest, closest, ...
and the active asteroid
Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jup ...
311P/PanSTARRS. The spacecraft will collect samples of the regolith of Kamo'oalewa.
Asteroid mining
The concept of asteroid mining was proposed in 1970s. Matt Anderson defines successful asteroid mining as "the development of a mining program that is both financially self-sustaining and profitable to its investors". It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth, or materials for constructing space habitat
A space habitat (also called a space settlement, space colony, spacestead, space city, orbital habitat, orbital settlement, orbital colony, orbital stead or orbital city) is a more advanced form of living quarters than a space station or habit ...
s. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing
Space manufacturing is the production of tangible goods beyond Earth. Since most production capabilities are limited to low Earth orbit, the term in-orbit manufacturing is also frequently used.
There are several rationales supporting in-space ...
and construction.
As resource depletion
Resource depletion is the consumption of a resource faster than it can be replenished. Natural resources are commonly divided between renewable resources and non-renewable resources (see also mineral resource classification). Use of either ...
on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
for profit, or using space-based resources to build solar-power satellites and space habitats
A space habitat (also called a space settlement, space colony, spacestead, space city, orbital habitat, orbital settlement, orbital colony, orbital stead or orbital city) is a more advanced form of living quarters than a space station or habi ...
, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depot
An orbital propellant depot is a cache of propellant that is placed in orbit around Earth or another body to allow spacecraft or the transfer stage of the spacecraft to be fueled in space. It is one of the types of space resource depots that ha ...
s.
From the astrobiological
Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI
The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other p ...
). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable.
Mining Ceres
Ceres most commonly refers to:
* Ceres (dwarf planet), the largest asteroid
* Ceres (mythology), the Roman goddess of agriculture
Ceres may also refer to:
Places
Brazil
* Ceres, Goiás, Brazil
* Ceres Microregion, in north-central Goiás ...
is also considered a possibility. As the largest body in the asteroid belt, Ceres could become the main base and transport hub for future asteroid mining infrastructure, allowing mineral resources to be transported to Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, and Earth. Because of its small escape velocity combined with large amounts of water ice, it also could serve as a source of water, fuel, and oxygen for ships going through and beyond the asteroid belt.[ Transportation from Mars or the Moon to Ceres would be even more energy-efficient than transportation from Earth to the Moon.
]
Threats to Earth
There is increasing interest in identifying asteroids whose orbits cross Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's, and that could, given enough time, collide with Earth. The three most important groups of near-Earth asteroid
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). ...
s are the Apollos
Apollos ( grc-gre, Ἀπολλώς) was a 1st-century Alexandrian Jewish Christian mentioned several times in the New Testament. A contemporary and colleague of Paul the Apostle, he played an important role in the early development of the c ...
, Amors, and Atens
The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids . The group is named after 2062 Aten, the first of its kind, discovered on 7 Januar ...
.
The near-Earth asteroid 433 Eros
Eros (minor planet designation: (433) Eros), provisional designation is a stony asteroid of the Amor group and the first discovered and second-largest near-Earth object with an elongated shape and a mean diameter of approximately . Visi ...
had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor
1221 Amor is an asteroid and near-Earth object on an eccentric orbit, approximately in diameter. It is the namesake of the Amor asteroids, the second-largest subgroup of near-Earth objects. It was discovered by Eugène Delporte at the Uccle Obs ...
, 1862 Apollo
1862 Apollo is a stony asteroid, approximately 1.5 kilometers in diameter, classified as a near-Earth object (NEO). It was discovered by German astronomer Karl Reinmuth at Heidelberg Observatory on 24 April 1932, but lost and not recovered until ...
, 2101 Adonis
2101 Adonis, ''provisional designation'': , is an asteroid on an extremely eccentric orbit, classified as potentially hazardous asteroid and near-Earth object of the Apollo group. ''Adonis'' measures approximately in diameter. Discovered by Eug ...
, and finally 69230 Hermes
69230 Hermes is a sub-kilometer sized asteroid and binary system on an eccentric orbit, classified as a potentially hazardous asteroid and near-Earth object of the Apollo group, that passed Earth at approximately twice the distance of the Moon on ...
, which approached within 0.005 AU of Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis
The Alvarez hypothesis posits that the Extinction event, mass extinction of the non-avian dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the Impact event, impact of a large asteroid on the ...
that an impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
crashing into Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. The U.S. military also declassified the information that its military satellite
A military satellite is an artificial satellite used for a military purpose. The most common missions are intelligence gathering, navigation and military communications.
The first military satellites were photographic reconnaissance missions. So ...
s, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.
