The trojan asteroid (3548) Eurybates, one of the targets of NASA’s upcoming Lucy mission, has a tiny moon.
This image of Eurybates and its moon (circled) was captured by Hubble’s Wide Field Camera 3 (WFC3) on September 14, 2018. Image credit: NASA / ESA / Hubble / Noll et al, arXiv: 2008.01858.
Trojan asteroids follow the same orbit as a planet, but stay either around 60 degrees ahead or 60 degrees behind along the orbit.
Earth has one trojan asteroid, 2010 TK7. Mars hosts at least nine, Uranus has two, and Neptune has 22 trojans. Jupiter has more than one million trojan asteroids larger than 1 km (0.62 miles).
Eurybates is classified as a carbonaceous Jupiter trojan, with a diameter of 63.9 km (39.7 miles).
The asteroid has a rotation period of 8.7 hours and orbits the Sun at a distance of 4.7-5.7 AU (astronomical units) once every 11 years and 10 months.
Discovered on September 19, 1973 by Dutch astronomers Ingrid and Cornelis van Houten, Eurybates is the parent body of the small family with 218 known members.
“Eurybates is the largest member of the only confirmed trojan collisional family — over 200 asteroids all traceable to, and probably fragments from, the same collision,” said Dr. Hal Levison, principal investigator of the Lucy mission and a researcher at the Southwest Research Institute.
Eurybates’ satellite was detected in images obtained with the NASA/ESA Hubble Space Telescope in September 2018 and January 2020.
This disovery makes it the fourth trojan asteroid with a directly observed companion.
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. Image credit: Southwest Research Institute.
“There are only a handful of known trojan asteroids with satellites, and the presence of a satellite is particularly interesting for Eurybates,” said Dr. Thomas Statler, Lucy program scientist at NASA Headquarters in Washington.
Designated S/2018 (3548) 1, the newly-discovered moon is approximately 1.2 km (0.75 miles) in diameter.
“This satellite is more than 6,000 times fainter than Eurybates, implying a diameter around 1 km (0.62 miles),” Dr. Levison said.
“If this estimate proves to be correct, it will be among the smallest asteroids visited.”
The separation of S/2018 (3548) 1 from Eurybates is between 1,700 and 2,300 km (1,056-1,429 miles), consistent with a large range of possible orbits.
“Lucy will fly by Eurybates in August 2027 at a distance of 1,000 km (621 miles) and closer than the projected distance of this satellite,” the astronomer said.
“The discovery of this satellite now, before launch, means that it may be possible to determine the satellite orbit prior to encounter, thus increasing the scientific yield of the mission by enabling complementary observations of this satellite during the flyby.”
The team’s paper was accepted for publication in the Planetary Science Journal.
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K.S. Noll et al. 2020. Detection of a Satellite of the Trojan Asteroid (3548) Eurybates — A Lucy Mission Target. Planet. Sci. J, in press; arXiv: 2008.01858
