All these observations will be complemented by continuing ground-based 

 observations at microwave frequencies. The potential for extremely high spatial 

 resolution using interferometers may prove to be valuable in interpreting the 

 lower-spatial-resolution infrared data, and distinguishing between several possible 

 parent species and production mechanisms. 



J ust as the infrared is best suited to studies of cometary grains, the ultraviolet 

 is the spectral region in which the most abundant atoms and small molecules 

 fluoresce. The primary objectives, therefore, of Space Telescope observations of 

 comets would concentrate on the photochemical evolution of the gaseous 

 component of the inner coma. There are few spectral signatures of simple 

 organic molecules in this part of the spectrum so that the presence of prebiotic 

 molecules must be inferred from observations of the spatial distribution of 

 dissociation products resulting from photochemical decomposition of the 

 heavier molecules. Observations of comets made over the past 6 years using the 

 IUE satellite have provided a wealth of new information about the chemistry 

 and evolution of the inner coma. These studies will be strongly enhanced with 

 the use of ultraviolet spectrographs on both HST and ASTRO. 



Suggestions for Further Reading 



Nature, vol. 321, 15 May 1986, p. 6067. 



Wilkening, L. L.; and Matthews, M. S., eds.: Comets. University of Arizona 

 Press, Tucson, 1982. 



3.6 Asteroids and Meteors 



Most of the 3500 asteroids for which orbits are known circle the Sun between 

 Mars and Jupiter and are found between 2.1 and 3.2 AU. In addition to this 

 main belt, there are important classes such as the Trojans at the L 4 and L s 

 points of the Sun-Jupiter system and the near-Earth asteroids such as the Amors, 

 Apollos, and Atens. The largest of the asteroids (1 -Ceres) is about 1000 km 

 across while the smallest for which physical data are available have diameters of 

 only 500 to 1000 m. 



Because of their small size it has been common to impute dull geological 

 histories to the asteroids as a group, but considerable evidence against such a 

 view can be marshalled. In particular, it has been demonstrated that at some 

 time during its 4.5-billion-year history, the asteroid 4-Vesta was hot enough for 

 lavas to have erupted onto its surface. 



On the basis of reflectances, colors, and other optical properties it has been 

 possible to divide asteroids into several groups. The most abundant of these are 

 the S-types (reflectances near 0.15 and colors similar to those of ordinary chon- 

 drites) and the C-types (very low reflectances, often less than 0.05, and colors 



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