101955 Bennu
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101955 Bennu (provisional designation ) is a carbonaceous asteroid in the
Apollo Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label= Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label ...
group discovered by the
LINEAR Linearity is the property of a mathematical relationship ('' function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear ...
Project on 11 September 1999. It is a
potentially hazardous object A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and is large enough to cause significant regional damage in the event of impact. They ar ...
that is listed on the
Sentry Risk Table Sentry is a highly automated impact prediction system operated by the JPL Center for NEO Studies (CNEOS) since 2002. It continually monitors the most up-to-date asteroid catalog for possibilities of future impact with Earth over the next 100+ y ...
and has the highest cumulative rating on the
Palermo Technical Impact Hazard Scale The Palermo Technical Impact Hazard Scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-Earth object (NEO). It combines two types of data—probability of impact and estimated kinetic yield— ...
. It has a cumulative 1-in-1,800 chance of impacting
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 ...
between 2178 and 2290 with the greatest risk being on 24 September 2182. It is named after the
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 ...
, the ancient Egyptian mythological bird associated with the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, creation, and rebirth. has a mean diameter of and has been observed extensively by the Arecibo Observatory planetary radar and the Goldstone
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary ...
. Bennu was the target of 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 carbona ...
mission which is intended to return its samples to Earth in 2023 for further study. On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey. It orbited the asteroid and mapped out Bennu's surface in detail, seeking potential sample collection sites. Analysis of the orbits allowed calculation of Bennu's mass and its distribution. In October 2020, OSIRIS-REx successfully touched down on the surface of Bennu, collected a sample using an extendable arm, secured the sample and prepared for a return trip to Earth. On 10 May 2021, OSIRIS-REx successfully completed its departure from the Bennu asteroid while still carrying the sample of the asteroid rubble.


Discovery and observation

Bennu was discovered on 11 September 1999 during a Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research (LINEAR). The asteroid was given the
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 cal ...
and classified as a near-Earth asteroid. Bennu was observed extensively by the Arecibo Observatory and the Goldstone
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA's interplanetary ...
using radar imaging as Bennu closely approached Earth on 23 September 1999.


Naming

The name ''Bennu'' was selected from more than eight thousand student entries from dozens of countries around the world who entered a "Name That Asteroid!" contest run by the
University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a public land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first university in the Arizona Territory. T ...
,
The Planetary Society The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, a ...
, and the LINEAR Project in 2012. Third-grade student Michael Puzio from
North Carolina North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and ...
proposed the name in reference to the Egyptian mythological bird
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 ...
. To Puzio, the OSIRIS-REx spacecraft with its extended
TAGSAM TAGSAM or Touch-and-Go Sample Acquisition Mechanism is a robotic arm on the OSIRIS-REx space probe for collecting a sample from asteroid 101955 Bennu. OSIRIS-REx was launched in 2016, and arriving at the asteroid in December 2018 with plans to stu ...
arm resembled the Egyptian deity, which is typically depicted as a heron. Its features will be named after birds and bird-like creatures in mythology.


