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Published Online October 12, 2006
Science DOI: 10.1126/science.1133622

Reports

Submitted on August 8, 2006
Accepted on October 4, 2006

Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

Steven J. Ostro 1*, Jean-Luc Margot 2, Lance A. M. Benner 1, Jon D. Giorgini 1, Daniel J. Scheeres 3, Eugene G. Fahnestock 3, Stephen B. Broschart 3, Julie Bellerose 3, Michael C. Nolan 4, Christopher Magri 5, Petr Pravec 6, Petr Scheirich 6, Randy Rose 1, Raymond F. Jurgens 1, Eric M. De Jong 1, Shigeru Suzuki 1

1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA.
2 Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853-6801, USA.
3 Department of Aerospace Engineering, University of Michigan, 1320 Beal Avenue, Ann Arbor, MI 48109-2140, USA.
4 Michael C. Nolan, Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA.
5 University of Maine at Farmington, 173 High Street-Preble Hall, Farmington, ME 04938, USA.
6 Astronomical Institute, Academy of Sciences of the Czech Republic, Fricova 1, CZ-25165 Ondrejov, Czech Republic.

* To whom correspondence should be addressed.
Steven J. Ostro , E-mail: ostro{at}reason.jpl.nasa.gov

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ~1.5-km-diameter primary (Alpha) is an unconsolidated gravitational aggregate with spin period ~ 2.8 h, bulk density ~2 g cm-3, porosity ~ 50%, and an oblate shape dominated by an equatorial ridge at the object's potential energy minimum. The ~0.5-km secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with radius ~2.5 km and period ~17.4 h, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.


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Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4.
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