CFHT, Current Image of the Week


November 22nd, 1999

The First Earth-based Detection of an Asteroidal Satellite

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The First Earth-based Detection of an Asteroidal Satellite
Credit: Merline et al. 1999, Nature, 401, 565 (October, 7th, 1999).

(click here to see the press release presenting the discovery)

On this image, Asteroid (45) Eugenia (at center, bright) is shown with a companion "moon" pictured at five different epochs. The motion of the companion satellite is clearly evident in this image,i.e., in the rest frame of (45) Eugenia. The satellite moves in the clockwise direction. Its orbit is nearly circular, but inclined by 46°. The best-fit orbit is shown in green. This is the first ground based detection of an asteroidal satellite and the only other case known is the Ida/Dactyl pair, discovered by spacecraft Galileo in August 1993. At the time of this discovery, Eugenia was 321 million kilometers away. The size of the orbit is 1190km. Eugenia has a diameter of 215km and its satellite 13km only. The orbital period is 4.7 days. The main result extracted from these images is the low density of Eugenia, only about 1.2g/cm³ ,i.e., only slightly more than water: a flying rubble-pile!

Technical description: This composite image was made from five(5) individual frames obtained with CFHT's Adaptive Optics Bonnette PUEO, equipped with the KIR near-infrared camera in the H-Band during the period November 13-19, 1998. The images have been carefully recentered , stacked, and deconvolved.

next week: Satellites, Rings and Arcs around planet Neptune

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.