NASA's Trojan asteroid-bound Lucy mission has been full of surprises recently and now it's just revealed one more. Asteroid DInkinesh was not originally in the mission's plans but it was close enough to pass by and it would be an excellent opportunity to test Lucy's scientific instruments before reaching Jupiter. Well, the flyby has exceeded all expectations by revealing that Dinkinesh is not just a single asteroid but three-in-one, and has the first known contact binary to orbit another asteroid.
The first surprise was revealed last week when the team on the ground downloaded the first flyby images and discovered the presence of a tiny moon around Dinkinesh. But the surprises kept coming. This unexpected moon turned out to not be a single asteroid but two. The moon is a contact binary made of two smaller objects touching each other, much like Arrokoth.
Despite contact binaries being fairly common in the Solar System, “We haven’t seen many up-close, and we’ve never seen one orbiting another asteroid," said John Spencer, Lucy deputy project scientist, in a statement.
"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!”
The first downlinked image revealing the little moon peering from behind the edge of the larger body was taken at Lucy's closest approach to Dinkinesh. However, due to a fortuitous alignment, its binary nature was hidden from the team. It was only as more data and images from the flyby were downloaded that the team spotted the second moonlet and could see properly what the system looked like.
“It is puzzling, to say the least,” said Hal Levison, principal investigator for Lucy. “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's science mission is to find "fossils" of planet formation, studying asteroids in the main belt and the so-called Trojans around Jupiter. It is named after the unfortunate human ancestor that revolutionized paleontology. Dinkinesh, which is the Ethiopian name for the Lucy fossil, translates to "you are marvelous".
“It’s truly marvelous when nature surprises us with a new puzzle,” said Tom Statler, Lucy program scientist from NASA HQ. “Great science pushes us to ask questions that we never knew we needed to ask.”
Lucy is currently on its way back towards Earth to use our planet as a slingshot gravity boost, which will give it the speed boost needed to fly by asteroid Donaldjohanson in 2025, and then on to the Trojan asteroids in 2027.