Lucy and the Trojan Asteroids of Jupiter

What Trojan asteroids are

NASA says a Trojan asteroid shares an orbit with a larger planet but does not collide with it, because Trojans gather around two stable locations called the L4 and L5 Lagrange points. Jupiter's Trojans are the most important group. NASA's Lucy page describes them as remnants of the early Solar System, trapped in stable orbits associated with Jupiter but not close to it. They form two loose swarms, one leading Jupiter and one trailing it.

Jupiter's swarms are the biggest known, but not the only ones. NASA also mentions Mars Trojans, Neptune Trojans and an Earth Trojan announced in 2011. JPL's database lists 16,472 Jupiter Trojans as of November 11, 2026, and that number changes as surveys find more. For how Trojans fit among all asteroid groups, see the guide to what asteroids are.

Why they stay in Jupiter's orbit

A Lagrange point is a place near a planet where the pull of the Sun and the pull of the planet combine so that a small body there keeps pace with the planet. L4 and L5 lie in the planet's orbit, one ahead and one behind. An asteroid near one of them is held in a broad loop around the point instead of drifting off.

The numbers show how well this works. Eurybates has a semi-major axis, a measure of orbit size, of 5.22 astronomical units (AU). One AU is the average Earth-Sun distance, about 149.6 million km (93 million miles), so that is roughly 780 million km (485 million miles) from the Sun. Patroclus has a very similar size of 5.21 AU. Both match the size of Jupiter's own path.

Trojans were found long before any spacecraft could reach them. JPL credits A. Kopff with the discovery of Patroclus on October 17, 1906, at Heidelberg, and C. J. van Houten with Eurybates on September 19, 1973, at Palomar.

Why scientists want to visit them

NASA's Lucy team calls Trojans primitive bodies that can help explain the history of the Solar System, and says Lucy is the first mission chosen to study them. Calling them primitive means they are thought to have changed little since early times, so they may hold clues that more altered bodies have lost. Lucy will visit several of them to compare their shapes, surfaces and moons.

The targets are varied. JPL lists Eurybates at about 64 km (40 miles) across and Patroclus at about 140 km (87 miles). Eurybates and Polymele each have a small moon, and Patroclus has a companion of its own, Menoetius. A comparison of several bodies will show how much Trojans differ from one another. The mission's other flybys, of main-belt asteroids, test the spacecraft first. The belt itself is described in the article on the asteroid belt.

Lucy's targets and schedule

Lucy launched on October 16, 2021. NASA's schedule lists the encounters below, in Eastern Time. Dates can shift, so check NASA's mission page for changes.

A contact binary is an asteroid made of two lobes that touch. NASA says Lucy will fly past each Trojan at speeds up to about 25,700 km/h (16,000 mph), coming within 480 to 970 km (300 to 600 miles) of each one. Lucy is a flyby mission, unlike the sample-return mission described in the story of OSIRIS-REx at Bennu.

What Lucy has done so far

The first flyby, Dinkinesh in November 2023, served as a systems test, according to NASA. The flyby of Donaldjohanson on April 20, 2025 was what NASA called a full dress rehearsal. Lucy passed about 960 km (600 miles) from it. NASA's first estimate put Donaldjohanson at about 8 km (5 miles) long and 3.5 km (2 miles) wide at its widest, and images show an elongated contact binary, two lobes joined by a narrow neck.

A June 18, 2026 release from the Southwest Research Institute adds details from the flyby data. It says Donaldjohanson resembles a peanut and tumbles on two axes, turning end over end about every 10.5 Earth days while wobbling around its horizontal axis about every 26.5 days. Lucy also found iron-rich clay minerals, which formed long ago in the presence of liquid water. The release gives a smaller size than NASA's first estimate, so treat the early numbers as preliminary.

The spacecraft is now between encounters. NASA's August 6, 2026 blog says that in the spring the team turned the L'LORRI camera toward Eurybates, Polymele, Leucus and Orus. The images were downlinked and processed by July. The tests help choose camera settings so surface details are not lost to darkness or glare. NASA lists Lucy as an active mission, and its schedule places the first Trojan encounter in August 2027. The wider list of missions to small bodies, past and planned, is in the article on missions to small bodies.

Common mistakes and myths

Myth: Lucy has already visited Trojans. It has not. Its only flybys so far were of main-belt asteroids. The first Trojan encounter is scheduled for August 2027.

Myth: Trojans are Jupiter's moons. They orbit the Sun, not Jupiter. They share Jupiter's path but sit far from the planet, in leading and trailing swarms.

Myth: only Jupiter has Trojans. NASA lists Mars and Neptune Trojans, and an Earth Trojan announced in 2011.

Myth: Trojan asteroids are all alike. Lucy's targets include a 64 km body, a 140 km body, and binary pairs with moons. The mission is designed to compare them.

Frequently asked questions

What is a Trojan asteroid?

It is an asteroid that shares a larger planet's orbit around the Sun without colliding with it, gathering near the planet's L4 or L5 Lagrange point. Jupiter has the most important group, with 16,472 known as of November 11, 2026.

When does Lucy reach its first Trojan?

NASA's schedule gives August 11, 2027, for the flyby of Eurybates and its satellite Queta, followed by Polymele on September 14, 2027.

How long does the Lucy mission last?

NASA describes a 12-year mission that visits eight Trojan asteroids. The final encounter, with Patroclus and Menoetius, is scheduled for March 3, 2033.

Where is Lucy now?

As of November 11, 2026, NASA lists Lucy as an active mission. Its August 2026 blog describes camera tests on four Trojan targets ahead of the 2027 flybys. Check NASA's mission page for the latest status.