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M. C. Nolan, A. A. Hine, E. S. Howell (Arecibo Observatory / NAIC), L. A. M. Benner, J. D. Giorgini, S. J. Ostro (JPL / Caltech), G. J. Black (U. Virginia), D. B. Campbell, J. L. Margot (Cornell), L. M. Carter (Smithsonian), C. Magri (U. Maine)
Radar observations allow characterization of the rotation states, sizes, and shapes of near-Earth asteroids. Radar imaging at 7--15 m resolution is routine, is comparable to that obtained by spacecraft, and can be applied to a much larger sample of objects. The diversity of objects observed is remarkable, and suggests a diversity of formation and modification mechanisms. Thus, understanding the near-Earth asteroid population requires a combination of spacecraft reconnaissance and ground-based observations.
Since 1999, we have observed approximately 100 near-Earth asteroids with the Arecibo planetary radar system. Of these objects, 17 are binary systems, about 40% appear to be spheroidal ``rubble piles'', about 15% are elongated ``shards'', about 10% are ``bifurcated'', and the remaining 35% are ``irregular''.
Rotation rates vary from ~30 seconds to more than a week. About 10% have circular polarization ratios greater than 0.75, indicating extremely rough surfaces at the wavelength scale (~13 cm). Not surprisingly, NEAs are typically rougher than the terrestrial planets at that scale.
Few correlations among these characteristics are evident. It is thus very difficult to make generalizations about the physical properties of near-Earth asteroids. Their most distinguishing feature is the variety of structural properties seen, which seems to require a variety of production and evolution paths (including diverse collisional histories) to near-Earth space.
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation.
Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.