Trojan asteroid
A Trojan asteroid shares a planet's orbit, sitting about 60 degrees ahead of or behind it.
A Trojan asteroid orbits the Sun in step with a planet, clustered around one of two balance points called Lagrange points L4 and L5. These lie about 60 degrees ahead of and behind the planet along its path, where the gravity of the Sun and planet combine to hold small bodies in place.
Jupiter has by far the largest population, with more than 10,000 known Trojans in two swarms. By tradition, those leading Jupiter are named after Greek heroes of the Trojan War and those trailing after Trojan heroes, with a few exceptions. Earth has at least two small Trojans, and Mars and Neptune have some too.
Jupiter's Trojans are mostly dark and reddish, possibly rich in carbon and ice. They may have formed in the outer Solar System and been captured as the giant planets moved. NASA's Lucy mission is the first to visit them, with flybys planned from 2027.
Related terms
Level: Enthusiast Teens and adults