The last abstract written about my research on stable orbits around Ida.

This abstract along with a poster was presented at DPS '95 in Hawaii in mid October.

Stable periodic orbits around Ida: Several fit Dactyl's motion

T.A. Hurford, D.D. Durda, R. Greenberg, J-M Petit, P. Geissler, W.F. Bottke, Jr. (Lunar and Planetary Lab, Univ. Arizona)

Asteroid Ida's satellite Dactyl was observed over 5 1/2 hours by the Galileo spacecraft imaging system (Belton et al., 1994, Science 265, 1543). The observed motion fits any member of a set of elliptical orbits characterized by a particular relationship between semi-major axis a and eccentricity e, where each orbit coresponds to a specific assumed mass for Ida (Byrnes and D'Amario, 1994, Galileo project report). Orbits in this set with a <~ 80 km (corresponding to Ida's density >~ 3 gm/cm3) are unstable over time scales of a few days (Petit et al., 1994, DPS). Stability over much longer timescales for orbits with a > 80 km is uncertain. In order for Dactyl to have existed in its current orbit over astronomical timescales, we expect that its orbit must be periodic.

We have investigated the longer term stability of candidate orbits by numerically integrating motion about a realistically shaped Ida (based on Thomas et al., 1995, Icarus in press). We find stable periodic orbits at the 5:1, 6:1, 7:1, etc. commensurabilities with Ida's rotation. These orbits have their pericenter locked to longitudes near 60 deg on Ida, not near the principal axes , because of Ida's irregular shape (The short axis runs from longitude 90 deg to 270 deg). The 5.5:1, 6.5:1, etc. commensurabilities are less strongly locked, again due to the irregular shape. If Dactyl's orbit is stabilized for the long term by commensurabilities, the mass (and hence density) of Ida is constrained to only a few specific values.