Physical characteristics

Bennu has a roughly spheroidal shape, resembling a spinning
top A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will usually wobble for a few ...
. Bennu's axis of rotation is tilted 178 degrees to its orbit; the direction of rotation about its axis is retrograde with respect to its orbit. While the initial ground based radar observations indicated that Bennu had a fairly smooth shape with one prominent boulder on its surface, high resolution data obtained by OSIRIS-REx revealed that the surface is much rougher with more than 200 boulders larger than on the surface, the largest of which is across. The boulders contain veins of high
albedo Albedo (; ) is the measure of the diffuse reflection of 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 that refl ...
carbonate mineral Carbonate minerals are those minerals containing the carbonate ion, . Carbonate divisions Anhydrous carbonates *Calcite group: trigonal ** Calcite CaCO3 ** Gaspéite (Ni,Mg,Fe2+)CO3 ** Magnesite MgCO3 ** Otavite CdCO3 **Rhodochrosite MnCO3 ** ...
s believed to have formed prior to the formation of the asteroid due to hot water channels on the much larger
parent body In meteoritics, a parent body is the celestial body from which originates a meteorite or a class of meteorites. Identification The easiest way to correlate a meteorite with a parent body is when the parent body still exists. This is the case fo ...
. The veins range from 3 to 15 centimeters wide, and can be over one meter in length, much bigger than carbonate veins seen in meteorites. There is a well-defined ridge along the equator of Bennu. The presence of this ridge suggests that fine-grained
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 terrestr ...
particles have accumulated in this area, possibly because of its low gravity and fast rotation (about once every 4.3 hours). Observations by the OSIRIS-REx spacecraft has shown that Bennu is rotating faster over time. This change in Bennu's rotation is caused by the
Yarkovsky–O'Keefe–Radzievskii–Paddack effect The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or YORP effect for short, changes the rotation state of a small astronomical body – that is, the body's spin rate and the obliquity of its pole(s) – due to the scattering of solar rad ...
, or the YORP effect. Due to the uneven emission of thermal radiation from its surface as Bennu rotates in sunlight, the rotation period of Bennu decreases by about one second every 100 years. Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of , which is in line with other studies. It has a low visible geometric albedo of . The
thermal inertia In thermodynamics, a material's thermal effusivity, thermal inertia or thermal responsivity is a measure of its ability to exchange thermal energy with its surroundings. It is defined as the square root of the product of the material's thermal co ...
was measured and found to vary by approximately 19% during each rotational period. It was based on this observation scientists (incorrectly) estimated a moderate
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 terrestr ...
grain size, ranging from several millimeters up to a centimeter, evenly distributed. No emission from a potential dust coma has been detected around Bennu, which puts a limit of 106 g of dust within a radius of 4750 km.
Astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. His ...
observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced 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  ...
, causing the
semimajor 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 lo ...
of its orbit to drift on average by  meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ =  kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is %, suggesting the interior has 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 or even hollows. The estimated mass is .


Photometry and spectroscopy

Photometric observations of Bennu in 2005 yielded a synodic rotation period of . It has a B-type classification, which is a sub-category of
carbonaceous Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up ...
asteroids. Polarimetric observations show that Bennu belongs to the rare F subclass of carbonaceous asteroids, which is usually associated with cometary features. Measurements over a range of phase angles showed a phase function slope of 0.040 magnitudes per degree, which is similar to other near-Earth asteroids with low albedo. Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI and/or CM carbonaceous chondrite meteorites, including carbonaceous-chondrite mineral
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
. Magnetite, a spectrally prominent water product but destroyed by heat, is an important proxy of astronomers including OSIRIS-REx staff.


Water

Predicted beforehand, Dante Lauretta (University of Arizona) then stated that Bennu is water-rich- already detectable while OSIRIS-REx was still technically in approach. Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx confirmed magnetite and the meteorite-asteroid linkage, dominated by
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 ...
. Phyllosilicates, among others, hold water. Bennu's water spectra were detectable on approach, reviewed by outside scientists, then confirmed from orbit. OSIRIS-REx observations have resulted in a (self-styled) conservative estimate of about 7 x 108 kg water in one form alone, neglecting additional forms. This is a water content of ~1 wt.%, and potentially much more. In turn this suggests transient pockets of water beneath Bennu's regolith. The surficial water may be lost from the collected samples. However, if the sample return capsule maintains low temperatures, the largest (centimeter-scale) fragments may contain measurable quantities of adsorbed water, and some fraction of Bennu's ammonium compounds.


Activity

Bennu is 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 ...
, sporadically emitting plumes of particles and rocks as large as ,. (not
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...
, defined as tens of micrometers). Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of
phyllosilicate 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 ...
s, pockets of subsurface water, and/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 mi ...
impacts. Before the arrival of OSIRIS-REx, Bennu had displayed polarization consistent with
Comet Hale-Bopp 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 are ...
and
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 small ...
, a rock comet. Bennu, Phaethon, and inactive Manx comets are examples of active asteroids. B-type asteroids displaying a blue color in particular, may be dormant comets. If the
IAU 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, outreach ...
declares Bennu to be a dual-status object, its comet designation would be P/ (LINEAR).


