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S. J. Ostro, L. A. M. Benner (JPL/Caltech), M. C. Nolan (Arecibo Observatory), J. D. Giorgini, R. F. Jurgens, R. Rose, D. K. Yeomans (JPL/Caltech)
We observed this object, the target of the MUSES-C sample-return mission, during its 2001 close approach at Arecibo on twelve dates during March 18 - April 9 and at Goldstone on nine dates during March 20 - April 2. We obtained delay-Doppler images with range resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. The Arecibo images are stronger, but the Goldstone sequences are about three times longer and therefore provide more rotation-phase coverage. The asteroid's circular polarization ratio at 13 cm, 0.27 +/- 0.04, is comparable to Eros' (0.22 +/- 0.06; Ostro et al. 1991, Astron. J. 102, 1490), so the cm-to-m surface roughness probably is comparable to that on Eros. The radar albedo, a crude indicator of near-surface bulk density, is within a factor of two of the Moon's albedo (~0.07) but significantly less than Eros' albedo (~0.25). A first approximation to the asteroid's shape is an ellipsoid 630 +/- 60 m long and 250 +/- 30 m wide. However, the ends are different distances from the center of mass and also have different curvatures, so the pole-on silhouette is not axisymmetric. The images show brightness features that probably are due to concavities, but otherwise the object looks fairly smooth at the scale of our imaging resolution. We used an optically derived synodic rotation period (12.15 +/- 0.03 h; T. Kwiatkowski, pers. comm.) to search for pole directions consistent with subjectively estimated epochs of two end-on orientations and one broadside orientation in the Arecibo images. The results suggest two preliminary pole solutions: either (320 +/- 30, -75 +/- 15) or (230 +/- 15, -5 +/- 15) deg. Our radar images are strong enough for reconstruction of a detailed physical model (Hudson 1993, Remote Sens. Rev. 8, 195).
Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by the Cornell University under a cooperative agreement with the National Science Foundation and with support from NASA.