  The Trojan asteroids provide a unique perspective on the history of Solar
System. As a large population of small bodies, they record important
gravitational interactions and dynamical evolution of the Solar System. In the
past decade, significant advances have been made in understanding physical
properties, and there has been a revolution in thinking about the origin of
Trojans. The ice and organics generally presumed to be a significant part of
Trojan compositions have yet to be detected directly, though low density of the
binary system Patroclus (and possibly low density of the binary/moonlet system
Hektor) is consistent with an interior ice component. By contrast, fine-grained
silicates that appear to be similar to cometary silicates in composition have
been detected, and a color bimodality may indicate distinct compositional
groups among the Trojans. Whereas Trojans had traditionally been thought to
have formed near 5 AU, a new paradigm has developed in which the Trojans formed
in the proto-Kuiper Belt, and they were scattered inward and captured in the
Trojan swarms as a result of resonant interactions of the giant planets.
Whereas the orbital and population distributions of current Trojans are
consistent with this origin scenario, there are significant differences between
current physical properties of Trojans and those of Kuiper Belt objects. These
differences may be indicative of surface modification due to the inward
migration of objects that became the Trojans, but understanding of appropriate
modification mechanisms is poor and would benefit from additional laboratory
studies. Many open questions remain, and the future promises significant
strides in our understanding of Trojans. The time is ripe for a spacecraft
mission to the Trojans, to turn these objects into geologic worlds that can be
studied in detail to unravel their complex history.
