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

doi:10.1126/science.1133622

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Originally published in Science Express on 12 October 2006
Science 24 November 2006:
Vol. 314. no. 5803, pp. 1276 - 1280
DOI: 10.1126/science.1133622

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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 Suzuki¹

High-resolution radar images reveal near-Earth asteroid (66391)1999 KW4 to be a binary system. The $~$ 1.5-kilometer-diameter primary(Alpha) is an unconsolidated gravitational aggregate with aspin period $~$ 2.8 hours, bulk density $~$ 2 grams per cubic centimeter,porosity $~$ 50%, and an oblate shape dominated by an equatorialridge at the object's potential-energy minimum. The $~$ 0.5-kilometersecondary (Beta) is elongated and probably is denser than Alpha.Its average orbit about Alpha is circular with a radius $~$ 2.5kilometers and period $~$ 17.4 hours, and its average rotation issynchronous with the long axis pointed toward Alpha, but librationaldepartures from that orientation are evident. Exotic physicaland dynamical properties may be common among near-Earth binaries.

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 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, Fri $c$ ova 1, CZ-25165 Ond $r$ ejov, Czech Republic.

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

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Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4.: D. J. Scheeres, E. G. Fahnestock, S. J. Ostro, J.-L. Margot, L. A. M. Benner, S. B. Broschart, J. Bellerose, J. D. Giorgini, M. C. Nolan, C. Magri, et al. (2006)
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