Lucy
A twelve-year tour of the Trojan asteroids, possible fossils of planet formation.
Key facts
- Agency
- NASA (United States)
- Type of mission
- Multiple asteroid flybys
- Target
- Main-belt asteroids and the Trojan asteroids of Jupiter
- Launched
- 16 October 2021
- Status
- On its way to the Trojan asteroids
The Trojan asteroids travel around the Sun in two great swarms, one ahead of Jupiter and one behind it, held in place by the balance of the Sun's and Jupiter's gravity. They are dark and reddish, and many scientists think they formed far out in the Solar System and were captured into these orbits when the giant planets shifted early in their history. If so, they are leftovers of the material that built the outer planets.
Lucy is the first mission to visit them. It is named after the famous fossil skeleton of an early human ancestor, because the Trojans may be fossils of the Solar System's own beginnings.
Surprises on the way
On 1 November 2023 Lucy flew past the small main-belt asteroid Dinkinesh, about 800 metres across, and found that it has a moon. The moon, named Selam, turned out to be two bodies touching each other, a contact binary. On 20 April 2025 it passed Donaldjohnson, about 8 kilometres long and also made of two lobes. Both encounters suggest that small asteroids are often loose collections of fragments, reshaped as they spin.
The Trojans
From 2027 to 2028 Lucy is due to fly past four Trojans in the leading swarm, Eurybates with its tiny moon Queta, Polymele, Leucus and Orus, then return past Earth and reach the trailing swarm in 2033 to visit the pair Patroclus and Menoetius, two bodies of about 100 kilometres orbiting each other.
Why it matters to meteorites
No meteorite has been firmly linked to a Trojan. Lucy will measure their colours, surfaces and densities, which can be compared with the darkest carbonaceous meteorites and with comets.
Key dates
| Date | What happened |
|---|---|
| 16 October 2021 | Lucy launched on an Atlas V rocket. |
| 16 October 2022 | First gravity-assist flyby of Earth. |
| 1 November 2023 | Flew past Dinkinesh and discovered its moon Selam. |
| 13 December 2024 | Second gravity-assist flyby of Earth. |
| 20 April 2025 | Flew past the asteroid Donaldjohnson. |
| August 2027 | Planned flyby of the Trojan Eurybates and its moon Queta. |
| March 2033 | Planned flyby of the Trojan pair Patroclus and Menoetius. |
See where this mission fits among all the others in the mission timeline.
Did you know?
Donaldjohnson is named after the scientist who discovered the Lucy fossil in 1974.
Lucy will visit more separate asteroids than any mission before it.
Related pages
Comets connected with this mission: Comet 3I/ATLAS. See all comets.