Surface features

All geological features on Bennu are named after various species of birds and bird-like figures in mythology. The first features to be named were the final four candidate OSIRIS-REx sample sites, which were given unofficial names by the team in August 2019. On 6 March 2020 the IAU announced the first official names for 12 Bennu surface features, including regiones (broad geographic regions), craters, dorsa (ridges), fossae (grooves or trenches) and saxa (rocks and boulders). Analysis showed that the particles making up Bennu's exterior are loosely packed and lightly bound to each other; "The spacecraft would have sunk into Bennu had it not fired its thrusters to back away immediately after it grabbed dust and rock from the asteroid's surface." Analysis also revealed that the Sun's heat fractures rocks on Bennu in just 10,000 to 100,000 years instead of millions of years as was thought before.


Candidate sample sites

On 12 December 2019, after a year of mapping Bennu's surface, a target site was announced. Named Nightingale, the area is near Bennu's north pole and lies inside a small crater within a larger crater. Osprey was selected as the backup sample site.


IAU named features

File:Bennu named surface features.png, Map of Bennu showing the locations of the IAU-named surface features File:Tour of Asteroid Bennu.webm, Narrated tour of Bennu's most prominent surface features File:Asteroid Bennu's Surprising Surface Revealed by OSIRIS-REx (long form).webm, Effects of OSIRIS-REx sampling attempt on Bennu


Origin and evolution

The carbonaceous material that composes Bennu originally came from the breakup of a much larger parent body—a
planetoid 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 ''mino ...
or a
proto-planet 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 ...
. But like nearly all other matter in the
Solar System The Solar System Capitalization 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 ...
, the origins of its minerals and atoms are to be found in dying stars such as red giants and supernovae. According to the accretion theory, this material came together 4.5 billion years ago during the
formation of the Solar System 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 int ...
. Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation inside a much larger
planetoid 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 ''mino ...
or a
proto-planet 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 ...
capable of producing the requisite pressure, heat and
hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...
(if need be)—into more complex minerals. Bennu probably began in the inner
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, c ...
as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family. Impactors on boulders of Bennu indicate that Bennu has been in near earth orbit (separated from 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, c ...
) for 1–2.5 million years. Subsequently, the orbit drifted as a result of 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  ...
and mean motion resonances with the giant planets, such as
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousandth t ...
and Saturn. Various interactions with the planets in combination with the Yarkovsky effect modified the asteroid, possibly changing its spin, shape, and surface features. Cellino et al. have suggested a possible cometary origin for Bennu, based on similarities of its spectroscopic properties with known comets. The estimated fraction of comets in the population of
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 is . This includes rock comet
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 small ...
, originally discovered as, and still numbered as an asteroid.


Orbit

Bennu currently orbits the Sun with a period of 1.20 years (437 days). Earth gets as close as about 480,000 km (0.0032  au) from its orbit around 23 to 25 September. On 22 September 1999 Bennu passed 0.0147 au from Earth, and six years later on 20 September 2005 it passed 0.033 au from Earth. The next close approaches of less than 0.09 au will be 30 September 2054 and then 23 September 2060, which will perturb the orbit slightly. Between the close approach of 1999 and that of 2060, Earth completes 61 orbits and Bennu 51. An even closer approach will occur on 25 September 2135 around 0.0014 au (see below). In the 75 years between the 2060 and 2135 approaches, Bennu completes 64 orbits, meaning its period will have changed to 1.17 years (427 days). The Earth approach of 2135 will increase the orbital period to about 1.24 years (452 days). Before the 2135 Earth approach, Bennu's maximum distance from the Earth occurs on 27 November 2045 at a distance of .


