title: A low density of 0.8 g cm(-3) for the Trojan binary asteroid 617 Patroclus.
abstract: 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 degrees ). The asteroid 617 Patroclus is the only known binary Trojan. 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 0.8(- 0.1)+0.2 g cm(-3). 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 System.
authors.affiliation: Department of Astronomy, University of California, 601 Campbell Hall, Berkeley, California 94720, USA. fmarchis@berkeley.edu
authors.forename: Franck
authors.initials: F
authors.lastname: Marchis
authors.forename: Daniel
authors.initials: D
authors.lastname: Hestroffer
authors.forename: Pascal
authors.initials: P
authors.lastname: Descamps
authors.forename: Jérôme
authors.initials: J
authors.lastname: Berthier
authors.forename: Antonin H
authors.initials: AH
authors.lastname: Bouchez
authors.forename: Randall D
authors.initials: RD
authors.lastname: Campbell
authors.forename: Jason C Y
authors.initials: JC
authors.lastname: Chin
authors.forename: Marcos A
authors.initials: MA
authors.lastname: van Dam
authors.forename: Scott K
authors.initials: SK
authors.lastname: Hartman
authors.forename: Erik M
authors.initials: EM
authors.lastname: Johansson
authors.forename: Robert E
authors.initials: RE
authors.lastname: Lafon
authors.forename: David
authors.initials: D
authors.lastname: Le Mignant
authors.forename: Imke
authors.initials: I
authors.lastname: de Pater
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Stomski
authors.forename: Doug M
authors.initials: DM
authors.lastname: Summers
authors.forename: Frédéric
authors.initials: F
authors.lastname: Vachier
authors.forename: Peter L
authors.initials: PL
authors.lastname: Wizinovich
authors.forename: Michael H
authors.initials: MH
authors.lastname: Wong