fundamental to theories of the origins of the constituents of interstellar clouds, 

 comets, meteorites, interplanetary dust, and all of the bodies in the solar system. 

 Experiments capable of yielding insight into the nature of these processes are 

 thus of great value in confirming or modifying various aspects of these theories. 



Nucleation, condensation, and growth of carbonaceous particles must occur 

 in the envelopes of carbon stars to yield the observed circumstellar dust and 

 molecules. Similar processes are thought to occur under conditions as diverse as 

 those in interstellar clouds and in the atmospheres of some of the outer planets 

 and their satellites; observational evidence points to the presence of fine-grained 

 dust (from less than 0.1 /urn to about 1 jum in diameter, presumably containing 

 varying proportions of hydrogen, carbon, nitrogen, and oxygen) in both types of 

 environments. Although there are some theoretical discussions of the properties 

 of dust based on remote spectrophotometric observations, the physical and 

 chemical character of the materials remains poorly understood, as does the 

 nature of the processes that produced them. 



Although theories of grain nucleation, condensation, and dust growth are 

 being developed, the complexities of the natural processes make them difficult 

 to model. The few experimental studies that have been conducted were per- 

 formed under conditions that do not permit scaling to relevant astrophysical 

 environments. A common feature of the processes in all the environments men- 

 tioned above is that grains form and evolve over substantial lengths of time while 

 suspended in a thin gas phase, largely, if not entirely, independent of other 

 grains. This condition should influence the rate of formation, the chemistry, 

 structure, morphology, and other characteristics of the dust. While this condi- 

 tion is difficult, if not impossible, to achieve in a terrestrial laboratory, it may 

 be effectively simulated under microgravity conditions. Experiments conducted 

 in Earth orbit would provide "space truth" for analogous experiments carried 

 out in terrestrial laboratories or on computers. Furthermore, they would yield 

 samples formed under well-defined conditions, the properties of which could be 

 readily determined and compared with those of natural material either remotely 

 sensed or obtained from meteorites, interplanetary dust, and dust returned from 

 a comet. 



Once grains are formed in the solar nebula, they must accrete to form the 

 larger planetesimal-sized objects thought to have been the building blocks of 

 planets. The rate and mechanism for planetesimal formation are believed to 

 depend on the size distribution, composition, and structure of the original 

 nebular dust. In theory, the ability of colliding grains to stick together depends 

 largely on short-range, Van der Waals interactions, although electrostatic and 

 ferromagnetic forces may come into play. It has been suggested that grains 

 endowed with mantles containing organic matter and/or icy components should 

 accrete and grow faster than others. Despite its implications for early solar 

 system history, this suggestion has never been tested experimentally. Micro- 

 gravity facilities would provide excellent opportunities for model studies of grain 



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