RADAR OBSERVATIONS OF ASTEROID 25143 ITOKAWA (1998 SF36)

Abstract

We observed 25143 Itokawa, the target of Japan's Hayabusa (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. Itokawa's average circular polarization ratio at 13 cm, 0.26 ± 0.04, is comparable to that of Eros, so its cm-to-m surface roughness probably is comparable to that on Eros. Itokawa's radar reflectivity and polarization properties indicate a near-surface bulk density within 20% of 2.5 g cm −3 . We present a preliminary estimate of Itokawa's shape, reconstructed from images with rather limited rotation-phase coverage, using the method of Hudson (1993) and assuming the lightcurve-derived spin period (12.132 hr) and pole direction (ecliptic long., lat. = 355°, −84°) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 × 312 × 276 m ± 10%. Itokawa's topography is very subdued compared to that of other asteroids, for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9° and everywhere are less than 27°. The radar-refined orbit allows accurate identification of Itokawa's close planetary approaches through 2170. If radar ranging planned for Itokawa's 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroid's orbit.