Patroclus
Key facts
- What it is
- Double Jupiter Trojan
- Size
- About 110 km across (each of the two)
- Discovered
- 1906, by August Kopff
- Spacecraft
- Lucy fly-by planned for 2033
- Orbit group
- Jupiter Trojan
- Average distance from the Sun
- 5.21 au (about 780 million km)
- Time to go round the Sun
- 11.9 years
- Closest and farthest from the Sun
- 4.48 to 5.93 au
- Tilt of its orbit
- 22.1°
- Spins once every
- 4.3 days
An au (astronomical unit) is Earth's average distance from the Sun, about 150 million km. Orbit figures are from open tracking data, figures last updated 1 Nov 2026.
Patroclus is not one asteroid but two. In 2001 astronomers found that it is a pair of nearly equal bodies, each about 110 kilometres across, orbiting each other while together they travel around the Sun in the trailing swarm of Jupiter's Trojans. The second body is named Menoetius, after the father of Patroclus in Greek legend.
The two circle each other every four and a quarter days or so, at a distance of about 680 kilometres. That orbit allowed astronomers to work out their combined mass, and so their density, which came out at less than that of water ice. Patroclus and Menoetius must therefore contain a lot of ice, a lot of empty space, or both. That is very different from the stony asteroids of the inner belt and fits the idea that the Trojans formed far out in the cold outer Solar System.
Large pairs like this are thought to be fragile. A close encounter with a planet would probably pull them apart, so the fact that this pair survives suggests it has lived quietly in its present place for billions of years. That makes it a valuable time capsule.
Patroclus was discovered in 1906 by August Kopff at Heidelberg, only months after the first Trojan, Achilles. It is the final target of NASA's Lucy mission, which is due to fly past the pair in 2033 and photograph both bodies.
By convention, Trojans in the trailing swarm are named after the Trojan side of the war, and those in the leading swarm after the Greeks. Patroclus, a Greek hero, is one of two famous exceptions.