Trojan asteroid

The Trojan asteroids are a large group of objects that share the orbit of the planet Jupiter around the Sun. Viewed from a coordinate system that is fixed on Jupiter, they appear to orbit one of the two Lagrangian points of stability, L4 and L5, that lie 60° ahead of and behind Jupiter in its orbit.
As originally defined, Trojan asteroids have a semi-major axis between 5.05 AU and 5.40 AU, and lie in elongated, curved regions around the two Lagrangian points. The term is sometimes used to refer to minor bodies with similar relationships to other major bodies, in which case the name of the major body is included, e.g. the Neptunian Trojan 2001 QR322.
A team from the Keck Observatory in Hawaii announced in 2006 that they had measured the density of the binary Trojan asteroid 617 Patroclus as being less than that of water ice, suggesting that the pair, and possibly all the Trojan objects, more closely resemble comets or Kuiper Belt objects in size and composition—water ice with a layer of dust—than they do the main belt asteroids.