This small asteroid has a tiny moon of its own

Polymele⁠—one of the smallest asteroids targeted by NASA's Lucy mission⁠—has an even smaller satellite.
A graphic showing the observed separation of asteroid Polymele from its discovered satellite. NASA's Goddard Space Flight Center

On August 16, the science team from NASA’s Lucy mission announced the discovery of a moon orbiting the asteroid Polymele. Named for one of the daughters of Peleus in Greek mythology, Polymele is only 17 miles long at its widest axis. By asteroid standards, 17 miles is a fairly small diameter making Polymele one of the smallest of the Trojan asteroids targeted by the mission.

According to NASA, Polymele was expected to pass in front of a star on March 27. This allowed the team to watch the star blink out when the asteroid briefly blocked it, a process is called occultation. These common occultation missions have been called “chasing an asteroid’s shadow” and often provide astronomers with incredibly valuable information.

This particular occultation day proved to be extra special. Spreading 26 teams of both professional and amateur astronomers across the occultation path, the Lucy team planned to gather data on Polymele’s location, size, and shape, with “unprecedented precision,” while the asteroid was outlined by the star behind it. 

“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,” said Marc Buie, Lucy occultation science lead at Southwest Research Institute, which is headquartered in San Antonio, via press release. “Those two observers detected an object around 200 km (about 124 miles) away from Polymele. It had to be a satellite.”

Polymele was about 480 million miles away from Earth at the time of observation. According to NASA, seeing something at this kind of distance is similar to “finding a quarter on the sidewalk in Los Angeles, while trying to spot it from a skyscraper in Manhattan.”

The team estimates that Polymele’s moon is roughly three miles in diameter and are 125 miles apart. This moon (or satellite) won’t be given an official name until the team determines its orbit. The naming could take a little while since the moon is too close to Polymele to be clearly seen with both Earth-based and Earth-orbiting telescopes. NASA says naming will have to wait until the Lucy team can spot the orbit on a future occultation attempt or when Lucy approached the asteroid in 2027.

The 12-year long Lucy mission launched in October 2021 and is the first mission to study Jupiter’s Trojan asteroids. The mission is named after the fossilized human ancestor called Lucy by her discoverers in 1974. Her 3 million year-old skeleton provided scientists with unique insight into humanity’s evolution. The mission’s goal is to “revolutionize our knowledge of planetary origins and the formation of the solar system.”