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Discovery of the triple asteroidal system 87 Sylvia


After decades of speculation1, the existence of binary asteroids has been observationally confirmed2,3, with examples in all minor planet populations4. However, no triple systems have hitherto been discovered. Here we report the unambiguous detection of a triple asteroidal system in the main belt, composed of a 280-km primary (87 Sylvia) and two small moonlets orbiting at 710 and 1,360 km. We estimate their orbital elements and use them to refine the shape of the primary body. Both orbits are equatorial, circular and prograde, suggesting a common origin. Using the orbital information to estimate its mass and density, 87 Sylvia appears to have a rubble-pile structure with a porosity of 25–60 per cent. The system was most probably formed through the disruptive collision of a parent asteroid, with the new primary resulting from accretion of fragments, while the moonlets are formed from the debris, as has been predicted previously5.

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Figure 1: One of the discovery images taken on 9 August 2004, showing 87 Sylvia and its two satellites, S/2001(87)1 and S/2004(87)1, in the K band (2.2 µm).
Figure 2: Positions of S/2001(87)1 and S/2004(87)1 around 87 Sylvia.
Figure 3: The apparent orbit of Sylvia's companions projected on the plane of the sky at different epochs in September 2004.


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The observations reported here are based on data collected at the European Southern Observatory, Chile. This work was partially supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under a cooperative agreement, and by the Chretien International Research Grant of the American Astronomical Society.Author Contributions All authors contributed equally to this work.

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Correspondence to Franck Marchis.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Table S1

This table contains the measurements performed on each observation for the analysis, such as the apparent shape of the primary (based on an ellipsoid fit in milli-arcsec), its orientation, the positions X and Y (in arcsec) of the moonlets and their sizes. (DOC 53 kb)

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Marchis, F., Descamps, P., Hestroffer, D. et al. Discovery of the triple asteroidal system 87 Sylvia. Nature 436, 822–824 (2005).

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