617 Patroclus — a binary Jupiter Trojan

Discovery and name

The object was discovered in the early 20th century by August Kopff; it was reported shortly after the first recognized Jupiter Trojan, making it an early entry in this class of objects. The name Patroclus derives from Greek mythology and follows the naming tradition used for many of the Jupiter co-orbitals. For more on the broader population see Trojan group, and for information about the discoverer see August Kopff.

Physical characteristics

Both components of the Patroclus system are of similar size, each roughly on the order of a hundred kilometres across, and the pair circle each other in a compact mutual orbit. Observations indicate a very low overall density compared with typical inner‑belt asteroids, implying substantial internal ice or a porous structure rather than solid rock. Studies suggesting an icy composition refer to these objects as being more similar to cometary bodies; see summaries of the icy interpretation at icy comet-like bodies and contrasting views of rocky asteroids.

Orbit and classification

Patroclus shares Jupiter's orbital path around the Sun at a stable Lagrange region. Like other Trojans, its long-term motion is tied to Jupiter's gravity, and the system's stability has invited study as a fossil record of the early Solar System. Its status as a binary makes it especially valuable because the mutual orbit allows astronomers to determine the system mass and thus estimate bulk density.

Importance and exploration

The low density and binary nature of Patroclus make it important for questions about the origin and migration of small bodies: an icy, porous composition supports ideas that some Trojans originated in the outer Solar System and were later captured. Because of its scientific interest, the Patroclus–Menoetius pair has been selected as a target for spacecraft reconnaissance, which will provide higher-resolution data on shape, composition and internal structure.

Notable facts

• One of the earliest-discovered Jupiter co-orbitals and the second known Trojan.
• Unusual among Trojans for being a near-equal-mass binary, allowing direct mass and density measurements.
• Evidence points toward a composition richer in ice than typical main-belt asteroids, influencing models of Solar System evolution.