NASA’s Lucy mission finds asteroid Polymele has a little companion orbiting it

NASA’s Lucy mission has found two asteroids for the price of one. Months of detailed observations have finally revealed that the asteroid Polymele has a tiny moon of its own.

Already on track to visit more asteroids than any previous mission, Lucy can now add another asteroid to its list.

At around 27 kilometres in length, Polymele was Lucy’s smallest target. But the discovery of a smaller object – around five kilometres wide – was a pleasant surprise. The asteroids are part of the Trojan group which share Jupiter’s orbit around the Sun.

Lucy’s team was waiting on March 27 for Polymele to pass in front of a star to see the star blink out as the asteroid briefly blocked, or occulted, it. This would allow the 26 teams of professional and amateur astronomers to measure the location, size and shape of Polymele more precisely than ever before. What they saw was a little bonus.

Using the occultation data, the team assessed that this satellite is roughly 3 miles (5 km) in diameter, orbiting Polymele, which is itself around 17 miles (27 km) along its widest axis. The observed distance between the two bodies was about 125 miles (200 km). Credit: NASA’s Goddard Space Flight Center.

“We were thrilled that 14 teams reported observing the star blink out as it passed behind the asteroid, but as we analysed the data, we saw that two of the observations were not like the others,” says Marc Buie, Lucy occultation science lead at the Texas-based Southwest Research Institute. “Those two observers detected an object around 200 kilometres away from Polymele. It had to be a satellite.”

As per planetary naming conventions, the baby asteroid will not be officially named until its orbit is determined. Being so close to its parent, the satellite can’t be seen clearly by Earth-based or even Earth-orbiting telescopes. So, the name will have to wait until the pair pass in front of another serendipitously placed star or until Lucy gets close enough to Polymele in 2027.

Polymele was 770 million kilometres from Earth when its little friend was observed. According to NASA’s release, that’s equivalent to finding a quarter on a footpath in Los Angeles while standing on a Manhattan skyscraper – 4,000 kilometres away!

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It’s also not the first time that NASA has found a satellite around an asteroid. Another of the Trojans, Eurybates (68 kilometres in diameter), was found in January 2021 by Hubble to have a satellite.

When it was launched on October 16, 2021, Lucy’s original 12-year mission brief was to visit seven asteroids – six of which were within the Trojan group. Although it hit a snag when one of the rocket’s two solar arrays only partially unfurled, NASA remained confident that Lucy’s power levels would be enough to keep the spacecraft healthy and functioning.

After months of brainstorming NASA engineers figured out a way of reopening the array. While still not completely unfurled, it is now generating almost all the power it would if it were fully latched.

Even so, Lucy is already exceeding expectations. Lucy is now expected to visit nine asteroids.

“Lucy’s tagline started out: 12 years, seven asteroids, one spacecraft,” says Lucy program scientist Tom Statler. “We keep having to change the tagline for this mission, but that’s a good problem to have.”

Asteroids hold vital clues to determining the history and formation of the solar system. It is possible that they may even hold the keys to the origins of life. Lucy is part of NASA’s aims to shed light on these mysteries.

The mission takes its name from the fossilised human ancestor (called “Lucy” by her discoverers) whose skeleton provided unique insight into humanity’s evolution.

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