Possible Earth impact

On average, an asteroid with a diameter of can be expected to impact Earth about every 130,000 years or so. A 2010 dynamical study by Andrea Milani and collaborators predicted a series of eight potential Earth impacts by Bennu between 2169 and 2199. The cumulative probability of impact is dependent on physical properties of Bennu that were poorly known at the time, but was found to not exceed 0.071% for all eight encounters. The authors recognized that an accurate assessment of 's probability of Earth impact would require a detailed shape model and additional observations (either from the ground or from spacecraft visiting the object) to determine the magnitude and direction of 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  ...
. The publication of the shape model and of astrometry based on radar observations obtained in 1999, 2005, and 2011 made possible an improved estimate of the Yarkovsky acceleration and a revised assessment of the impact probability. In 2014, the best estimate of the impact probability was a cumulative probability of 0.037% in the interval 2175 to 2196. This corresponds to a cumulative score on the Palermo scale of −1.71. If an impact were to occur, the expected kinetic energy associated with the collision would be 1,200 megatons in
TNT equivalent TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a ...
(for comparison, TNT equivalent of
Little Boy "Little Boy" was the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II, making it the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress ''Enola Gay'' p ...
was approx 0.015 megaton). The 2021 orbit solution extended the virtual impactors from the year 2200 to the year 2300 and slightly increased the cumulative Palermo impact scale to −1.42. The solution even included the estimated masses of 343 other asteroids and represents about 90% of the total mass of 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 ...
.


2060/2135 close approaches

Bennu will pass from Earth on 23 September 2060, while 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 ...
's average orbital distance (''Lunar Distance, LD'') is
today Today (archaically to-day) may refer to: * Day of the present, the time that is perceived directly, often called ''now'' * Current era, present * The current calendar date Arts, entertainment, and media Films * ''Today'' (1930 film), a 1930 ...
and will be 384,404 km in 50 years time. It will be too dim to be seen with common binoculars. The close approach of 2060 causes divergence in the close approach of 2135. On 25 September 2135, the Earth approach distance is ±20 thousand km. There is no chance of an Earth impact in 2135. The 2135 approach will create many lines of variations and Bennu may pass through a gravitational keyhole during the 2135 passage which could create an impact scenario at a future encounter. The keyholes are all less than ~20 km wide with some keyholes being only 5 meters wide.


2182

The most threatening virtual impactor is on 24 September 2182 when there is a 1 in 2,700 chance of an Earth impact, but the asteroid could be as far as the Sun is from Earth. To impact Earth on 24 September 2182 Bennu needs to pass through a keyhole roughly 5 km wide on 25 September 2135.Table 3. Impact dates, keyhole centers and widths in the 2135 B-plane
(Farnocchia2021) The table reports the zeta coordinate on the B-plane, which is not the same thing as the miss distance during the 2135 encounter.
The next two biggest risks occur in 2187 (1:14,000) and 2192 (1:26,000). There is a cumulative 1 in 1,800 chance of an Earth impact between 2178 and 2290.


Long term

Lauretta et al. reported in 2015 their results of a computer simulation, concluding that it is more likely that 101955 Bennu will be destroyed by some other cause:
The orbit of Bennu is intrinsically dynamically unstable, as are those of all NEOs. In order to glean probabilistic insights into the future evolution and likely fate of Bennu beyond a few hundred years, we tracked 1,000 virtual "Bennus" for an interval of 300 Myr with the gravitational perturbations of the planets Mercury–Neptune included. Our results ... indicate that Bennu has a 48% chance of falling into the Sun. There is a 10% probability that Bennu will be ejected out of the inner Solar System, most likely after a close encounter with Jupiter. The highest impact probability for a planet is with Venus (26%), followed by the Earth (10%) and Mercury (3%). The odds of Bennu striking Mars are only 0.8% and there is a 0.2% chance that Bennu will eventually collide with Jupiter.


Meteor shower

As an active asteroid with a small
minimum orbit intersection distance Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits o ...
from Earth, Bennu may be the
parent body In meteoritics, a parent body is the celestial body from which originates a meteorite or a class of meteorites. Identification The easiest way to correlate a meteorite with a parent body is when the parent body still exists. This is the case fo ...
of a weak meteor shower. Bennu particles would radiate around 25 September from the southern constellation of
Sculptor Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...
. The meteors are expected to be near the naked eye limit and only produce a Zenith hourly rate of less than 1.


