Letter

Nature 439, 565-567 (2 February 2006) | doi:10.1038/nature04350

A low density of 0.8 g cm-3 for the Trojan binary asteroid 617 Patroclus

Franck Marchis1, Daniel Hestroffer2, Pascal Descamps2, Jérôme Berthier2, Antonin H. Bouchez3, Randall D. Campbell3, Jason C. Y. Chin3, Marcos A. van Dam3, Scott K. Hartman3, Erik M. Johansson3, Robert E. Lafon3, David Le Mignant3, Imke de Pater1, Paul J. Stomski3, Doug M. Summers3, Frédéric Vachier2, Peter L. Wizinovich3 and Michael H. Wong1

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 stable Lagrange points of the Jupiter–Sun system (leading and following Jupiter by 60°). The asteroid 617 Patroclus is the only known binary Trojan1. The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system's centre of mass, describing a roughly circular orbit. Using this orbital information, combined with thermal measurements to estimate the size of the components, we derive a very low density of Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com. The components of 617 Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the Solar System2.

  1. Department of Astronomy, University of California, 601 Campbell Hall, Berkeley, California 94720, USA
  2. Institut de Mécanique Céleste et de Calculs des Éphémérides, UMR CNRS 8028, Observatoire de Paris, 77 Avenue Denfert-Rochereau, F-75014 Paris, France
  3. W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, Hawaii 96743, USA

Correspondence to: Franck Marchis1 Correspondence and requests for materials should be addressed to F.M. (e-mail: Email: fmarchis@berkeley.edu).

Received 19 September 2005; Accepted 17 October 2005

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