All of these considerations helped spur the launch of highly efficient surveys, consisting of charge-coupled device ( CCD) cameras and computers directly connected to telescopes. , it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.[ A list of teams using such systems includes:
* ]Lincoln Near-Earth Asteroid Research
The Lincoln Near-Earth Asteroid Research (LINEAR) project is a collaboration of the United States Air Force, NASA, and the Massachusetts Institute of Technology's Lincoln Laboratory for the systematic detection and tracking of near-Earth objects ...
(LINEAR)
* Near-Earth Asteroid Tracking
Near-Earth Asteroid Tracking (NEAT) was a program run by NASA and the Jet Propulsion Laboratory, Astronomical survey, surveying the sky for near-Earth objects. NEAT was conducted from December 1995 until April 2007, at Ground-based Electro-Optica ...
(NEAT)
* Spacewatch
The Spacewatch Project is an astronomical survey that specializes in the study of minor planets, including various types of asteroids and comets at University of Arizona telescopes on Kitt Peak near Tucson, Arizona, in the United States. The Sp ...
* Lowell Observatory Near-Earth-Object Search
Lowell Observatory Near-Earth-Object Search (LONEOS) was a project designed to discover asteroids and comets that orbit near the Earth. The project, funded by NASA, was directed by astronomer Ted Bowell of Lowell Observatory in Flagstaff, Arizo ...
(LONEOS)
* Catalina Sky Survey
Catalina Sky Survey (CSS; obs. code: 703) is an astronomical survey to discover comets and asteroids. It is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States.
CSS focuses on the search ...
(CSS)
* Pan-STARRS
The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical c ...
* NEOWISE
Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
* Asteroid Terrestrial-impact Last Alert System
The Asteroid Terrestrial-impact Last Alert System (ATLAS) is a robotic astronomical survey and early warning system optimized for detecting smaller near-Earth objects a few weeks to days before they impact Earth.
Funded by NASA, and developed an ...
(ATLAS)
* Campo Imperatore Near-Earth Object Survey
The CINEOS program (Campo Imperatore Near-Earth Object Survey), started in 2001, is dedicated to the discovery and follow-up of near-Earth objects (NEOs), namely asteroids and comets which periodically approach or intersect the Earth's orbit. In ...
(CINEOS)
* Japanese Spaceguard Association
The is a not-for-profit organization based in Tokyo, Japan. Its formal status under the Japanese law is .
Missions
The aims of the Japan Spaceguard Association (JSGA) echoes that of The Spaceguard Foundation and other spaceguard movements: to ...
* Asiago-DLR Asteroid Survey
The Asiago-DLR Asteroid Survey (ADAS; obs. code: 209) was an astronomical survey conducted in the early 2000s to search for comets and asteroids, with special emphasis on near-Earth objects. The Minor Planet Center directly credits ADA ...
(ADAS)
, the LINEAR system alone had discovered 147,132 asteroids. Among the surveys, 19,266 near-Earth asteroids have been discovered including almost 900 more than in diameter.
In April 2018, the B612 Foundation
The B612 Foundation is a private nonprofit foundation headquartered in Mill Valley, California, United States, dedicated to planetary science and planetary defense against asteroids and other near-Earth object (NEO) impacts. It is led mainl ...
reported "It is 100 percent certain we'll be hit y a devastating asteroid
Y, or y, is the twenty-fifth and penultimate Letter (alphabet), letter of the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. According to some auth ...
but we're not 100 percent sure when." In June 2018, the US National Science and Technology Council
The National Science and Technology Council (NSTC) is a council in the Executive Branch of the United States. It is designed to coordinate science and technology policy across the branches of federal government.
History
The National Science and ...
warned that America is unprepared for an asteroid impact event, and has developed and released the ''"National Near-Earth Object Preparedness Strategy Action Plan"'' to better prepare. According to expert testimony in the United States Congress
The United States Congress is the legislature of the federal government of the United States. It is bicameral, composed of a lower body, the House of Representatives, and an upper body, the Senate. It meets in the U.S. Capitol in Washing ...
in 2013, NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
would require at least five years of preparation before a mission to intercept an asteroid could be launched.
The United Nations declared 30 June as International Asteroid Day
__NOTOC__
Asteroid Day (also known as International Asteroid Day) is an annual global event which is held on June 30, the anniversary of the Tunguska event in 1908 when a meteor air burst levelled about of forest in Siberia, Russia.
Asteroid ...
to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia
The Tunguska event (occasionally also called the Tunguska incident) was an approximately 12-TNT equivalent, megaton explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russian Empire, Rus ...
, on 30 June 1908.
Chicxulub impact
The Chicxulub crater is an impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
buried underneath the Yucatán Peninsula
The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula in southeastern Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north ...
in Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
. Its center is offshore near the communities of Chicxulub Puerto and Chicxulub Pueblo
Pueblo ( ) is a town, and surrounding municipality of the same name, in the Mexican state of Yucatán.
