RADAR RECONNAISSANCE OF NEAR-EARTH ASTEROIDS

Radar is a powerful astronomical technique for characterizing near-Earth objects and for refining their orbits. The Arecibo and Goldstone radars can image near-Earth asteroids (NEAs) with resolutions as fine as four meters, which greatly exceeds the finest resolution available from any ground- or space-based optical telescope. Radar images can reveal an object’s size, shape, spin state, and features on its surface such as craters, valleys, ridges, lineaments, and even boulders. Among NEAs larger than 200 m in diameter, radar imaging has revealed that ~14% are binary systems, that ~15% are contact binaries, and that objects with boulders and oblate shapes and are common. Radar can determine the masses and bulk densities of binary NEAs through Kepler’s 3^rd law and solitary NEAs through detection of the Yarkovsky effect. Multiple opportunities for high-resolution radar imaging occur annually.

Radar imaging has revealed stony and metallic objects, spheroids, shapes that are elongated and irregular, rubble piles, rotation periods ranging from a few minutes to weeks, objects whose rotation is accelerating, non-principal axis rotators, and surfaces that are extraordinarily rough.

Radar is invaluable for refining asteroid orbits. Range-Doppler measurements yield astrometry with precision as fine as 4 m in range and 1 mm/s in velocity. Radar astrometry can add centuries to the interval over which we can predict close Earth approaches and dramatically refine collision probability estimates. Radar astrometry regularly prevents newly-discovered objects from being lost.

A sequence of radar images can be inverted to estimate an asteroid’s 3D shape and spin state, which constrains its formation and evolution and which can be very useful for spacecraft planning and navigation. Radar observations have supported multiple spacecraft missions to asteroids and comets such as NEAR-Shoemaker, Hayabusa, EPOXI, Dawn, OSIRIS-REx, and Chang’e 2.

The number of asteroids observed by radar has more than tripled in the last three years. With a recent two-fold improvement in resolution, discoveries occur frequently and the field is poised for an explosion of exciting results.