NASA’s Lucy became the first spacecraft to reach Jupiter’s Trojan asteroid region

NASA’s Lucy has crossed into the Jupiter Trojan asteroid region five years after its Oct. 16, 2021 launch, the first spacecraft ever to do so. The probe has traveled more than 2.2 billion miles to reach a swarm of primitive bodies that share Jupiter’s orbit around the Sun, and its first close encounter with a Trojan asteroid is still more than nine months away.

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Katherine Kretke of Southwest Research Institute wrote NASA’s account of the arrival and reported that the spacecraft and its instruments are operating as expected after adapting to cooler temperatures and dimmer light. Kretke’s post, published Oct. 28 and updated the next day, also notes that teams are finalizing encounter plans for the first target, and it frames the crossing, which the post counts as five years after launch, as a milestone on the way to the encounters, not as the science itself.

Two islands of stability ahead of and behind Jupiter

Jupiter’s Trojans occupy two large regions NASA calls islands of stability, where thousands of asteroids are held in place by the combined gravity of the Sun and Jupiter. The group at Lagrange point 4 leads Jupiter in its orbit and the group at Lagrange point 5 trails it. Trojans sit about 484 million miles from Jupiter, roughly the distance between Jupiter and the Sun, and they are separated from one another by more than 6 million miles on average. The NASA Science blog post adds that engineers track the approach with radio signals and images of surrounding stars.

Lagrange points are the places where the pull of the Sun and Jupiter, combined with orbital motion, lets small bodies hold station, which is why asteroids collect at L4 and L5 instead of drifting away. Until now astronomers have studied the Trojans only from Earth, according to NASA’s explainer featuring Lucy scientist Audrey Martin of Northern Arizona University, and the explainer calls them pristine asteroids thought to come from the outer solar system.

Lucy’s design targets a region more than five times farther from the Sun than Earth, so the spacecraft has to work in cooler temperatures and dimmer light than comparable missions near home. With an average separation above 6 million miles between Trojans, a spacecraft passing through the region does not see crowds of rocks, only a vast and mostly empty volume with targets chosen well in advance.

Flyby schedule from Eurybates to Patroclus and Menoetius

The first encounter, with the Trojan Eurybates, will occur on Aug. 11, 2027. Polymele follows in September 2027, Leucus in April 2028 and Orus in November 2028. Lucy then returns to Earth in 2030 for a third and final gravity assist that sends it toward the trailing group, where it will explore the binary pair Patroclus and Menoetius in March 2033. NASA’s Lucy mission page describes eight Trojan targets and a pair of main-belt asteroids, Dinkinesh and Donaldjohanson. The trajectory visualization from NASA’s Scientific Visualization Studio shows the path.

Eurybates comes first and gives the shortest look. Lucy will study the asteroid for only a few hours. A camera rehearsal ran in spring 2026: Lucy observed four Trojan targets, Eurybates, Polymele, Leucus and Orus, with its high-resolution L’LORRI camera to pick the best settings for the flybys, and downlinking and processing finished in July, according to NASA’s August 6 mission post.

Close approaches will be tight. The same post gives a closest-approach distance of 300 to 600 miles and a flyby speed of up to 16,000 mph, which leaves little time to correct a pointing error. The whole mission is a 12-year journey past eight Trojan asteroids, with the flybys running from August 2027 to November 2028 before the return to Earth in 2030.

Primitive bodies and the 4.5-billion-year-old planet record

Scientists want the Trojans because the bodies are primitive. Studying them up close is expected to refine theories of how the planets formed about 4.5 billion years ago and how they reached their current arrangement. Because Trojans hold their positions through the gravity of the Sun and Jupiter, they have stayed in place for a very long time, and the surfaces and compositions Lucy records should show what material was available in the outer solar system when the giant planets settled. NASA’s explainer says the bodies have been stable for billions of years, so they may hold clues to how the solar system formed and evolved.

One caveat on the milestone. Lucy has entered the region, not reached an asteroid, and everything about Trojan surfaces, shapes and moons still waits on August 2027. The first full-resolution images of Eurybates will be the first real test of what the long approach was for. Until then, engineers keep tracking Lucy by radio signals and images of surrounding stars as the distance to the first target closes, and the later targets, through Patroclus and Menoetius in March 2033, depend on the spacecraft staying healthy for another six years and more.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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