At the census of 2010, the town had a population of 4,080 people.
Chicxulub is most famous for being near the geographic center of the C ...
, after which the crater is named. It was formed when a large asteroid, about in diameter, struck the Earth. The crater is estimated to be in diameter and in depth. It is one of the largest confirmed impact structures on Earth, and the only one whose peak ring
A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the crater's center, ...
is intact and directly accessible for scientific research.
In the late 1970s, geologist Walter Alvarez
Walter Alvarez (born October 3, 1940) is a professor in the Earth and Planetary Science department at the University of California, Berkeley. He is most widely known for the theory that dinosaurs were killed by an asteroid impact, developed in c ...
and his father, Nobel Prize–winning scientist Luis Walter Alvarez
Luis Walter Alvarez (June 13, 1911 – September 1, 1988) was an American experimental physicist, inventor, and professor who was awarded the Nobel Prize in Physics in 1968 for his discovery of resonance states in particle physics using the h ...
, put forth their theory that the Cretaceous–Paleogene extinction was caused by an impact event. The main evidence of such an impact was contained in a thin layer of clay present in the K–Pg boundary in Gubbio, Italy. The Alvarezes and colleagues reported that it contained an abnormally high concentration of iridium, a chemical element rare on earth but common in asteroids. Iridium levels in this layer were as much as 160 times above the background level. It was hypothesized that the iridium was spread into the atmosphere when the impactor was vaporized and settled across the Earth's surface among other material thrown up by the impact, producing the layer of iridium-enriched clay. At the time, consensus was not settled on what caused the Cretaceous–Paleogene extinction and the boundary layer, with theories including a nearby supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
, climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, or a geomagnetic reversal
A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's field has alternated ...
. The Alvarezes' impact hypothesis was rejected by many paleontologists, who believed that the lack of fossils found close to the K–Pg boundary—the "three-meter problem"—suggested a more gradual die-off of fossil species.
There is broad consensus that the Chicxulub impactor was an asteroid with a carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...
composition, rather than a comet. The impactor was around in diameter—large enough that, if set at sea level, it would have reached taller than Mount Everest
Mount Everest (; Tibetan: ''Chomolungma'' ; ) is Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border runs across its summit point. Its elevation (snow heig ...
.
Asteroid deflection strategies
Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet.[C. D. Hall and I. M. Ross, "Dynamics and Control Problems in the Deflection of Near-Earth Objects", ''Advances in the Astronautical Sciences, Astrodynamics 1997'', Vol. 97, Part I, 1997, pp. 613–631.] These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception, rendezvous, or remote station).
Strategies fall into two basic sets: fragmentation and delay. Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is approximately 12,750 km in diameter and moves at approx. 30 km per second in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.
" Project Icarus" was one of the first projects designed in 1967 as a contingency plan in case of collision with 1566 Icarus
1566 Icarus ( ; ''provisional designation'': ) is a large near-Earth object of the Apollo group and the lowest numbered potentially hazardous asteroid. It has is an extremely eccentric orbit (0.83) and measures approximately in diameter. In 196 ...
. The plan relied on the new Saturn V
Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, with multistage rocket, three stages, and powered with liquid-propellant r ...
rocket, which did not make its first flight until after the report had been completed. Six Saturn V rockets would be used, each launched at variable intervals from months to hours away from impact. Each rocket was to be fitted with a single 100-megaton nuclear warhead
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb ...
as well as a modified Apollo Service Module
The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship ...
and uncrewed Apollo Command Module
The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship ...
for guidance to the target. The warheads would be detonated 30 meters from the surface, deflecting or partially destroying the asteroid. Depending on the subsequent impacts on the course or the destruction of the asteroid, later missions would be modified or cancelled as needed. The "last-ditch" launch of the sixth rocket would be 18 hours prior to impact.
Fiction
Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.
See also
* List of asteroid close approaches to Earth
This is a list of examples where an asteroid or meteoroid travels close to the Earth. Some are regarded as potentially hazardous objects if they are estimated to be large enough to cause regional devastation.
Near-Earth object detection technol ...
* List of exceptional asteroids
The following is a collection of lists of asteroids of the Solar System that are exceptional in some way, such as their size or orbit. For the purposes of this article, "asteroid" refers to minor planets out to the orbit of Neptune, and includes ...
* Lost minor planet
A minor planet is "lost" when today's observers cannot find it, because its location is too uncertain to target observations. This happens if the orbital elements of a minor planet are not known accurately enough, typically because the observat ...
* Meanings of minor-planet names
This is a list of minor planets which have been officially named by the Working Group Small Body Nomenclature (WGSBN) of the International Astronomical Union (IAU). The list consists of partial pages, each covering a number range of 1,000 bodies ci ...
Notes
References
Further reading
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External links
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{{Authority control
Minor planets