Discovery of a moon orbiting the asteroid 45 Eugenia


Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously1,2,3. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13 km, and an orbital period of about 4.7 days with a separation of 1,190 km from Eugenia. Using a previously determined4 diameter for Eugenia, we estimate that its bulk density is about 1.2 g cm-3, which is similar to that of the C-type asteroid Mathilde5,6. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

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Figure 1: Discovery image, showing the asteroid 45 Eugenia and its satellite S/1998(45)1.
Figure 2: How the orbit of the satellite around 45 Eugenia would appear if it were observed face-on.
Figure 3: Orbit of the satellite around 45 Eugenia as observed.


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W.J.M., L.M.C., C.D., F.R., F.M. and G.D. are visiting astronomers at the Canada-France-Hawaii Telescope (CFHT), operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique of France and the University of Hawaii. This work was done using observing time allocated through the University of Hawaii and granted by the CFHT Corporation, and was supported by funding from NASA and the NSF. We thank the staff of CFHT, particularly J.-L. Beuzit, for their logistical support; M. Northcott and J. E. Graves for development of a prototype instrument; and W. Colwell for assistance with data analysis tools.

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Correspondence to W. J. Merline.

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Merline, W., Close, L., Dumas, C. et al. Discovery of a moon orbiting the asteroid 45 Eugenia. Nature 401, 565–568 (1999).

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