DACTYL - GALILEO OBSERVATIONS OF IDAS SATELLITE

Galileo's flyby of 243 Ida in August 1993 led to the discovery of a sm all satellite, Dactyl, some 85 km from the asteroid's center. From Ear th at mean opposition, the satellite is a V = +20.3 mag object (some 6 .7 magnitudes fainter than Ida). Forty-seven images of the satellite a t 18 different observing times were played back, including one multico lor sequence in which the satellite is resolved adequately to distingu ish surface markings (similar to 105 m/pxl) and three higher resolutio n single-color views (89, 39, and 24 m/pxl). The satellite, mean radiu s = 0.7 km, is an elongated, but not angular body with principal diame ters of 1.6 x 1.4 x 1.2 km. In the highest resolution view, the longes t axis points approximately in the direction of Ida, and its shortest axis is perpendicular to the orbital plane. The spin period is slow (> 8 hr?) and may be synchronous. The satellite shows no conspicuous sha rp edges and is much less irregular in shape than Ida. Limb profiles a re remarkably smooth over distances of 200-300 m. The geometric albedo s of the two objects are similar (0.20 vs 0.21), as are the 0.4-1.0-mu m colors. Like Ida, Dactyl is an S-asteroid, but has a slightly deepe r 1-mu m band than Ida (by 5-8%). While no identical regions (in color ) are seen on Ida, the color difference is consistent with color varia tions reported within the Koronis family and may be due to a slightly higher pyroxene/olivine ratio on the satellite. More than a dozen crat ers ranging from less than or similar to 90 to 280 m diameter are visi ble in the best image (39 m/pxl at 470 phase). The largest contains an off-centered, positive relief feature some 75 m across. The image inc ludes an intriguing crater chain, but no grooves, ridges, or sharp edg es are evident. In terms of limb roughness, Dactyl is much smoother th an Ida, but comparable to the two satellites of Mars, Phobos and Deimo s. While the satellite's origin is uncertain, a likely scenario would have the satellite date from the breakup of the Koronis family. It is interesting that crater densities on the satellite are similar to thos e on Ida itself. (C) 1996 Academic Press, Inc.