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W.J. Merline (SwRI), L.M. Close (ESO), C. Dumas (JPL), C.R. Chapman (SwRI), F. Roddier (U.~Hawaii), F. M\'enard (CFHT), W. Colwell (SwRI), D.C. Slater (SwRI), G. Duvert (Obs.~Grenoble, France), C. Shelton (W.M. Keck Obs.), T. Morgan (NASA HQ)
On 1998 November 1, we performed the first imaging of an asteroidal satellite from Earth-based observatories, when we detected a moon of asteroid (45)\thinspace Eugenia. Provisionally designated S/1998\thinspace (45)\thinspace 1, the satellite has a diameter of about 13\thinspace km and orbits the 215-km primary at a distance of about 1200\thinspace km. The orbit is near-circular and has a period of 4.7 days. Our observations show the object on 5 nights, spanning two continuous orbital cycles, and it was again detected two months later. At the time of discovery, the satellite was found to be about 6 magnitudes fainter than Eugenia, with a separation of 0.75\thinspace \arcsec; the orbit was inclined to the line-of-sight by 45{\char'27}. Our analysis suggests that the orbit is prograde ( i.e., the orbit and the primary spin are in the same sense) and lies approximately in Eugenia's equatorial plane.
Our measurements yield a tentative (because of the uncertain size of Eugenia) bulk density of this C-like asteroid of about 1.2\thinspace g\thinspace cm-3, implying that it may be structurally and/or compositionally similar to the C-type asteroid Mathilde, whose density was determined from the flyby of the NEAR spacecraft to be 1.3\thinspace g\thinspace cm-3. We discuss the implications of these findings for the composition and structure of Eugenia and for the mechanisms for satellite formation. We place these findings in the context of an additional 25 asteroids for which we have similar data, and in the context of asteroid families and taxonomic types.
These observations were made using near-infrared (H-band at 1.65\thinspace \mu) direct imaging, with the adaptive optics system (PUEO) of the 3.6-m Canada-France-Hawaii Telescope (CFHT) on Mauna Kea. This is the first positive detection from a comprehensive program, funded by NASA and NSF, to survey up to 200 asteroids for the presence of companions. The program is carried out using the adaptive optics systems of CFHT and the Mt. Wilson 100" telescope.