Asteroids

Radar imaging of near-Earth asteroids can provide dramatic images with resolutions down to 8 m, comparable to the images obtained by the Galileo and NEAR-Shoemaker spacecraft. The rotation rate, shape and reflectivity give us information about the asteroids' density, internal structure, and surface properties. The images also show surface features such as impact craters, and irregularities, which can often be traced across the surface as the object rotates. A series of images can be used to derive a three-dimensional shape model. In the case of 216 Kleopatra, shown below, that shape can be highly irregular, and is a strong constraint on formation mechanisms. The shape of some very small objects is surprisingly spherical, which suggests a rubble pile with no internal strength. Two near-Earth binary asteroid systems have now been discovered by radar. Both 2000 DP107 and 2000 UG11 were determined to be binary systems from a combination of Goldstone and Arecibo radar observations. These close pairs of asteroids must be recently formed, perhaps by tidal forces on a previous close approach to the Earth. The lifetime of such a binary system against collisional disruption is quite short. Main-belt asteroids have also been studied, and images with resolution comparable to those obtained by the Hubble Space Telescope have been obtained.��

Asteroid 2000 UG11 is the second near-Earth binary asteroid known. The primary, in the upper part of the image, is about 230m diameter. The secondary, the bright dot below, is about 100m, and separated by at least 300m from the primary. The orbital period is 19.5 +/- 1.5 hours. This image of 2000 UG11 was obtained on 2000 Nov 7. Range increases downwards, delay increases to the left. The range resolution is 15m per pixel. The cross-range resolution is approximately the same, but depends on the viewing geometry, which is not yet completly determined.��

Asteroid 2001 GQ2 was imaged by the Arecibo radar system during April, 2001 by M. Nolan. This sequence of images shows the asteroid appearance during about 1.5 hours on 29 April. The first image is in the upper left, with time increasing across and then down. Each image is a sum spanning about 12 minutes. The image resolution is about 15m, and the asteroid is estimated to be 400m in diameter. In each image, range increases to the right, and doppler frequency increases downward. CCD images obtained by M. Nicolini, using a 0.4m telescope at the Observatory of Cavezzo, showed that the rotation period was longer than 8 hours (no maximum or minumum in 2 hours). The radar data confirms that the rotation period is probably about 10 hours.��

These radar images of main-belt asteroid 216 Kleopatra were obtained in November, 1999, about one hour apart (Ostro et al 2000, Science). The top row are the data, which are delay-Doppler images with range increasing down, frequency to the right. The middle row are simulated radar data, using the shape model derived. The bottom row shows the shape model, as it would appear illuminated in the plane of the sky. The model was derived from 12 images on four days, together with a diameter from an earlier stellar occulation. The size bar in the lower left is 100 km.��