NASA's Lucy probe scores a threefer as it flies by first target in 12-year mission

Scientists shocked to find Dinkinesh is orbited by two touching asteroids

Dinkinesh, the first asteroid encountered by NASA's Lucy spacecraft, is being orbited by a smaller binary pair, and is the first object of its kind to be found by astronomers.

Initial images of Dinkinesh beamed back by the probe revealed a smaller space rock peeking out from behind it. Scientists were pleasantly surprised to reclassify the 790-meter-wide asteroid as a binary system – but they were wrong. 


Dinkinesh (left) and the contact binary (right). Credit: NASA/Goddard/SwRI/Johns Hopkins APL ... Click to enlarge

Upon closer inspection, they have found that that Dinkinesh is orbited by not one but two smaller asteroids that have formed a peanut-shaped body. The pair are bound so closely together that their surfaces are touching, to form a type of system known as a contact binary. 

"Contact binaries seem to be fairly common in the solar system," John Spencer, Lucy's deputy project scientist and a researcher at the Southwest Research Institute (SwRI), explained in a statement this week. 

"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," he added.

Launched on October 16, Lucy will study Trojan asteroids. These mysterious rocky objects share Jupiter's orbit, and are scattered across two different clusters.

They are fragments of material left over from the creation of the solar system, and NASA believes they hold vital clues to how the largest planets formed more than 4.5 billion years ago. Dinkinesh was billed as the first space rock Lucy would pass over its 12-year journey to visit ten different asteroids – three in the main belt and seven Trojans.

But the surprise reveal uncovered during its first pit stop will mean that Lucy has actually visited three asteroids already, and will go on to study 11 different objects in total (if we're counting the contact binary as a single object) if the mission goes to plan. 

"It's truly marvelous when nature surprises us with a new puzzle," enthused Tom Statler, Lucy's program scientist and researcher working at NASA. "Great science pushes us to ask questions that we never knew we needed to ask."

Astronomers aren't quite sure how the contact binary, which is estimated to be 220 meters across, formed to end up orbiting Dinkinesh like a double moon.

"It is puzzling, to say the least," said Hal Levison, principal investigator for Lucy and a planetary scientist at the SWRI. "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."

Both Lucy and Dinkinesh are named for a fossilized hominid female found in Ethiopia in 1974 of the species Australopithecus afarensis, dating back some 3.2 million years. Dink'inesh is the Amharic name given to the individual, whom the excavation team who discovered her nicknamed Lucy.

As for Lucy the spacecraft, it is currently zooming back towards Earth to perform a gravity assist maneuver in December 2024. That will send it flying back through the main asteroid belt to reach its next target: 52246 Donaldjohanson, an asteroid named after the American paleontologist, Donald Johanson, who discovered the Lucy's fossilized remains back in 1974.

After that, the Lucy spacecraft will continue to venture deeper into the solar system to get closer to the Trojan asteroids. ®

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