The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 Lagrange points of the Jupiter-Sun system (leading and following Jupiter by 60 degrees). The asteroid 617 Patroclus is the only known binary Trojan (Merline et al. 2001). The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system centre of mass, describing roughly a circular orbit. Using the orbital parameters, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8 g/cc. The components of 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.
@article{arxiv.astro-ph/0602033,
title = {A low density of 0.8 g/cc for the Trojan binary asteroid 617 Patroclus},
author = {Franck Marchis and Daniel Hestroffer and Pascal Descamps and Jerome Berthier and Antonin H. Bouchez and Randall D. Campbell and Jason C. Y. Chin and Marcos A. van Dam and Scott K. Hartman and Erik M. Johansson and Robert E. Lafon and David Le Mignant and Imke de Pater and Paul J. Stomski and Doug M. Summers and Frederic Vachier and Peter L. Wizinovich and Michael H. Wong},
journal= {arXiv preprint arXiv:astro-ph/0602033},
year = {2010}
}