Micrometeoroid

Micrometeorite, collected from the Antarctic snow, was a micrometeoroid before it entered the Earth's atmosphere

Amicrometeoroidis a tinymeteoroid:a small particle of rock in space, usually weighing less than agram.Amicrometeoriteis such a particle that survives passage throughEarth's atmosphereand reaches Earth's surface.

The term "micrometeoroid" was officially deprecated by theIAUin 2017, as redundant to meteoroid.

Origins and orbits

See also:Cosmic dust

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 theSolar System.Micrometeoroids are extremely common in space. Tiny particles are a major contributor tospace weatheringprocesses. When they hit the surface of theMoon,or any airless body (Mercury,theasteroids,etc.), the resulting melting and vaporization causes darkening and other optical changes in theregolith.

Micrometeoroids have less stableorbitsthan meteoroids, due to their greatersurface areatomassratio. Micrometeoroids that fall to Earth can provide information on millimeter scale heating events in thesolar nebula.Meteoritesandmicrometeorites(as they are known upon arrival at the Earth's surface) can only be collected in areas where there is no terrestrialsedimentation,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 intoEarth's atmosphere.Collection of such particles by high-flying aircraft began in the 1970s,since which time these samples of stratosphere-collectedinterplanetary dust(calledBrownlee particlesbefore their extraterrestrial origin was confirmed) have become an important component of theextraterrestrial materialsavailable for study in laboratories on Earth.

Historical studies

In 1946 during theGiacobinidmeteor shower,Helmut Landsbergcollected several small magnetic particles that were apparently associated with the shower.Fred Whipplewas intrigued by this and wrote a paper that demonstrated that particles of this size were too small to maintain their velocity when they encountered theupper atmosphere.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 withFletcher Watsonof theHarvard Observatory,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 fromcomettails, 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 theinterplanetary dust cloudor other objects made up from this material, like comets.

Flux

Lunar sample 61195 fromApollo 16textured with "zap pits" from micrometeorite impacts

The early studies were based exclusively on optical measurements. In 1957,Hans Petterssonconducted one of the first direct measurements of the fall of space dust on Earth, estimating it to be 14,300,000 tons per year.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-orbitingApollocapsules and for missions to the Moon. To determine whether the direct measurement was accurate, a number of additional studies followed, including thePegasus satellite program,Lunar Orbiter 1,Luna 3,Mars 1andPioneer 5.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.TheSurveyor Programdetermined that the surface of the Moon is relatively rocky.Most lunar samples returned during theApollo Programhave micrometeorite impacts marks, typically called "zap pits", on their upper surfaces.

Effect on spacecraft operations

See also:Space debris

Electron micrograph image of an orbital debris hole made in the panel of theSolar Maxsatellite

Micrometeoroids pose a significant threat tospace exploration.The average velocity of micrometeoroids relative to aspacecraftin orbit is 10 kilometers per second (22,500 mph). Resistance to micrometeoroid impact is a significant design challenge for spacecraft andspace suitdesigners (SeeThermal Micrometeoroid Garment). 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 tosandblasting.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 interminal ballistics(although accelerating objects up to such velocities is difficult; current techniques includelinear motorsandshaped charges). The risk is especially high for objects in space for long periods of time, such assatellites.They also pose major engineering challenges in theoretical low-cost lift systems such asrotovators,space elevators,and orbital airships.

Spacecraft micrometeoroid shielding

The "energy flash" of ahypervelocityimpact during a simulation of what happens when a piece of orbital debris hits a spacecraft in orbit

Whipple's work pre-dated theSpace Raceand 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 theWhipple shield,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.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 thatceramic fibrewoven shields offer better protection to hypervelocity (~7 km/s) particles thanaluminiumshields of equal weight.Another modern design usesmulti-layer flexible fabric,as inNASA's design for its never-flownTransHabexpandable space habitation module,and theBigelow Expandable Activity Module,which was launched in April 2016 and attached to theISSfor two years of orbital testing.

See also

• Extraterrestrial materials
• Interplanetary dust cloud

External links

• Melted Crumbs from Asteroid Vestaarticle about micrometeorites collected in Antarctica in Planetary Science Research Discoveries educational journal