The Jupiter trojan asteroid 15094 Polymele and its small satellite are among the targets of NASA’s Lucy mission, which will approach the asteroid system on September 15, 2027.
An artist’s impression of the asteroid Polymele. Image credit: NASA’s Goddard Space Flight Center.
Polymele is a P-type Jupiter L4 trojan asteroid approximately 27 km (17 miles) in diameter.
This asteroid was discovered on November 17, 1999 by astronomers with the Catalina Sky Survey at Mount Lemmon Observatory in Arizona.
The object was named after Polymele from Greek mythology, the wife of Menoetius and the mother of Patroclus.
Polymele has a rotation period of 5.9 hours and possibly a spherical shape.
It orbits the Sun at a distance of 4.7-5.7 astronomical units (AU) once every 11 years and 9 months.
Polymele is planned to be visited by NASA’s asteroid-hunting spacecraft Lucy which launched in 2021.
The encounter is scheduled for September 15, 2027. The goal is to fly within approximately 400 km (250 miles) at closest approach in order to enable determination of the asteroid’s mass to within 25%.
A graphic showing the observed separation of Polymele from its moon. Image credit: NASA’s Goddard Space Flight Center.
On March 27, 2022, Lucy’s science team discovered that Polymele has a satellite of its own.
On that day, Polymele was expected to pass in front of a star, allowing the researchers to observe the star blink out as the asteroid briefly blocked, or occulted, it.
By spreading 26 teams of professional and amateur astronomers across the path where the occultation would be visible, they planned to measure the location, size, and shape of Polymele with unprecedented precision while it was outlined by the star behind it.
These occultation campaigns have been enormously successful in the past, providing valuable information to the mission on its asteroid targets, but this day would hold a special bonus.
“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 Dr. Marc Buie, Lucy occultation science lead at the Southwest Research Institute.
“Those two observers detected an object around 200 km (about 124 miles) away from Polymele. It had to be a satellite.”
Using the occultation data, the scientists assessed that Polymele’s moon is roughly 5 km (3 miles) in diameter.
The observed distance between the two bodies was approximately 200 km (125 miles).
Following planetary naming conventions, the satellite will not be given an official name until the team can determine its orbit.
“Lucy’s tagline started out: 12 years, seven asteroids, one spacecraft,” said Lucy team member Dr. Tom Statler, a researcher at NASA Headquarters.
“We keep having to change the tagline for this mission, but that’s a good problem to have.”
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