BULK-DENSITY OF ASTEROID 243-IDA FROM THE ORBIT OF ITS SATELLITE DACTYL

DURING its reconnaissance of the asteroid 243 Ida, the Galileo spacecr aft returned images of a second object, 1993(243)1 Dactyl(1)-the first confirmed satellite of an asteroid. Sufficient data were obtained on the motion of Dactyl to determine its orbit as a function of Ida's mas s. Here we apply statistical and dynamical arguments to constrain the range of possible orbits, and hence the mass of Ida. Combined with the volume of Ida(2), this yields a bulk density of 2.6 +/- 0.5 g cm(-3). Allowing for the uncertainty in the porosity of Ida, this density ran ge is consistent with a bulk chondritic composition, and argues agains t some (but not all) classes of meteoritic igneous rock types that hav e been suggested as compositionally representative of S-type asteroids like Ida.