Radar contributions to asteroid astrometry and dynamics

Abstract

Almost every radar observation of asteroids has measured some characteristics of the distribution of echo power in time delay (range) and Doppler frequency (radial velocity). Such measurements, which are orthogonal to optical plane-of-sky angular astrometry, are also done in the well-known, “absolute” reference frame of the planetary ephemerides, often with fractional precision that is much finer than can be achieved with other kinds of groundbased observations; they therefore can dramatically refine orbits and ephemerides. This statement is most true for near-Earth asteroids (NEAs), which constitute the most prolifically observed radar targets and are likely to be observed in increasing abundance as instrumental capabilities improve and CCD-based, NEA search programs expand. Moreover, studies of closely approaching NEAs benefit tremendously from the inverse fourth-power dependence of echo strength on target distance. For these reasons, I devote the bulk of this paper to NEAs.