50 



radiation. The size of the coma is remarkable as measured by the 

 light emitted by the atomic hydrogen in it. The coma may be 

 >1X10 6 km in radius. The tail, composed of dust grains and charged 

 molecules, is even larger, >100X10 6 km in length in some cases. 

 When comets become visible in the inner solar system they can be, 

 spatially, the largest objects in the sky, bigger than the Sun itself. 



As mentioned above, comets are believed to be material con- 

 densed and accreted in the outer regions of the primitive solar 

 nebula. Thus, a relationship may exist between interstellar matter 

 and the dust and molecules that make up comets. If one compares 

 the molecules observed in interstellar clouds (table IV- 1) and in the 

 coma and tails of comets (table IV-2) there do seem to be similarities 

 between the populations. For example, both contain cyanide, and 

 derivatives with a "CN" group. It is also possible that comets are 

 related to some of the carbonaceous meteorites in that the latter 

 objects, less rich than comets in various forms of the volatile ele- 

 ments and organic matter, may be derived from remnants of volatile- 

 depleted, moribund comets. It is appropriate to note that if comets 

 do not contain relatively unaltered interstellar matter, and if they 

 formed at the outer edge of the solar nebula, where temperatures 

 were sufficiently low to condense gases like carbon dioxide and 

 water, then the presence of parent organic molecules in comets is 

 difficult to understand. No widely accepted model exists for the 

 chemical reactions that could have occurred in the solar nebula to 

 yield the chemistry of comets. Indeed, in the absence of direct 

 observations of the nucleus, our knowledge of comet chemistry is 

 unfortunately sparse and model-dependent. Since comets may 

 represent a chemical evolutionary link between the primitive solar 

 nebula and the interstellar medium but are poorly understood, their 

 direct study by space probes constitutes a high-priority objective for 

 many scientists. 



Unlike comets which have only been observed from afar, meteo- 

 rites are rock samples of extraterrestrial origin that have survived 

 passage through the atmosphere to the Earth's surface and are avail- 

 able for direct examination. Preserved in these objects are chemical, 

 mineralogical, and structural information about the nature of the 

 environments and the processes involved in their formation. Indeed, 

 recent discoveries of anomalies in the isotopic composition of some 

 elements (e.g., oxygen, aluminum, magnesium, noble gases) in 



