A micrometeoroid is a tiny

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 ...

: a small particle of rock in space, usually weighing less than a

gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure wate ...

. A

micrometeorite A micrometeorite is a micrometeoroid that has survived entry through the Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more abundant, and different in composition. T ...

is such a particle that survives passage through

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

and reaches Earth's surface. The term "micrometeoroid" was officially deprecated by 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 ...

in 2017, as redundant to meteoroid.

Origins and orbits

Micrometeoroids are very small pieces of rock or metal broken off from larger chunks of rock and debris often dating back to the birth of the

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 ...

. Micrometeoroids are extremely common in space. Tiny particles are a major contributor 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 ...

processes. When they hit the surface of 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 ...

, or any airless body ( Mercury, the

asteroid An asteroid is a minor planet of the 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 or icy bodies with no atmosphere. ...

s, etc.), the resulting melting and vaporization causes darkening and other optical changes in the

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 ...

. Micrometeoroids have less stable

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

s than meteoroids, due to their greater surface area to

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

ratio. Micrometeoroids that fall to Earth can provide information on millimeter scale heating events in the

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 ...

Meteorites 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 moon. When the original object en ...

and

micrometeorites A micrometeorite is a micrometeoroid that has survived entry through the Earth's atmosphere. Usually found on Earth's surface, micrometeorites differ from meteorites in that they are smaller in size, more abundant, and different in composition. T ...

(as they are known upon arrival at the Earth's surface) can only be collected in areas where there is no terrestrial

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

, typically polar regions. Ice is collected and then melted and filtered so the micrometeorites can be extracted under a microscope. Sufficiently small micrometeoroids avoid significant heating on entry into

. Collection of such particles by high-flying aircraft began in the 1970s, since which time these samples of stratosphere-collected

interplanetary dust The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System. ...

(called ''Brownlee particles'' before their extraterrestrial origin was confirmed) have become an important component of the

extraterrestrial materials Extraterrestrial material refers to natural objects now on Earth that originated in outer space. Such materials include cosmic dust and meteorites, as well as samples brought to Earth by sample return missions from the Moon, asteroids and comet ...

available for study in laboratories on Earth.

Historical studies

In 1946 during the Giacobinid meteor shower,

Helmut Landsberg Helmut Erich Landsberg (1906–1985) was a noted and influential climatologist. He was born in Frankfurt, Germany, February 9, 1906 and died December 6, 1985 in Geneva, Switzerland while attending a meeting of the World Meteorological Orga ...

collected several small magnetic particles that were apparently associated with the shower.

Fred Whipple Fred Lawrence Whipple (November 5, 1906 – August 30, 2004) was an American astronomer, who worked at the Harvard College Observatory for more than 70 years. Amongst his achievements were asteroid and comet discoveries, the " dirty snowball" h ...

was intrigued by this and wrote a paper that demonstrated that particles of this size were too small to maintain their velocity when they encountered the

upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

. Instead, they quickly decelerated and then fell to Earth unmelted. In order to classify these sorts of objects, he coined the term " micro-meteorite".

Velocities

Whipple, in collaboration with

Fletcher Watson Fletcher G. Watson (c. 1912 – 7 May 1997) was an American professor of science education at the Harvard Graduate School of Education (1946–77), where he served as the Henry Lee Shattuck Professor of Education (1966–77). Watson was a founder ...

of the

Harvard Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, United St ...

, led an effort to build an observatory to directly measure the velocity of the meteors that could be seen. At the time the source of the micro-meteorites was not known. Direct measurements at the new observatory were used to locate the source of the meteors, demonstrating that the bulk of material was left over from

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 ...

tails, and that none of it could be shown to have an extra-solar origin. Today it is understood that meteoroids of all sorts are leftover material from the formation of the Solar System, consisting of particles from the

interplanetary dust cloud The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System ...

or other objects made up from this material, like comets.

Flux

The early studies were based exclusively on optical measurements. In 1957, Hans Pettersson conducted one of the first direct measurements of the fall of space dust on Earth, estimating it to be 14,300,000 tons per year.Hans Pettersson, "Cosmic Spherules and Meteoritic Dust." ''Scientific American'', Volume 202 Issue 2 (February 1960), pp. 123–132. This suggested that the meteoroid flux in space was much higher than the number based on telescope observations. Such a high flux presented a very serious risk to the high-orbiting

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= ...

capsules and for missions to the Moon. To determine whether the direct measurement was accurate, a number of additional studies followed, including the

Pegasus satellite program The Pegasus Project was a NASA initiative to study the frequency of micrometeoroid impacts on spacecraft by means of a constellation of three satellites launched in 1965. All three Pegasus satellites were launched by Saturn I rockets, and remaine ...

Lunar Orbiter 1 The 1966 Lunar Orbiter 1 robotic spacecraft mission, part of NASA's Lunar Orbiter program, was the first American spacecraft to orbit the Moon. It was designed primarily to photograph smooth areas of the lunar surface for selection and verifica ...

Luna 3 Luna 3, or E-2A No.1 ( rus, Луна 3}) was a Soviet spacecraft launched in 1959 as part of the Luna programme. It was the first mission to photograph the far side of the Moon and the third Soviet space probe to be sent to the neighborhood of th ...

Mars 1 Mars 1 Mars 1, also known as 1962 Beta Nu 1, Mars 2MV-4 and Sputnik 23, was an automatic interplanetary station launched in the direction of Mars on November 1, 1962, the first of the Soviet Mars probe program, with the intent of flying by the ...

and

Pioneer 5 ''Pioneer 5'' (also known as Pioneer P-2, and Able 4, and nicknamed the "Paddle-Wheel Satellite") was a spin-stabilized space probe in the NASA Pioneer program used to investigate interplanetary space between the orbits of Earth and Venus. It ...