OSIRIS-REx

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 carbona ...
mission of NASA's
New Frontiers program The New Frontiers program is a series of space exploration missions being conducted by NASA with the purpose of furthering the understanding of the Solar System. The program selects medium-class missions which can provide high science returns. ...
was launched towards on 8 September 2016. On 3 December 2018, the spacecraft arrived at the asteroid Bennu after a two-year journey. One week later, at the
American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, atmospheric, ocean, hydrologic, space, and planetary scientists and enthusiasts that according to their website includes 130,000 people (not members). AGU's a ...
Fall Meeting, investigators announced that OSIRIS-REx had discovered spectroscopic evidence for hydrated minerals on the surface of the asteroid, implying that liquid water was present in Bennu's parent body before it split off. On 20 October 2020, OSIRIS-REx descended to the asteroid and " pogo-sticked off" it while successfully collecting a sample. OSIRIS-REx is expected to return samples to Earth in 2023 via a capsule-drop by parachute, ultimately, from the spacecraft to the Earth's surface in
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
on 24 September. On 7 April 2021, OSIRIS-REx completed its final flyover of the asteroid and began slowly drifting away from it. On 10 May 2021, the departure was completed with OSIRIS-REx while still managing to contain the asteroid sample.


Selection

The asteroid Bennu was selected from over half a million known asteroids by the OSIRIS-REx selection committee. The primary constraint for selection was close proximity to Earth, since proximity implies low
impulse Impulse or Impulsive may refer to: Science * Impulse (physics), in mechanics, the change of momentum of an object; the integral of a force with respect to time * Impulse noise (disambiguation) * Specific impulse, the change in momentum per uni ...
(Δv) required to reach an object from Earth orbit. The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of . Furthermore, the candidate asteroid for a sample-return mission must have loose regolith on its surface, which implies a diameter greater than 200 meters. Asteroids smaller than this typically spin too fast to retain dust or small particles. Finally, a desire to find an asteroid with pristine carbon material from the early Solar System, possibly including volatile molecules and
organic compound 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. T ...
s, reduced the list further. With the above criteria applied, five asteroids remained as candidates for the OSIRIS-REx mission, and Bennu was chosen, in part for its potentially hazardous orbit.


Gallery

File:Bennu shape model.gif, A compilation of radar images of asteroid Bennu (left) and a corresponding 3D shape model. (right) File:Bennu 200px.png, This picture, taken by the OSIRIS-REx spacecraft on November 2, 2018, was part of a sequence of frames collected to show asteroid 101955 Bennu rotating. Bennu is approximately 200 pixels wide in this shot. File:1999 RQ36.ogv, Animation of OSIRIS-REx collecting a sample from the surface of Bennu. File:Bennu 75cm shape model.stl, 3D shape model of Bennu made from OSIRIS-REx images.


See also

*
List of minor planets and comets visited by spacecraft The following tables list all minor planets and comets that have been visited by robotic spacecraft. List of minor planets visited by spacecraft A total of 17 minor planets (asteroids, dwarf planets, and Kuiper belt objects) have been visit ...
* , provisional target of the cancelled
Asteroid Redirect Mission The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013; the mission was later cancelled. The Asteroid Retrieval Robotic ...
*
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 ...
, an asteroid being studied JAXA concurrent with the NASA mission to 101955 Bennu *
73P/Schwassmann–Wachmann 73P/Schwassmann–Wachmann, also known as Schwassmann–Wachmann 3, is a periodic comet that has a 5.4 year orbital period and that has been actively disintegrating since 1995. It last came to perihelion (closest approach to the Sun) in March 20 ...
, a disintegrating-
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 ...


References


External links


Video (2:53)
– Asteroid Bennu Mission Overview (
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
; 11 May 2021).
Video (01:12) – Asteroid Bennu ejecting material into space
(
CNN CNN (Cable News Network) is a multinational cable news channel headquartered in Atlanta, Georgia, U.S. Founded in 1980 by American media proprietor Ted Turner and Reese Schonfeld as a 24-hour cable news channel, and presently owned by ...
; 5 December 2019)
Video (01:32) – OSIRIS REx's approach to asteroid Bennu
(
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
; 7 January 2019)
Earth Impact Risk Summary: 101955 1999 RQ36
(Years: 2175–2199) –
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
near-Earth object site
NEODyS-2 Ephemerides for 2135
(step size: 10 days) * * *
Nominal and impacting solution for 2182
* * {{DEFAULTSORT:101955 Apollo asteroids Potentially hazardous asteroids Active asteroids Discoveries by LINEAR
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 ...
Potential impact events caused by near-Earth objects Minor planets visited by spacecraft
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 ...
Articles containing video clips 19990911 B-type asteroids (SMASS)