Lucy reaches Eurybates
NASA's Lucy spacecraft is due to fly past Eurybates on 12 August 2027, the first close look any spacecraft will get at a Trojan asteroid, one of the swarms that share Jupiter's orbit.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 12 August 2027 NASA's Lucy spacecraft is due to fly past the asteroid Eurybates. Eurybates belongs to the Trojan asteroids. These asteroids travel around the Sun along the same path as Jupiter, but they gather about 60 degrees ahead of the planet or 60 degrees behind it. At those spots the pulls of the Sun and Jupiter balance in a way that lets asteroids stay for an extremely long time.
For that reason, scientists think the Trojans have kept material from the time the solar system formed in fairly original condition, like leftover building pieces stored in a freezer. Lucy left Earth on 16 October 2021 on a 12-year journey planned to pass one asteroid after another. No single spacecraft has set out to visit so many asteroids before.
The name comes from ‘Lucy’, a fossil of an early human relative found in Ethiopia in 1974. That fossil helped scientists understand where humans came from, and the hope is that the Trojans will help explain where planets came from. Eurybates is the first Trojan on Lucy's list and, at about 64 km across, one of the larger ones.
Eurybates has a small moon called Queta. It was discovered by the Hubble Space Telescope in 2018, after Lucy's targets had been chosen. By measuring how far away the moon circles and how long each orbit takes, scientists can work out how heavy Eurybates is. Weight and size together give density, and density tells whether the asteroid is solid rock or a loose pile of pieces with gaps between them.
Mission goals
How the Trojans ended up where they are is still not settled. Many scientists think that early in the solar system's history the giant planets, such as Jupiter and Saturn, shifted their orbits, and that objects from much farther out were trapped in the spots ahead of and behind Jupiter at that time. If so, the Trojans were not born near Jupiter at all; they moved in from far away.
Clues lie in their colour and make-up. Some Trojans are quite red and others less so. Nobody yet knows whether that difference means they came from different places, or simply that their surfaces have been changed by different amounts of sunlight and space particles over billions of years. That is why Lucy looks at several of them with the same instruments: the answer depends on comparing them.
Eurybates is the largest piece of a family of fragments left by a big collision long ago. The collision may have exposed material from inside, so its surface could give the first clue to what Trojan asteroids are made of beneath their outer layers.
Mission chronology and background
The first Trojan asteroid, Achilles, was discovered in 1906 by the German astronomer Max Wolf. As more were found they were named after heroes of the Trojan War, and the whole group became known as the Trojans. Thousands are known today, but they are far away and dark, so ground-based telescopes show them only as tiny points.
Before heading to the Trojans, Lucy practised twice in the main asteroid belt. In November 2023 it passed the small asteroid Dinkinesh and found an unexpected moon, which itself turned out to be two pieces stuck together. In April 2025 it flew past the asteroid Donaldjohanson.
The schedule continues after Eurybates. Lucy is to pass Polymele in September 2027 and Leucus and Orus in 2028, then swing past Earth again at the end of 2030. Finally, in March 2033, it is planned to visit Patroclus and Menoetius, a pair of asteroids orbiting each other in the swarm that trails behind Jupiter.
Where to see it — sky and simulator
This flyby is not something you can see in the sky. Eurybates is a dark asteroid about as far away as Jupiter, well beyond the reach of amateur telescopes. Lucy also spends the flyby taking measurements and sends the data home over the following days, so the pictures are released days later rather than live.
Instead, set the orbit view to August 2027 and follow Lucy. It shows how the probe passed Earth twice to enlarge its orbit until it climbed out to Jupiter's distance from the Sun. It also shows that Jupiter is far from the flyby point, because the Trojans sit 60 degrees ahead of the planet rather than beside it. When you find bright Jupiter in the night sky, Lucy is working far ahead of it along the same path.
See this for yourself — replay the moment in the orbit view
FAQ
Where exactly are the Trojan asteroids?
On Jupiter's own orbit, in two swarms about 60 degrees ahead of and 60 degrees behind the planet. They are not moons of Jupiter; they circle the Sun just as Jupiter does.
Can I see it with a telescope?
No. Eurybates is a dark asteroid at Jupiter's distance, beyond amateur equipment. Jupiter, travelling the same path, is easy to find with the naked eye.
Why does the moon Queta matter?
Measuring Queta's orbit gives the weight of Eurybates. Add the size and you get density, which shows whether it is solid rock or a loose pile of pieces.
Where does Lucy go next?
It passes several more Trojans and ends its planned mission in 2033 at Patroclus and Menoetius, a pair of asteroids orbiting each other in the trailing swarm.
Related
- Psyche arrives at a metal world
- Hayabusa2's extended mission
- Lucy — object index
- Jupiter — object index
- Psyche — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.