THE DISCOVERY AND ORBIT OF 1993-(243)1-DACTYL

Dactyl was discovered in solid state imaging (SSI) data on February 17 , 1994, during the long playback of approach images from the Galileo s pacecraft's encounter with the asteroid 243 Ida. Forty-seven images of the Ida-Dactyl pair were obtained, A detailed search for other satell ites was made, No confirmed detections were made, all other candidate features being consistent with radiation hits. We deduce a manifold of osculating two-body orbits that approximate Dactyl's motion over the observed orbital are depending on the assumed mass of Ida. At the time of Galileo's encounter, Dactyl was found to be 85 km from the center of Ida, moving at similar to 6 m . sec(-1) in the same direction as Id a's retrograde spin, The inclination of its orbit is similar to 172 de grees in Ida's equatorial system (IAU definition), It was not possible to obtain a definitive orbit or measure of Ida's mass from the observ ed motion even though supplemental techniques (search for Dactyl's sha dow on Ida, changes in angular diameter and brightness, and attempts t o determine the spin of Dactyl) were explored. The influence of Ida's irregular gravitational field and solar perturbations on two-body moti on are evaluated and found to be undetectable in the observed orbital are. These effects may, however, strongly influence the motion over or bital time scales. Limits to the value of Ida's gravitation parameter, GM,are derived. A robust lower limit, GM > 0.0023 km(3) . sec(-2), is obtained by requiring Dactyl's orbit to be bound. Hubble Space Telesc ope observations, which show no evidence of Dactyl on a hyperbolic orb it, excludes values of GM in the range 0.00216 < GM < 0.0023 km(3) . s ec(-2). An upper limit, GM < 0.0031 km(3) . sec(-2), deduced by requir ing that the orbital motion has a long lifetime in a realistic approxi mation to Ida's gravitational field, is illustrated with numerical cal culations. Ida's mass is therefore constrained to the range 4.2 +/- 0. 6 x 10(19) g, which, together with a volume of 16,100 +/- 1900 km(3) ( Thomas P.C., M.J.S.Belton, B. Carcich, C. R. Chapman, M. E. Davies, R. Sullivan, and J. Veverka 1996. Icarus 120, 20-32.) yields a bulk dens ity of 2.6 +/- 0.5 g . cm(-3) (Belton, M. J. S., C. R. Chapman, P. C. Thomas, M. E. Davies, R. Greenberg, K. Klaasen, D. Byrnes, L. D'Amario , S. Synnott, T. V. Johnson, A. McEwen, W. Merline, D. R. Davis, J-M. Petit, A. Storrs, J. Veverka, and B. Zellner 1995. Nature 374, 785-788 .). (C) 1996 Academic Press, Inc.