NASA's Lucy mission set its sights on one asteroid. It found two

On Wednesday, NASA's Lucy spacecraft zoomed by its first asteroid target -- and scientists on the mission were shocked to discover that the rock, named Dinkinesh, was actually two rocks. The binary consists of a larger, primary asteroid and a smaller "moon" orbiting around it, as seen in images that Lucy captured of the pair.

"We knew this was going to be the smallest main belt asteroid ever seen up close," Keith Noll, an astronomer and Lucy project scientist at the NASA Goddard Space Flight Center, said in a news release. "The fact that it is two makes it even more exciting."

Lucy's flyby was a pit stop for more ambitious targets: two groups of asteroids called the Trojan swarms. The Trojans, leftover chunks from the outer planets' formation, are locked in stable orbits of the sun along the same path as the planet Jupiter. Lucy will visit nine additional space rocks through 2033, part of NASA's broader effort to glean knowledge about our celestial neighborhood.

"The Trojans are the last big population of objects that we have not yet seen close up," said Thomas Statler, a NASA planetary scientist on the mission. "And Lucy is going to do that for the first time."

NASA named the mission after a skeleton discovered in 1974 in Ethiopia that revolutionised scientists' understanding of human evolution. Similarly, "we're hoping that looking at these fossils of planetary origin will give us insight into the origins of our solar system," Statler said.

Lucy's encounter with Dinkinesh was serendipitous. When the mission launched in 2021, the previously unnamed asteroid was not part of Lucy's space tour. But the mission team found that with a minor adjustment to Lucy's course in May, the spacecraft could pass within 264 miles of the space rock, which was given the Amharic name for the Lucy skeleton, Dinkinesh.

The focus of this encounter wasn't scientific discovery, according to Hal Levison, a planetary scientist at the Southwest Research Institute and the Lucy mission's principal investigator. Instead, he said, it was an in-flight test of Lucy's asteroid tracking system. Minutes before its closest approach, which occurred about 12:55 p.m. Eastern time Wednesday, Lucy "locked on" to Dinkinesh and automatically adjusted itself to keep the rock in its field of view.

Lucy sped past Dinkinesh at 10,000 mph while its instruments captured images of the asteroid's surface and measured the rock's composition and structure. Once finished, Lucy's antenna pivoted back to the eagerly waiting science team on Earth.

Preliminary studies of Lucy's first images of the binary asteroid pair indicate that the bigger rock is about half a mile wide, while its satellite is about 0.15 miles wide.

Amy Mainzer, an astronomer at the University of Arizona who is not involved in the Lucy mission, said that studying Dinkinesh could help explain how asteroids similar in size migrated close to Earth, some near enough to potentially pose a threat to our planet.

But Lucy's goals lie far beyond the vicinity of Earth. After looping around the sun and having a rendezvous with another main belt asteroid -- this one named after Donald Johanson, one of the paleontologists who discovered the Lucy skeleton -- the spacecraft will reach the Trojans in front of Jupiter in 2027. Another solar loop will take it to the swarm of asteroids trailing Jupiter in 2033.

The Trojans are "actually very different from one another," Levison said. "And that's not what we expected when we started studying them." Data that reveals more information about the conditions in which they formed might contain clues supporting a theory that the outer planets first emerged much closer to the sun, and eventually scattered into more stable orbits farther away.

But no matter what secrets the Trojans hold, the mission team expects them to add to the knowledge that space rocks reveal about our cosmic beginnings. "There is no such thing as just another asteroid," Statler said. "Each one is carrying with it a memory of a different part of the history of our solar system."

By piecing together this story, he added, "we get an understanding of where we came from at a molecular level, and how we are coupled to our solar system, and to our universe."