A low density of 0.8 g cm -3 for the Trojan binary asteroid 617 Patroclus

• Published in: Nature Vol. 439; no. 7076; pp. 565 - 567
• Main Authors: Bouchez, Antonin H, Lafon, Robert E, Campbell, Randall D, Hestroffer, Daniel, Wong, Michael H, Marchis, Franck, van Dam, Marcos A, Wizinovich, Peter L, Johansson, Erik M, Hartman, Scott K, Descamps, Pascal, Stomski, Paul J, Vachier, Frédéric, Berthier, Jérôme, Mignant, David Le, de Pater, Imke, Summers, Doug M, Chin, Jason C. Y
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 02.02.2006; Nature Publishing Group
• Subjects: Asteroids; Asteroids (minor planets); Astronomy; Earth, ocean, space; Exact sciences and technology; Orbits; Planets, their satellites and rings. Asteroids; Solar system; Stars & galaxies; Binary systems; Density; Asteroids
• ISSN: 0028-0836; 1476-4687; 1476-4679
• DOI: 10.1038/nature04350

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°). 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.