discerned, as has been the case with meteorites. Through such windows it may 

 be possible, in ways not available with meteorites, to elucidate the pathways 

 taken by biogenic materials from their origins in stars to their incorporation in 

 some of the earliest objects formed in the solar system. 



Meteorites found on the surface of the Earth have been the traditional source 

 of primordial extraterrestrial materials, and they have been, and will continue to 

 be, valuable windows into the past. However, there are two major limitations 

 with this source of samples. One is that the stress of atmospheric entry prevents 

 structurally weak materials from surviving in pieces as large as conventional 

 meteorites. This exclusion effect prevents typical cometary matter from surviv- 

 ing as recoverable macroscopic samples. Studies of the fragmentation of meteors 

 in cometary meteor showers have shown that typical cometary material is more 

 than an order of magnitude weaker than the most friable of the recovered 

 meteorites. The second limitation with meteorites is that, with the exception of 

 the lunar meteorites, it has not been possible to associate collected samples with 

 their sources. It is widely believed that conventional meteorites are fragments of 

 asteroids, but it is not possible to prove this or to identify the specific parent 

 bodies of recovered samples. There are many distinct families of meteorites, and 

 it is unfortunate that their measured chronologies and detailed physical and 

 chemical properties cannot be associated with specific bodies or locations within 

 the solar system. 



An approach for overcoming the problems of the atmospheric exclusion 

 effect and the unknown origin of meteoritic samples is to collect meteoroids 

 directly in space. From Earth orbit this is not practical for meteoroids as large as 

 conventional meteorites because the flux is too low, but it is possible to collect 

 the much more abundant submillimeter particles, cosmic dust. The flux of 

 0.1 -mm particles is 1 m~ 2 yr _1 and that of 0.01 -mm particles is 1 m~ 2 day -1 . 

 With large areas and long exposure, it should be possible to collect a few par- 

 ticles of millimeter size and large numbers of smaller samples. Dust-collection 

 experiments in space began during the Mercury program, but progress was slow, 

 partly because of limitations on the size and exposure times of collectors that 

 could be exposed and then returned to the laboratory. This situation changed 

 dramatically in April 1 984 with the launch of the LDEF. Opportunities for long 

 exposures with Earth return will be common in the future, and it is likely that 

 considerable progress can be made in the meteoroid collection field. 



4.1 State of Knowledge 



The most detailed information on the properties of individual interplanetary 

 dust grains has come from laboratory studies of 5- to 50-jum particles that have 

 been collected in the stratosphere with U2 aircraft. The extraterrestrial nature of 

 the samples has been proven by detection of trapped solar wind particles and 

 tracks of solar cosmic rays, unique indicators of exposure to space. 



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