After the Southwest Research Institute-led Lucy mission flew past the asteroid Dinkinesh, the team discovered that it is even more "marvelous" as its newly discovered satellite is now shown to be a double-lobed moonlet. As NASA's Lucy spacecraft continued to return data acquired during its first asteroid encounter on Nov. 1, 2023, the team discovered that Dinkinesh's surprise satellite is itself a contact binary, made of two smaller objects touching each other.
In the first image of Dinkinesh and its satellite taken at closest approach, the two lobes of the contact binary lined up, one behind the other, appearing to be one body from Lucy's point of view. When the team downlinked additional images captured after the closest encounter, the data revealed that Dinkenesh has a double moonlet.
"Contact binaries seem to be fairly common in the solar system," said John Spencer, Lucy deputy project scientist, of the Boulder, Colorado, branch of the San Antonio-based SwRI. "We haven't seen many up close, and we've never seen one orbiting another asteroid. We'd been puzzling over odd variations in Dinkinesh's brightness that we saw on approach, which gave us a hint that Dinkinesh might have a moon of some sort, but we never suspected anything so bizarre!"
Lucy's primary goal is to survey the never-before-visited Jupiter Trojan asteroids. The mission team added the first encounter with this small, main-belt asteroid in January 2023. The Dinkinesh encounter would serve as an inflight test of the spacecraft's novel terminal tracking system, which keeps tabs on the target as the spacecraft buzzes past. The excellent performance of that system at Dinkinesh allowed the team to capture multiple perspectives on the system, characterizing the asteroids' shapes and making this unexpected discovery.
"It is puzzling, to say the least," said SwRI's Hal Levison, the Lucy principal investigator. "I would have never expected a system that looks like this. In particular, I don't understand why the two components of the satellite have similar sizes. This is going to be fun for the scientific community to figure out."
Lucy took the second image about six minutes after closest approach from approximately 1,010 miles (1,630 km) away. The spacecraft traveled around 960 miles (1,500 km) between the two released images.
"It's truly marvelous when nature surprises us with a new puzzle," said Tom Statler, Lucy program scientist from NASA headquarters in Washington. "Great science pushes us to ask questions that we never knew we needed to ask."
The team is continuing to downlink and process the remainder of the encounter data. Dinkinesh and its satellite pair are the first asteroids Lucy has explored. Over Lucy's 12-year journey, that tally will include eight target asteroids with three known satellites among them, including the newly discovered contact binary.
After skimming the inner edge of the main asteroid belt, Lucy is now heading back toward Earth for a gravity assist in December 2024. That close flyby will propel the spacecraft back through the main asteroid belt, where it will observe the asteroid Donaldjohanson in 2025 and continue its journey to reach the Trojan asteroids in 2027.
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