Two types of experiments are envisioned: releasing water vapor and selected 

 gases, singly or as mixtures, from canisters and releasing a meter-sized ball of 

 water ice laden with either gas or dust or both. The measurement objective in 

 both types of experiments would be to follow spectroscopically in the ultravio- 

 let, visible, or infrared (or some combination of wavelength regions) the changes 

 in composition in the coma and tail as functions of time and distance from 

 release. An additional objective in the case of the ice ball would be to determine 

 the physical evolution of the ice and dust with respect to chemical reactions at 

 the surface and to formation of dust mantle, ejection of grains or grain aggre- 

 gates from the surface or interior, and their disaggregation and dissipation over 

 time. Embedded thermal sensors could be used to follow the thermal evolution 

 of the interior. 



The interaction of the unattenuated solar flux with the volatiles and dust 

 would be examined, as would the processes of sublimation and photolysis. The 

 interaction of the Earth's magnetic field and ionosphere moving past the sub- 

 limed and ionized gases at about 7 km/sec could be studied as analogs to pro- 

 cesses that occur at a real comet because of solar wind interaction with the 

 cometary ionosphere. 



For both types of experiments, selecting the mode(s) of observation (ground- 

 based, airborne, spaceborne, or combinations thereof) to follow the resulting 

 course of chemical evolution of nucleus, coma, and tail would have to take into 

 account the time scales available for observations of each feature; the require- 

 ments for spatial, spectral, and temporal resolution; and the need to synchronize 

 the release with the onset and continuation of observations. 



One major problem in either type of experiment is the interference of the 

 Earth's atmosphere. The coma of an artificial comet cannot be studied at the 

 orbit of the Space Station (~300 km) because of the reactivity of a large flux of 

 oxygen atoms (~4X10 14 cm -2 sec -1 ) and a somewhat smaller flux of nitrogen 

 molecules. By comparison, the flux of photons of wavelength less than 2000 A 

 is only 1.1 X10 13 cm -2 sec -1 . Therefore, the gas or ice ball should be released at 

 an altitude of ~1000 km. At this altitude, the fluxes of oxygen and hydrogen 

 atoms are only ~6X10 9 and ~3X10 10 cm" 2 sec -1 , respectively; the flux of 

 thermal ions is ~6X10 9 and of 100 MeV photons only 10 3 cm" 2 sec -1 . All of 

 these are much smaller than the flux of photons with wavelength less than 

 2000 A; and, therefore, the coma chemistry is not likely to be much affected by 

 them. 



For the gas-release experiments, several canisters, each containing a selected 

 pressurized gas, would be placed at the appropriate orbit. The water canister 

 should be heated to a high temperature in order to supply water vapor at the 

 required rate. The valve of each canister should be operated separately, enabling 

 the formation of various gas mixtures and also imitating to some extent the 

 gradual release of gases from a gas-laden ice ball. 



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