The space rock has been found orbiting Polymele, a Trojan asteroid close to Jupiter. Found some 770 million kilometres from the Earth, it was spotted by Nasa’s Lucy mission which was launched in 2021 to study two large asteroid clusters in the hopes of finding the primordial material that formed the solar system’s outer planets.
The new moon was detected 200 kilometres away from Polymele and is roughly five kilometres in diameter, approximately one third of its Trojan host.
Lucy itself, which takes its name from a fossilised human ancestor, is only 14 metres from tip to tip in size - most of which is huge solar panels used to power the spacecraft.
Its instruments, and a two-metre-high gain antenna needed to communicate with Earth, are all on the much smaller spacecraft body.
As scientists have not yet determined the moon’s orbit, it will probably not be named until 2027 when Lucy is expected to approach the asteroid.
Polymele’s moon is too close to be seen by Earth-based or Earth-orbiting satellites, and even spotting it from Lucy’s distance was unlikely; Nasa compares it to finding a quarter on a sidewalk in Los Angeles – while trying to spot it from a skyscraper in Manhattan.
This is not the first time that Lucy has detected an unexpected satellite. In January of 2021, the team used the Hubble Space Telescope to discover that one of the Trojan asteroids, Eurybates, also has a small satellite.
This new satellite means Lucy is on track to visit nine asteroids on its 12-year voyage.
“Lucy’s tagline started out: 12 years, seven asteroids, one spacecraft,” said Lucy program scientist Tom Statler at Nasa Headquarters in Washington.
“We keep having to change the tagline for this mission, but that’s a good problem to have.”
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