Radar Astrometry of Asteroids, Comets and Natural Satellites

Ostro S.J.

JPL/Caltech

The tally of radar-detected targets includes the Moon, Venus, Mercury, Mars, Saturn's rings, the Galilean satellites, Phobos, Titan, 6 comets, 37 mainbelt asteroids (MBAs), and 37 near-Earth asteroids (NEAs, almost all of which are Earth-crossers). Almost every radar observation of each of these targets has measured some characteristics of the distribution of echo power in time delay (range) and Doppler frequency (radial velocity), often with fractional precision that is much finer than can be achieved with other kinds of groundbased observations. For example, the state of the art in asteroid radar is conveyed by the following table:

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                        Asteroid Radar Measurement Precision

                           Range         Radial Velocity
                          (meters)         (meters/sec)  
                       ______________    ________________


 Best Radar Resolution            ~10               0.0001

 Asteroid "size"               ~1 000            0.01 to 1

 Asteroid "location"    ~10 000 000 000              10000

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In addition to delay-Doppler astrometry, it is possible to use Goldstone transmissions to illuminate a target so the Very large Array can make synthetic aperture image of the echoes. G-VLA observations have yielded angular astrometry with uncertainties of a few hundredths of an arcsec.

The Arecibo and Goldstone radars are responsible for almost all published delay-Doppler astrometry. However, radar astronomy has a long history in Russia and, during the past few years, two-station asteroid radar observations have used international combinations of antennas in Russia, Germany, and Japan.

The upgraded Arecibo telescope, expected to become operational later this year, will be over an order of magnitude more sensitive than any other radar on the Earth. It should be technically capable of routine ranging to the Martian satellites, the Galilean satellites, the largest Saturnian satellites, some comets, many dozens of main-belt asteroids, and increasingly large numbers of Earth-crossing asteroids.