would be adequate in many cases. Ultimately, the collection of large or other- 

 wise rare particle types will require several square meters exposed tor at least a 

 year. On an operational Space Station one can envision a 10- to 100-m 2 area 

 exposed continuously with astronaut recovery of selected modules that are hit. 

 Development of a contamination monitoring system for the collection environ- 

 ment should take place in parallel with any collection program. 



6.3 In Situ Investigations 



Spacecraft in Earth orbit, in particular the Space Shuttle and Space Station, 

 allow experiments to be done that are not possible on the ground. Space labora- 

 tories provide a unique combination of conditions that cannot be duplicated in 

 terrestrial laboratories. Four different types of experiments have been identi- 

 fied as being able to take advantage of the special opportunities provided in 

 Earth orbit; three of these are highlighted here. 



6.3.1 Studies of Suspended Dust Grains 



Detailed studies of the fundamental physical and chemical processes leading 

 to the formation, condensation, and aggregation of dust grains, and the processes 

 occurring on the surfaces of grains in circumstellar shells, in interstellar dust 

 clouds, and in prestellar nebula, can be best done on a space platform in Earth 

 orbit. Such experiments are directly relevant to the growth of planetesimals and, 

 ultimately at least, the terrestrial planets. Increasing interest is being shown in 

 the constituents of these environments, and the mechanisms of molecular and 

 grain synthesis that may occur there. Whether organic molecules or carbona- 

 ceous grains that originated in these environments could have survived intact 

 during the accretion of the Earth is not known. However, the analysis of meteo- 

 rites and cosmic dust clearly shows that molecules and carbonaceous grains of 

 extraterrestrial origin can survive entry into the atmosphere of the contempo- 

 rary Earth. The proposed experiments may therefore also be relevant to Earth- 

 based exobiological research: what organic compounds should one use in labora- 

 tory simulations of reactions that may have been important for the origin of 

 life? 



6.3.2 Artificial Comets 



Interest in comets is presently intense as the many exciting results from the 

 study of comet Halley are being analyzed. As is normal in scientific investiga- 

 tions, many new questions are arising from these studies. A deeper understand- 

 ing of the chemical and physical processes that occur at the interface between 

 gas-and-dust-laden ice and space, under the influence of solar heating, is not only 



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