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Abstract

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 . 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.

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Acknowledgements

This work was supported by the National Science Foundation Science and Technology Center for Adaptive Optics and by the National Aeronautics and Space Administration (NASA) issue through the Science Mission Directorate Research and Analysis programmes. Most of the data were obtained at the W. M. Keck observatory, which is operated as a scientific partnership between the California Institute of Technology, the University of California and NASA. Additional observations were obtained at the Gemini Observatory (acquired through the Gemini Science Archive). Author Contributions F.M. and the IMCCE researchers processed, analysed and interpreted the data. The 2004–2005 campaign of observations with Keck LGS adaptive optics was conducted by the team from the W. M. Keck Observatory, and other University of California at Berkeley researchers.

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  1. Department of Astronomy, University of California, 601 Campbell Hall, Berkeley, California 94720, USA

    • Franck Marchis
    • , Imke de Pater
    •  & Michael H. Wong
  2. Institut de Mécanique Céleste et de Calculs des Éphémérides, UMR CNRS 8028, Observatoire de Paris, 77 Avenue Denfert-Rochereau, F-75014 Paris, France

    • Daniel Hestroffer
    • , Pascal Descamps
    • , Jérôme Berthier
    •  & Frédéric Vachier
  3. W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, Hawaii 96743, USA

    • Antonin H. Bouchez
    • , Randall D. Campbell
    • , Jason C. Y. Chin
    • , Marcos A. van Dam
    • , Scott K. Hartman
    • , Erik M. Johansson
    • , Robert E. Lafon
    • , David Le Mignant
    • , Paul J. Stomski
    • , Doug M. Summers
    •  & Peter L. Wizinovich

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Correspondence to Franck Marchis.

Supplementary information

Excel files

  1. 1.

    Supplementary Table 1

    This Supplementary Table summarizes the measurements performed on each observation for the analysis, such as the relative positions of the components (X, Y based on a fit by a gaussian function in arcsec), the difference of brightness in magnitude, and the residual mean square fitting error of our model.

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https://doi.org/10.1038/nature04350

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