Ostro has applied techniques of radar astronomy to small solar system bodies [c.f. Ostro 1993]. This has revolutionized our ability to study the position, shape, size and spin state of some of the asteroids and the microscopic to topographic-scale of their geological surfaces. The high precision of radar time/frequency measurements has resulted in an iterative process between the calculation of the position of an asteroid or comet, and the timing of the radar signal sent to the asteroid. The result is a highly precise ephemerides for asteroids studied by radar [ Ostro et al., 1991a]. The positive consequences for these asteroids as mission targets are recognized in decreased navigation uncertainties.
Radar evidence for a metallic composition of the near-Earth asteroid 1986 DA was presented by Ostro et al., [1991b] using Arecibo Observatory's 13-cm radar. An unmodulated, continuous wave was transmitted to the asteroid and the distribution of echo power as a function of frequency was measured. The resulting estimate of the asteroid's radar albedo is larger than those of other radar-detected asteroids. The interpretation preferred by Ostro et al. is that this small asteroid is most likely to be free of regolith and its high radar reflectivity implies a surface rich in metallic iron. The possibility of observing additional asteroids with similarly high reflectivity is advancing our ability to remotely sense the surface composition of small body surfaces.
In 1992 there was an extremely close approach to Earth of the near-Earth asteroid 4179 Toutatis. Ostro et al., [1993], using simultaneous radar return reception at two antennas, acquired interferometric data producing the highest radar resolution images of an asteroid to date (Figure 3).
It appears that the asteroid consists of two irregular lobes in
close contact. Whether or not the lobes are connected
is under study. Craters as small as
100 meters are resolved and the ratio of same-sense to
opposite-sense polarization, 
0.3, is indicative of a rough
surface at centimeter to decameter scales. Time resolved data
enables the study of the asteroid's spin rate which for Toutatis is
slow (the average rotation rate for asteroids is 
8-10 hrs).
With more such data, the range of size, shape, spin and radar
albedo of both asteroids and comets will be explored, and studies of
the radar properties of large numbers of asteroids can be studied.
Because the radar signal is proportional to the fourth root of the
range of the radar beam, opportunities to study comets have
been fewer in the past.
With upgraded facilities these opportunities will increase. This
technique is proving to be quite powerful as equipment is upgraded
and techniques for their interpretation are explored.