Elliptical duet rides the Kuiper belt

Researchers announced last year that a previously known member of the Kuiper belt–a reservoir of frozen bodies that lies beyond Neptune’s orbit–has its own moon (SN: 6/9/01, p. 360: Nine Planets, or Eight?). Follow-up observations with the Hubble Space Telescope now reveal that the two components of this Kuiper belt curiosity, dubbed 1998 WW31, pivot about each other in the most elongated orbit of any two bodies in the solar system.

During their 570-day mutual orbit, the two objects vary in separation from 4,000 to 40,000 kilometers, report Christian Veillet of the Canada France Hawaii Telescope in Kamuela, Hawaii, and his colleagues in the April 18 Nature. Over the past year, astronomers have discovered six other Kuiper belt pairs, but they haven’t observed the objects long enough to determine whether the orbits are as elongated as that of 1998 WW31.

The precision with which Hubble discerned the motion of the two dim, closely orbiting bodies that make up 1998 WW31 enabled Veillet’s team to estimate the binary system’s total mass: It’s about one-five-thousandth as heavy as Pluto and its moon Charon, which some astronomers believe are escapees from the Kuiper belt.

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