NASA's Lucy mission discovers a small moon orbiting a Trojan asteroid

The Lucy spacecraft hasn't even reached its first target, and the team is already making discoveries.
Chris Young
An artist's impression of Lucy
An artist's impression of Lucy


NASA's Lucy mission team made a surprise discovery years before their spacecraft is set to reach its target destination, a post from NASA reveals. They discovered that the smallest asteroid the probe is traveling to investigate has an even smaller moon in orbit.

The Lucy mission is heading towards Jupiter, where it will observe a group of asteroids known as the Trojans, which follow in the wake of the gas giant. The Lucy mission is set to break records by visiting more asteroids than any previous spacecraft.

Trojan asteroids are space rocks that are locked into orbit near larger space objects – typically planets. Two trojan asteroids are known to be orbiting near Earth, with one, called 2020 XL5, having been confirmed by observations in February this year.

NASA's Lucy mission: Observing Polymede from Earth

The Lucy spacecraft won't reach its first target until 2025, but the Lucy team has already made the impressive discovery of the moon orbiting an asteroid called Polymele. They were able to make observations of the asteroid as it aligned between Earth and a distant star, casting a strong silhouette.

The new discovery of the moon orbiting Polymede means the Lucy spacecraft will observe more space rocks than originally anticipated. "Lucy's tagline started out: 12 years, seven asteroids, one spacecraft," said Lucy program scientist Tom Statler from NASA headquarters in Washington. "We keep having to change the tagline for this mission, but that's a good problem to have."

The measurements were taken earlier this year, after which scientists and astronomers analyzed the findings. Twenty-six teams made observations on March 26th from different parts of the Earth. Taking observations from different vantage points, the international teams were able to collaborate to precisely measure the shape, size, and location of Polymele.

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"We were thrilled that 14 teams reported observing the star blink out as it passed behind the asteroid, but as we analyzed the data, we saw that two of the observations were not like the others," Marc Buie, from the Lucy team at the Southwest Research Institute, said in NASA's press statement. "Those two observers detected an object around 200 km (about 124 miles) away from Polymele. It had to be a satellite."

NASA's Lucy may uncover more than 8 Trojan asteroids

The Lucy team's analysis showed that the tiny moon is only about 3 miles (5 kilometers) across. By comparison, the asteroid Polymede has a 17-mile (27-kilometer) diameter.

This isn't actually the first time the Lucy team has found a satellite orbiting one of its target asteroids. In January 2021, Hubble Space Telescope images revealed that the Trojan asteroid Eurybates has a small orbiting moon.

The Lucy mission is one of several asteroid missions that are uncovering new valuable information on ancient space rocks holding secrets of the early universe. Japanese space agency JAXA's Hayabusa-2 mission, for example, recently showed that Earth's water may have come from asteroids, while NASA's Double Asteroid Redirection Test (DART) is making its way to an asteroid named Dimorphos to test whether a spacecraft can successfully alter a hazardous space rock's trajectory.

The 1.5-ton Lucy spacecraft, which launched from Cape Canaveral Space Force Station aboard an Atlas V rocket on October 16th, set out to observe eight trojan asteroids. Based on the latest findings, it could observe more space objects and collect more valuable data on Jupiter's ancient Trojan asteroids than originally anticipated.