Lucy reaches Eurybates

Humanity's first visit to the Trojan asteroids sharing Jupiter's orbit — frozen leftovers of planet formation.

The moment of Lucy reaches Eurybates: https://nowsky.org/e/lucy-eurybates — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

In August 2027 NASA's Lucy spacecraft sweeps past Eurybates, an asteroid in Jupiter's Trojan swarm. The Trojans share Jupiter's orbit around the Sun but sit roughly sixty degrees ahead of or behind the planet, held there by gravity. Those positions are stable over very long spans, which is why the material parked in them is thought to be relatively untouched leftovers from the era when the planets formed.

Lucy left Earth in 2021 on a twelve-year itinerary designed to pass a whole series of Trojans in turn. No spacecraft has ever visited so many asteroids. The name comes from the hominin fossil: as one skeleton reshaped the story of human origins, these asteroids are expected to say something decisive about planetary ones.

Eurybates is the first Trojan on the list. It is among the larger targets on Lucy's tour, measured in tens of kilometres across, and its surface is unusually dark. What makes it interesting is that it is the largest member of a collisional family — a group of fragments left by an ancient smash-up — which raises the possibility that its interior is now its surface.

It also has a small moon, Queta, picked up in Hubble observations well after the target was chosen. The moon is not a bonus so much as a key measurement. Timing its orbit yields the mass of the main body, and mass with size gives density — the number that separates a solid rocky object from a loose, possibly ice-bearing rubble pile.

Why it matters

The Trojans matter because how they got there is still argued over. The leading account says the giant planets migrated early in the solar system's history, and that the shuffle captured bodies from farther out into the stable points ahead of and behind Jupiter. If that is right, the Trojans are not local at all — they are immigrants from the outer solar system parked in convenient gravitational harbours.

The evidence lies in colour and composition. The swarm mixes redder and less-red objects, and nobody has settled whether that split records two different birthplaces or simply different amounts of surface weathering over billions of years. That is precisely why Lucy flies past several of them with the same set of instruments: the comparison is the experiment.

Eurybates is the starting point of that comparison and carries a question of its own. As the largest piece of a collisional family, how much ice-related material shows on its surface gives the first direct hint of what these objects are made of on the inside. This is a look at construction material from four and a half billion years ago.

How to watch

There is nothing here to see in the sky. Eurybates is a dark asteroid out near Jupiter's distance, far beyond amateur equipment, and the encounter itself is over in hours. The spacecraft also spends the pass observing rather than talking, so images are not live: they trickle down to Earth over the following days and are released after that.

The orbit-view replay is the better way to understand it. Set the clock to August 2027 and follow Lucy, and you see a trajectory that climbed outward through repeated passes near Earth and across the asteroid belt until it reached Jupiter's distance from the Sun. The screen also makes one thing immediately clear: Jupiter is nowhere near the encounter point, because the Trojans lead the planet by sixty degrees along the same orbit.

For naked-eye observing, the realistic substitute is Jupiter itself. It is among the brightest objects in the night sky and stands out even from a city, and ordinary binoculars show its four large moons strung out in a line. Picture Lucy working far ahead of that planet along the same orbital track, and the distances stop being abstract.

See this for yourself — replay the moment in the orbit view

Background

The Trojans entered the record in 1906 with the discovery of Achilles, and as more turned up they were named after figures from the Trojan War until the label stuck to the whole population. Thousands are now catalogued, but they are distant and dark, never more than points of light from the ground, and for well over a century no spacecraft went anywhere near them.

Lucy rehearsed in the main asteroid belt before heading out. It flew past small asteroids to exercise its instruments and its tracking procedure, and one of those targets turned out to have an unexpected satellite of its own. It was a useful reminder of how much a few hours of close approach can change.

The itinerary continues past Eurybates. Lucy is planned to pass several more objects in the leading swarm, then cross over to the group trailing Jupiter and finish at a pair of bodies orbiting each other there. Each encounter is a matter of hours; what joins them together is years of quiet cruising.

FAQ

Where exactly are the Trojan asteroids?

They sit in Jupiter's own orbit around the Sun, gathered into two clouds about sixty degrees ahead of the planet and sixty degrees behind it. Those are gravitationally stable points where objects can remain for extremely long periods. They are not moons of Jupiter and do not orbit the planet at all.

Can I see this with a telescope?

Eurybates is a dark asteroid at Jupiter's distance, effectively out of reach for amateur instruments. The encounter is not broadcast live either; images are released after the data has been downlinked over the following days. Jupiter, which shares that orbit, is easy to find with the naked eye.

Why does the moon Queta matter?

Measuring how far the moon orbits and how long it takes gives the mass of the main body directly. Combine that with the size measured during the flyby and you get density — the decisive clue to whether the object is solid rock or a loosely bound, possibly ice-rich structure.

Where does Lucy go after this?

Eurybates is the first of the Trojan stops. Lucy then passes several more objects in the leading swarm before crossing to the trailing swarm, where the planned mission ends at a pair of bodies that orbit each other. Years of cruise separate one encounter from the next.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.