An asteroid is a rocky body which is gravitationally bound to and orbits the Sun. The majority of asteroids lie in a region called the asteroid belt which is located between the orbits of Mars and Jupiter. Some of the largest asteroids are listed in the table below.

Asteroids whose orbits have been determined carefully enough so that their position on the sky as a function of time can be predicted are assigned permanent numbers and names by the Minor Planet Center. At present, there are more than 8,700 numbered asteroids (Minor Planet Center, Schmadel 1997). Astroid names are chosen by the discoverer, and some noteworthy names assigned to asteroids are 3834 Zappafrank, 2906 Caltech, 2404 Antarctica, 5102 Benfranklin, 7850 Buenos Aires, 10221 Kubrick, etc.

Asteroids are placed in families having similar orbits, with the most common families being the Amor, Apollo, and Aten groups, each named after a prominent asteroid in a given group. The approximate number of known members of each group is listed in the following table (Minor Planet Center). Earth-crossing asteroids are called Apollo asteroids. A table of large Apollo asteroids is contained in Sky & Telescope (March 1990).

The physical properties of an asteroid may be measured in several ways. Rotation periods are deduced from periodic brightness variations in the light curves. The albedo (reflectivity), temperature, and radius of an asteroid may be determined by measuring the infrared and visible fluxes at Earth. To see how this can be done, let A be the albedo, r be the radius, T be the temperature (in Kelvin ), d be the distance to the Earth, and be the energy flux from the Sun. Then the flux of visible light reflected from the asteroid is given by

while the infrared radiation emitted from the asteroid is

with the Stefan-Boltzmann constant and is a factor depending on the thermal inertia and rate of rotation. Combining (1) and (2) with the energy balance equation

then gives three equations in three unknowns, which can be immediately solved to yield

The radius can be estimated by measuring the duration of a stellar occultation from several observatories. Several observatories are needed, since otherwise it is unknown which part of the circular shadow was observed.

More sophisticated techniques can also be brought to bear on the study of asteroid properties. A technique called speckle interferometry can capture an image of the asteroid for 0.01 second, freezing the Earth's atmosphere and allowing the angular size of an asteroid to be determined. An image can also be reconstructed by superimposing many such images. Bouncing radar off asteroids can reveal information about their size and rotation, although interpretation of the resulting "Doppler-delay" images requires some care (Hudson and Ostro 1994, 1995; Ostro et al. 1995).

One of the best techniques for gathering data about asteroids is spacecraft flybys. The Galileo spacecraft had encounters with the two asteroids Ida and Gaspra while on its way to Jupiter. Amazingly, Ida was found to have a smaller asteroid as a companion moon, and this object has been named Dactyl.

Observations of the rotation periods of a sample of 107 asteroids by A. Harris and J. Young has revealed a pronounced lack of rapid rotators, suggesting that asteroids may be rubble piles held together only by self-gravity (Kerr 1996).