One of the goals of the Lucy mission found a satellite

The scientific team of the Lucy mission reported the discovery of a satellite near the asteroid Polymele. Thus, the already record number of small Solar System bodies that the probe will visit has increased to nine. 

Polymele asteroid in the artist’s image. Source: NASA’s Goddard Space Flight Center

The Lucy probe was launched last autumn. Its target is the Trojan asteroids of Jupiter. This is how astronomers call two large groups of asteroids that are constantly located in the vicinity of the Lagrange points L₄ and L₅ of the Sun-Jupiter system. According to one of the modern models, many Trojans are actually Kuiper Belt objects that moved into the current orbit during the migration of gas giants that occurred several hundred million years after the birth of the Sun and the completion of the formation of planets. 

Ahead of Lucy’s upcoming visit, astronomers are using every opportunity to gather more information about its goals. This will allow us to better plan the program of upcoming observations. One such opportunity came on March 27, 2022, when the 27-kilometer asteroid Polymele eclipsed the background star. This event was observed by 26 teams of astronomers. During the subsequent analysis of the data, they unexpectedly found two brightness dips: one corresponded to the Polymele, the second to its previously unknown moon.

Data on the coverage of the background star Polymele by the asteroid. You can see two dips in brightness corresponding to the asteroid and its moon. Source: NASA’s Goddard Space Flight Center

Astronomers estimate that the diameter of the Polymele moon is about 5 km. The two asteroids are separated by a distance of 200 km. We will find out exactly what this pair is on September 15, 2027, when Lucy makes its close flyby. The current plan assumes that the probe will pass at a distance of 415 km from Polymele. But it is possible that after the moon is detected, the mission staff will change the flight plan. 

Diagram of the Polymele asteroid system. Source: NASA’s Goddard Space Flight Center 

This is not the first time that a moon has been discovered at one of Lucy’s targets. Earlier, astronomers found a companion near the asteroid Eurybates. Therefore, taking into account the latest find, the original slogan of the mission “Twelve years, seven asteroids, one spacecraft” now sounds like “Twelve years, nine asteroids, one spacecraft”.

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