similar to carbonaceous chondrites). By the mid-1980s more than half a dozen 

 "types" had been identified and their distributions charted as a function of dis- 

 tance from the Sun. In part, the observed pattern has been interpreted to reflect 

 the temperature gradient believed to have controlled the condensation of solids 

 in the early solar nebula. For instance, in accordance with such a model the 

 probably more volatile-rich and carbonaceous objects are relatively more abun- 

 dant at distances farther from the Sun than the asteroid belt. 



A major breakthrough during the past decade has been the development of 

 two techniques for the routine determination of diameters (and therefore, 

 reflectances). These radiometric and polarimetric techniques have been verified 

 in several instances in which asteroid diameters could be obtained directly from 

 observations of stellar occultations. As a result, the average diameters of hun- 

 dreds of asteroids are known reasonably well, as are the average absolute reflec- 

 tances of their surfaces. A complication arises in that many asteroids are known 

 to have irregular shapes (as deduced from observed periodic fluctuations in their 

 brightness). 



Asteroids are seen as distant point objects from Earth, and any compositional 

 information is derived from measurements of how their surfaces scatter sunlight. 

 Such spectral-reflectance techniques have been developed extensively over the 

 past two decades. Characteristic absorption features make it possible to identify 

 some key minerals, and although matching is not always exact, a striking corre- 

 spondence between the spectra of certain asteroids and some meteorites has 

 been established. The lack of precise matching must have several causes. First, 

 there are measurement uncertainties in both the asteroid and the meteorite data. 

 Second, and more importantly, it is very unlikely that meteorites provide a full 

 sample of surface materials present in the asteroid belt. Finally, it must be 

 realized that asteroid surfaces have undergone long histories of space weathering, 

 whereas space-weathered samples of meteoroid surfaces are not available for 

 laboratory measurements. In spite of such difficulties, it has proved possible to 

 characterize the likely compositions of many asteroid surfaces and to examine 

 the distribution of various compositions as a function of distance from the Sun. 

 In most cases spectral-reflectance measurements are restricted to the global 

 properties of asteroid surfaces, although careful observations made at different 

 phases of an asteroid's rotation period have recently documented evidence of 

 variations in composition on the surfaces of a few asteroids such as 4-Vesta and 

 8-Flora. 



Spectral-reflectance measurements provide data on surface composition only; 

 they contain no direct information on the internal makeup of the body. Such 

 information can sometimes be inferred from an accurate measurement of the 

 mean density. Unfortunately, approximated masses have been determined from 

 orbital perturbations for only four asteroids so far, and the determination of 

 additional masses must await close spacecraft flybys. Such flybys will also pro- 

 vide accurate measurements of volumes, and will represent a future source of 

 very precise density determinations for asteroids. 



42 



