Shapes, Rotations, Photometric and Internal Properties of Jupiter Trojans

The Jupiter Trojans, being trapped around the stable L4 and L5 Jupiter Lagrangian points, are thought to be more primitive than the Main Belt asteroids. They are believed to have originated from a range of heliocentric distances in the trans-Neptunian region, to have subsequently been scattered inwards, and finally captured in their current location during the phase of Giant Planet migration. As a consequence, their bulk composition is expected to reflect that of the protoplanetary disk at the time and location of their formation. The photometric properties of Trojans appear to have a bi-modal distribution. A few Trojans have been discovered to be binary systems, suspected contact binaries, or to possess moonlets, which has revealed consistently low bulk densities (around 1× 10^3 kg m^-3 ) for those systems. Those estimates, together with the presence of a spin barrier between 4 and 4.8 h rotation period, suggest that low densities are a general property of the population, similar to that of cometary nuclei. Current Trojan physical properties provide clues that relate to their formation that can, in turn, be traced back to the origin of the solar system. We review here our current knowledge on the physical properties of Trojans and the methods used for their determinations. Most of these methods are based on Earth-bound observations, and are limited by the large distance to these objects. The next breakthrough will be made possible by the Lucy mission, which, by visiting several Trojans during a tour through both clouds, will address many open questions and probably raise new ones. The combination of the ground truth for select objects provided by Lucy with the context view given by the Earth-bound observations will result in powerful synergy.