Campbell, Donald B. Department of Astronomy, Cornell University, Ithaca, New York.
Ostro, Steven J. Formerly, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California.
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A powerful astronomical technique that uses radar to obtain unique information about bodies in the solar system. Comparing the properties of the radar echo to those of the transmitted signal can provide information about the target's distance, radial velocity, topography and surface electrical and physical properties. For a small body such as a near-Earth asteroid, radar can provide detailed information about the orbit, size, shape, surface bulk density, and spin vector, and, if it is discovered to be a binary system, the mass and density of both bodies. Achieving the desired measurement objectives requires tailoring the properties of the coherent transmitted signal, primarily the time or frequency modulation. Through measurements of the distribution of echo power in time delay and Doppler-shifted frequency, radar achieves spatial resolution of a planetary target despite the fact that the radar beam is typically much larger than the angular extent of the target. This capability is used to image the surfaces of the Moon and terrestrial planets and is particularly valuable for imaging asteroids and comets, which appear as largely unresolved point sources through optical telescopes. Moreover, the centimeter-to-meter wavelengths used in radar astronomy readily penetrate cometary comas and the optically opaque clouds that conceal the surface of Venus, and can also penetrate into icy and dry surfaces to study subsurface properties. See also: Asteroid; Comet; Moon; Planet; Venus
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