NASA uses occult means to spot tiny moon orbiting asteroid

Lucy probe is five years away from getting a closeup of Polymele's satellite

NASA scientists working on the Lucy mission, humanity's first exploration of the "Trojan" asteroids that orbit Jupiter's Lagrange points, have found a moon around one of the asteroids the probe will visit.

Lucy launched in 2021 to engage in a 12-year journey resulting in the flyby of seven Trojan asteroids and one in the solar systems' main asteroid belt. The Trojans are believed to have formed by the same process as planets so boffins are hoping it will tell them more about how our solar system began.

But in late March, NASA was able to add one more celestial body to the list scheduled for observation – a moon orbiting the smallest of the mission's Trojan asteroid targets, Polymele.

The moon was discovered during a campaign to study it while it passed in front of a star, thus blocking out the star's light, a tactic that was expected to lead to understanding the asteroid's location, size and shape with deep precision. The blockage, or occultation, revealed the satellite.

"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. "Those two observers detected an object around 200km (about 124 miles) away from Polymele. It had to be a satellite."

The Lucy team reckons the satellite is about 3 miles (5km) in diameter, while Polymele itself is only 17 miles (27km) at its widest. Lucy has a long way to go to meet Polymele and its as-yet unnamed satellite as they are around 480 million miles away (770 million km). In travel time for the spacecraft, that means five years.

"Those distances are roughly equivalent to finding a quarter on a sidewalk in Los Angeles – while trying to spot it from a skyscraper in Manhattan," said NASA in its announcement of the discovery on Wednesday.

Polymele is the second of the mission-targeted Trojan asteroids to have a moon spotted. A satellite was found orbiting Eurybates in January 2020 via the Hubble Space Telescope.

Not everything has been Champagne and roses for the mission. A few hours after launch, it became apparent the probe's solar arrays did not fully deploy, a problem that could impede the spacecraft's power system. As its cameras weren't pointed at the solar arrays, the flight ops team was handicapped when it came to diagnosing and solving the problem.

Over months, computer models and existing data helped boffins develop commands to deploy and stabilize the array and carry on for the next 11 years. ®

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