Trojan asteroid Patroclus and its companion Menoetius : Comets in disguise ?

A French-American team composed of researchers of Institut de mécanique céleste (Observatoire de Paris, France), University of California, Berkeley (USA) and W. M. Keck observatory (Hawaii, USA) announce in the February 2, 2006 issue of Nature the determination of a very low density for the binary system 617 Patroclus-Menoetius. Determining the orbit of the binary system, made of two components of 112 km and 122 kilometer in diameter, gives the opportunity to estimate for the first time the mass and the density of a Jupiter-trojan asteroid. The surprising low density, approximately 0.8 g/cm3, had never been measured for asteroids located at such distance from the Sun (5.2 astronomical units, approximately 780 million kilometers). This result suggests that the two companions have a composition close to that of comets and then should come from the outer part of the solar system.