Most planetary missions spend their cruise years quietly. NASA's Lucy has used its cruise to make discoveries. The spacecraft launched on Oct. 16, 2021. On Oct. 19, 2026, the Southwest Research Institute (SwRI), which leads the science team, marked five years of operations. By then Lucy had already returned results that changed how researchers think about small bodies, and it was less than a year from the encounters it was built for.
The first of those is Eurybates, a Jupiter Trojan asteroid. Lucy is scheduled to fly past it on Aug. 12, 2027. That will be the first close look any spacecraft has had at a member of this population.
A mission built around a population nobody has visited
The Jupiter Trojans are two swarms of asteroids that share Jupiter's orbit around the Sun. One swarm runs ahead of the planet and the other trails behind it. NASA describes them as remnants of the early solar system. Lucy is the first mission sent to explore them.
The mission plan is long. Over 12 years, Lucy is to explore three main-belt asteroids and eight Trojan targets, which NASA breaks down as five asteroids and three moons. Principal investigator Hal Levison of SwRI describes the full plan as visits to 11 asteroids. SwRI says three main-belt asteroids have been visited so far.
NASA's Goddard Space Flight Center manages the mission. Lockheed Martin built the spacecraft, and SwRI's Boulder office hosts the principal investigator. Lucy is the 13th mission in NASA's Discovery Program. It is named for the Lucy hominin fossil found in Ethiopia in 1974, the fossil of a human ancestor.
Rehearsal one: Dinkinesh, and a moon nobody expected
The main-belt flybys were placed along Lucy's route partly as practice: a way to test the spacecraft and its operations team on real targets before the Trojans. The first, Dinkinesh, took place on Nov. 1, 2023. It produced the mission's first major surprise.
Lucy found that Dinkinesh has a satellite, now named Selam. Selam turned out to be a contact binary, two lobes resting against each other, orbiting a larger asteroid. "We have never seen one orbiting another asteroid," said John Spencer, the mission's deputy project scientist, in SwRI's release.
This finding made a practical point: small bodies can be more complicated than ground-based observations suggest. A flyby has to be ready for structure that wasn't in the pre-encounter models. That is a useful thing to learn before reaching a population that has never been seen up close.
Rehearsal two: Donaldjohanson's peanut and its smooth neck
The second main-belt target, Donaldjohanson, followed on April 20, 2025. Lucy passed at about 650 miles (1,000 kilometers). The asteroid, about 5 miles (8 kilometers) wide, was another contact binary. Deputy principal investigator Simone Marchi pointed to its smooth neck, where the two lobes meet.
Results published in Science in June 2026 describe an object with complicated motion. It rotates end-over-end every 10.5 days and wobbles about its long axis every 26.5 days. The team concluded that the contact binary formed after a collision about 155 million years ago.
Two flybys, two contact binaries, one of them a moon. That is a small sample, but the pattern matters. Lucy's team has now processed, interpreted and published results on irregular, multi-lobed bodies with odd spin states. That experience should carry over to the Trojan flybys.
Getting there: a route shaped by Earth
Getting a spacecraft from Earth to Jupiter's orbit and back through both Trojan swarms takes a lot of orbital maneuvering, and much of it comes from Earth's gravity. EarthSky's account of the first five years lists the key steps:
- October 2022: first Earth gravity assist, passing about 220 miles (350 kilometers) above the surface.
- November 2023: Dinkinesh flyby and the discovery of Selam.
- December 2024: second Earth gravity assist, which changed Lucy's speed relative to the Sun by more than 16,000 mph.
- April 2025: Donaldjohanson flyby.
- August 2027: first Trojan encounter, Eurybates.
- December 2030: another Earth flyby.
- March 2033: arrival at the second Trojan swarm.
The December 2030 Earth flyby is the hinge of the mission. After the four Trojan systems of the first swarm (Eurybates and its satellite Queta, Polymele and its satellite, Leucus and Orus, from August 2027 to November 2028), Lucy loops back to the inner solar system for a gravity assist that slingshots it to the other swarm, where it visits the binary pair Patroclus and Menoetius in March 2033.
Why It Matters
Lucy is the first mission to explore the Jupiter Trojans. NASA describes them as leftovers from the early solar system. If that is right, then visiting several of them, spread across both swarms, gives a sample of the material and conditions that existed when the planets formed. One spacecraft can collect that kind of comparison only because it visits several targets in turn.
The main-belt results raise the stakes for the Trojan flybys. Contact binaries turned up at both of Lucy's practice targets, including one orbiting another asteroid, and Spencer notes that contact binaries seem fairly common in the solar system, though few have been seen up close. Whether the Trojans show the same shapes, and whether they record the same kind of collisional history seen at Donaldjohanson, is exactly what Lucy is equipped to test.
There is also an operational point. Lucy reaches Eurybates with two real flybys of difficult targets behind it. Its team has already worked through the full cycle of encounter, analysis and peer-reviewed results. When Lucy reaches its first Trojan next August, it will be doing work its team has done before, at a target no one has seen.