. These showed that the rate of meteors passing into the atmosphere, or flux, was in line with the optical measurements, at around 10,000 to 20,000 tons per year. The

Surveyor Program The Surveyor program was a NASA program that, from June 1966 through January 1968, sent seven robotic spacecraft to the surface of the Moon. Its primary goal was to demonstrate the feasibility of soft landings on the Moon. The Surveyor craft w ...

determined that the surface of the Moon is relatively rocky.Snelling, Andrew and David Rush
"Moon Dust and the Age of the Solar System."
''Creation Ex-Nihilo Technical Journal'', Volume 7, Number 1, 1993, p. 242. Most lunar samples returned during the Apollo Program have micrometeorite impacts marks, typically called "zap pits", on their upper surfaces.

Effect on spacecraft operations

Micrometeoroids pose a significant threat to

space exploration Space exploration is the use of astronomy and space technology to explore outer space. While the exploration of space is carried out mainly by astronomers with telescopes, its physical exploration though is conducted both by robotic spacec ...

. The average velocity of micrometeoroids relative to a

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

in orbit is 10 kilometers per second (22,500 mph). Resistance to micrometeoroid impact is a significant design challenge for spacecraft and

space suit A space suit or spacesuit is a garment worn to keep a human alive in the harsh environment of outer space, vacuum and temperature extremes. Space suits are often worn inside spacecraft as a safety precaution in case of loss of cabin pressure, ...

designers (''See

Thermal Micrometeoroid Garment An (Integrated) Thermal Micrometeoroid Garment (TMG or ITMG) is the outer layer of a space suit. The TMG has three functions: to insulate the suit occupant and prevent heat loss, to shield the occupant from harmful solar radiation, and to protec ...

''). While the tiny sizes of most micrometeoroids limits the damage incurred, the high velocity impacts will constantly degrade the outer casing of spacecraft in a manner analogous to

sandblasting Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove su ...

. Long term exposure can threaten the functionality of spacecraft systems. Impacts by small objects with extremely high velocity (10 kilometers per second) are a current area of research in

terminal ballistics Terminal ballistics (also known as wound ballistics) is a sub-field of ballistics concerned with the behavior and effects of a projectile when it hits and transfers its energy to a target. Bullet design (as well as the velocity of impact) larg ...

. (Accelerating objects up to such velocities is difficult; current techniques include linear motors and

shaped charge A shaped charge is an explosive charge shaped to form an explosively formed penetrator (EFP) to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, ini ...

s.) The risk is especially high for objects in space for long periods of time, such as satellites. They also pose major engineering challenges in theoretical low-cost lift systems such as rotovators,

space elevator A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system, often depicted in science fiction. The main component would be a cable (also called a space tethe ...

s, and orbital airships.

Spacecraft micrometeoroid shielding

Whipple's work pre-dated the

space race The Space Race was a 20th-century competition between two Cold War rivals, the United States and the Soviet Union, to achieve superior spaceflight capability. It had its origins in the ballistic missile-based nuclear arms race between the tw ...

and it proved useful when space exploration started only a few years later. His studies had demonstrated that the chance of being hit by a meteoroid large enough to destroy a spacecraft was extremely remote. However, a spacecraft would be almost constantly struck by micrometeorites, about the size of dust grains. Whipple had already developed a solution to this problem in 1946. Originally known as a "meteor bumper" and now termed the

Whipple shield The Whipple shield or Whipple bumper, invented by Fred Whipple, is a type of hypervelocity impact shield used to protect crewed and uncrewed spacecraft from collisions with micrometeoroids and orbital debris whose velocities generally range be ...

, this consists of a thin foil film held a short distance away from the spacecraft's body. When a micrometeoroid strikes the foil, it vaporizes into a plasma that quickly spreads. By the time this plasma crosses the gap between the shield and the spacecraft, it is so diffused that it is unable to penetrate the structural material below.Brian Marsden
"Professor Fred Whipple: Astronomer who developed the idea that comets are 'dirty snowballs'."
''The Independent'', 13 November 2004. The shield allows a spacecraft body to be built to just the thickness needed for structural integrity, while the foil adds little additional weight. Such a spacecraft is lighter than one with panels designed to stop the meteoroids directly. For spacecraft that spend the majority of their time in orbit, some variety of the Whipple shield has been almost universal for decades. Later research showed that ceramic fibre woven shields offer better protection to hypervelocity (~7 km/s) particles than

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

shields of equal weight. Another modern design uses multi-layer flexible fabric, as in

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 ...

's design for its never-flown

TransHab TransHab was a concept pursued by NASA in the 1990s to develop the technology for expandable habitats inflated by air in space. Specifically, TransHab was intended as a replacement for the already existing rigid International Space Station crew H ...

expandable space habitation module,Kim Dismukes (curator)
"TransHab Concept"
, NASA, 27 June 2003. Retrieved 10 June 2007. and the

Bigelow Expandable Activity Module The Bigelow Expandable Activity Module (BEAM) is an experimental expandable space station module developed by Bigelow Aerospace, under contract to NASA, for testing as a temporary module on the International Space Station (ISS) from 2016 to ...

, which was launched in April 2016 and attached to the

ISS The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (J ...

for two years of orbital testing.

Footnotes

External links

Melted Crumbs from Asteroid Vesta
article about micrometeorites collected in Antarctica in Planetary Science Research Discoveries educational journal {{meteorites Meteoroids Cosmic dust Planetary science Space hazards tr:Mikrometeor