3548 Eurybates is a Jupiter trojan, one of the family of objects that have moved within the Lagrange points around Jupiter for billions of years (the term is libration, meaning these asteroids actually oscillate around the Lagrange points). Consider them trapped objects, of consequence because they have so much to tell us about the early Solar System. The Lucy mission aims to visit both populations (the ‘Greeks’ and the ‘Trojans’) at Jupiter’s L4 and L5 Lagrangians when it heads for Jupiter following launch in 2021.

Image: During the course of its mission, Lucy will fly by six Jupiter Trojans. This time-lapsed animation shows the movements of the inner planets (Mercury, brown; Venus, white; Earth, blue; Mars, red), Jupiter (orange), and the two Trojan swarms (green) during the course of the Lucy mission. Credit: Astronomical Institute of CAS/Petr Scheirich (used with permission).

Right now the focus is on Eurybates as mission planning continues, for we’ve just learned thanks to the Hubble instrument’s Wide Field Camera 3 that this asteroid has a moon, an object more than 6,000 times fainter than Eurybates itself. According to mission principal investigator Hal Levison (Southwest Research Institute), that implies a diameter of less than 1 kilometer. The tiny moon will be among the smallest asteroids visited by Lucy, which is intended to perform flybys of six trojans, as well as a main belt asteroid along the way.

Thomas Statler is a Lucy program scientist at NASA headquarters in Washington:

“There are only a handful of known Trojan asteroids with satellites, and the presence of a satellite is particularly interesting for Eurybates. It’s the largest member of the only confirmed Trojan collisional family – roughly 100 asteroids all traceable to, and probably fragments from, the same collision.”

It took three tries with Hubble to confirm the satellite’s existence, a tricky job given the object’s faintness and unknown orbit around the much brighter Eurybates. Now the task is to figure out when it will become visible again, for Eurybates won’t be observable until well clear of the Sun, which won’t happen until June. No major changes to the existing flight planning are needed to incorporate the small moon into the mission, but refining its orbit around the asteroid will help scientists schedule the best observing time during the Eurybates encounter.

Image: This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white), (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years. Credit: Southwest Research Institute.

We already have one binary to look forward to, for Patroclus, in the L5 cloud, has a small satellite called Menoetius. Note how stuffed with interesting things these Lagrangian points seem to be. We have D-type asteroids like Patroclus, which likely have water ice in the interior, as well as C- and P- class asteroids, the latter darker and bearing more similarities to Kuiper Belt objects than main belt asteroids. All are thought to be rich in volatiles. Our explorations here should offer insights into primordial planet-building materials in the early Solar System.