Nature 439, 943-945 (23 February 2006) | doi:10.1038/nature04547

Discovery of two new satellites of Pluto

H. A. Weaver1, S. A. Stern2, M. J. Mutchler3, A. J. Steffl2, M. W. Buie4, W. J. Merline2, J. R. Spencer2, E. F. Young2 and L. A. Young2

Pluto's first known satellite, Charon, was discovered1 in 1978. It has a diameter (approx1,200 km) about half that of Pluto2, 3, 4, 17, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful5, 6, 7, but they were not sensitive to objects 150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites6. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approx38 days (P1) and approx25 days (P2).

  1. The Johns Hopkins University Applied Physics Laboratory, Space Department, 11100 Johns Hopkins Road, Laurel, Maryland 20723-6099, USA
  2. Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, Colorado 80302, USA
  3. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
  4. Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, Arizona 86001, USA

Correspondence to: H. A. Weaver1 Correspondence and requests for materials should be addressed to H.A.W. (Email: hal.weaver@jhuapl.edu).

Received 18 November 2005; Accepted 22 December 2005


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