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


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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Figure 1: 617 Patroclus observed with the Keck 10-m telescope on the Mauna Kea summit in Hawaii and its adaptive optics system.
Figure 2: Several good-fit orbits of the 617 Patroclus components estimated using 2004–2005 Keck LGS adaptive optics data.
Figure 3: Relationship between the sample density and the porosity of 617 Patroclus for various compositions.


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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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Correspondence to Franck Marchis.

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Supplementary information

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. (XLS 18 kb)

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Marchis, F., Hestroffer, D., Descamps, P. et al. A low density of 0.8 g cm-3 for the Trojan binary asteroid 617 Patroclus. Nature 439, 565–567 (2006).

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