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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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  1. Weidenschiling, S. J., Paolicchi, P. & Zappala, V. in Asteroids II (eds Binzel, R. P., Gehrels, T. & Matthews, M. S.) 643–658 (Univ. Arizona Press, Tucson, 1989)

    Google Scholar 

  2. Chapman, C. R. et al. Discovery and physical properties of Dactyl, a satellite of asteroid 243 Ida. Nature 374, 783–785 (1995)

    Article  ADS  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  4. Merline, W. J. L., et al. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 289–312 (Univ. Arizona Press, Tucson, 2002)

    Google Scholar 

  5. Michel, P., Benz, W., Tanga, P. & Richardson, D. C. Collision and gravitational reaccumulation: forming asteroid families and satellites. Science 294, 1696–1700 (2001)

    Article  ADS  CAS  Google Scholar 

  6. Marchis, F. et al. A three-dimensional solution for the orbit of the asteroidal satellite of 22 Kalliope. Icarus 165, 112–120 (2003)

    Article  ADS  Google Scholar 

  7. Marchis, F., Descamps, P., Berthier, J., Hestroffer, D. & de Pater, I. Fine analysis of 121 Hermione, 45 Eugenia, and 90 Antiope binary asteroid systems with AO observations. Bull. Am. Astron. Soc. Div. Planet. Sci. 36, 33.23 (2004)

    Google Scholar 

  8. Marchis, F. et al. Mass and density of asteroid 121 Hermione from an analysis of its companion orbit. Icarus (in the press)

  9. Brown, M. E., Margot, J. L., de Pater, I. & Roe, H. S/2001(87)1. IAU Circ. No. 7588 (2001).

  10. Storrs, A. et al. S/2001 (87) 1. IAU Circ. No. 7590 (2001).

  11. Descamps, P. Orbit of a visual binary system. Bull. Am. Astron. Soc. Div. Dynam. Astron. 35, 04.01 (2005)

    Google Scholar 

  12. Margot, J. L. & Brown, M. E. Discovery and characterization of binary asteroids 22 Kalliope and 87 Sylvia. Bull. Am. Astron. Soc. Div. Planet. Sci. 33, 52.02 (2001)

    Google Scholar 

  13. Kaasalainen, M., Torppa, J. & Piironen, J. Models of twenty asteroids from photometric data. Icarus 159, 369–395 (2002)

    Article  ADS  Google Scholar 

  14. Tedesco, E. F., Noah, P. V., Noah, M. & Price, S. D. The supplemental IRAS minor planet survey. Astron. J. 123, 1056–1085 (2002)

    Article  ADS  Google Scholar 

  15. Bus, S. J. & Binzel, R. P. Phase II of the small main-belt asteroid spectroscopic survey. Icarus 158, 146–177 (2002)

    Article  ADS  Google Scholar 

  16. Britt, D. T., Yeomans, D., Housen, K. & Consolmagno, G. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 485–500 (Univ. Arizona Press, Tucson, 2002)

    Google Scholar 

  17. Clark, B. E. et al. Spectroscopy of X-type asteroids. Icarus 128, 3070–3081 (2004)

    CAS  Google Scholar 

  18. Durda, D. D. The formation of asteroidal satellites in catastrophic collisions. Icarus 120, 212–219 (1996)

    Article  ADS  Google Scholar 

  19. Doressoundiram, A., Paolicchi, P., Verlicchi, A. & Cellino, A. The formation of binary asteroids as outcomes of catastrophic collisions. Planet. Space Sci. 45, 757–770 (1997)

    Article  ADS  Google Scholar 

  20. Chauvineau, B., Farinella, P. & Mignard, F. Planar orbits about a triaxial body—Application to asteroidal satellites. Icarus 105, 370–384 (1993)

    Article  ADS  Google Scholar 

  21. Scheeres, D. J. The effect of C22 on orbit energy and angular momentum. Celest. Mech. Dynam. Astron. 73, 339–348 (1999)

    Article  ADS  Google Scholar 

  22. Petit, J.-M., Durda, D. D., Greenberg, R., Hurford, T. A. & Geissler, P. E. The long-term dynamics of Dactyl's orbit. Icarus 130, 177–197 (1997)

    Article  ADS  Google Scholar 

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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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