NASA’s Lucy Reaches Trojan Asteroid Region 5 Years After Launch

NASA's Lucy Reaches Trojan Asteroid Region 5 Years After Launch

Having traveled more than 2.2 billion miles since it launched on Oct. 16, 2021, NASA's Lucy is the first spacecraft to enter a region of primitive solar-system bodies known as the Jupiter Trojan asteroids.

Although there aren't well-defined boundaries in space, mission engineers know that Lucy is approaching Jupiter's Lagrange point 4 (L4) by tracking Lucy's location through the spacecraft's radio signals and images of surrounding stars. This region is one of two large and unexplored "islands of stability", where thousands of asteroids share Jupiter's orbit around the Sun, guided by the combined gravitational pull of the Sun and the gas-giant planet.

The L4 point is surrounded by the group of asteroids that travel ahead of Jupiter in its orbit. The second group, around Lagrange point 5, trails behind the planet. Lucy will visit asteroids in both regions. Trojans in these clusters are as far from Jupiter as the planet is from the Sun, about 484 million miles.

Because the Trojans are separated from each other by more than 6 million miles on average, they will appear to Lucy as pinpricks of light moving against a field of stars until the spacecraft bears down on its chosen targets. Lucy's first encounter with the Trojan Eurybates will occur on Aug. 11, 2027.

While several previous spacecraft have visited Jupiter, some of them using its gravity to travel even farther - a maneuver called a gravity assist-none have had the opportunity to visit the Jupiter Trojans, given how far they are from the planet. Only Lucy, a spacecraft specifically on a mission to explore this population, has had a reason to travel to the regions these asteroids occupy.

Scientists expect that studying Trojans up close will help them hone their theories on how our solar system's planets formed 4.5 billion years ago, and how they ended up in their current configuration.

As Lucy cruises deeper into space, it has adapted to the gradually cooling temperature and dimming light. Lucy's design is optimized for this region of space - more than five times farther from the Sun than Earth is - and the spacecraft and instruments continue to operate as expected. With less than a year until Lucy's first asteroid encounter, scientists are busy putting the final touches on the encounter plans. That work includes ensuring that the spacecraft, instruments, and team members are prepared to make the most of the few hours Lucy will spend studying Eurybates up close.

After flying by Eurybates, Lucy will visit Polymele (September 2027), Leucus (April 2028), and Orus (November 2028). The spacecraft then will return to Earth in 2030 for a third and final gravity assist, which will send it toward Jupiter's trailing group of Trojans, where it will explore the binary pair Patroclus and Menoetius in March 2033.

By Katherine Kretke
Southwest Research Institute

Download the Lucy trajectory visualization.

More from Lucy