617 Patroclus

Lore & Background

Patroclus was the second trojan asteroid discovered and the only member of the Trojan camp named after a Greek figure, as the convention of naming one camp after Greek characters and the other after Trojan characters had not yet been established. It was long thought to be one of the largest Jupiter trojans, with a diameter on the order of 150 km, but in 2001 it was discovered to be a binary system of two similarly sized objects. The larger component retains the name Patroclus (110–115 km diameter), while the secondary, slightly smaller (100–105 km), was named Menoetius.

The binary components orbit their center of mass in about 4.283 days at a distance of 680 km in a roughly circular orbit. Later measurements refined the co-orbital period to 103.5 hours. In 2013, both bodies occulted a magnitude 8.8 star, observed by 41 observers across the USA, yielding sizes equivalent to 113 km and 104 km diameter spheres. The system's density (0.88 g/cm³) is less than water, suggesting an icy, comet-like composition rather than rocky. It is a dark D-type asteroid and a slow rotator, with a rotation period between 102.8 and 103.5 hours.

Reader's Guide

617 Patroclus holds significance as the first binary trojan asteroid discovered, revealing that such systems exist in Jupiter's Lagrange points. Its low density (0.88 g/cm³) and icy composition challenge earlier assumptions that trojans are primarily rocky, aligning them more closely with comets. This supports the hypothesis that many Jupiter trojans are small planetesimals captured during the migration of the giant planets 3.9 billion years ago, as proposed by A. Morbidelli and colleagues in 2005. The Patroclus–Menoetius system is a scheduled target for NASA's Lucy mission, which will perform a flyby in 2033, offering a rare opportunity to study a binary trojan up close. Its naming history—being the only Greek-named object in the Trojan camp—reflects the early, unstandardized conventions for naming Jupiter trojans.