Thermal emission has been detected from the four largest asteroids:
Ceres, Pallas, Vesta, Hygiea, at 
1mm to 20cm and
from a few smaller asteroids at 2cm. Such measurements
place constraints on the thermal and electrical properties of those
objects' surface layers. The existing observations indicate that the
asteroids studied so far are covered with fine-grained regoliths,
which are expected to result from meteoroid bombardment (``sand
blasting'') of the surfaces over 100's of millions of years.
Differences in the microwave spectra of Ceres, Pallas, Vesta, and
Hygiea suggest variations in the dielectric properties of those
regoliths and/or variations in the regolith thicknesses. However,
those few objects represent just a tiny fraction of the thousands of
main-belt asteroids, most of which are beyond the detectability of the
current VLA. (A 100-km-diameter object at 1 AU distance from the
Earth represents a flux of 
microJy at 2cm,
decreasing to 
microJy at 8.4 GHz). In addition, there is an
important gap in the wavelength coverage from 1 mm to 1 cm. Over this
range, the spectra show a distinct drop in brightness temperature.
The proposed upgrades in continuum sensitivity and the expansion of
wavelength coverage should make a large number of these objects
detectable at several wavelengths. Of particular importance is the
40-50 GHz band, which samples the wavelength gap between 1 mm and
1 cm.