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Model suggests asteroid Donaldjohanson formed 150 million years ago

New Southwest Research Institute-led modeling indicates the main belt asteroid (52246) Donaldjohanson may have formed about 150 million years ago when a larger parent asteroid broke apart; its orbit and spin properties have undergone significant evolution since.

When NASA's Lucy spacecraft flies by this approximately three-mile-wide space rock on April 20, 2025, the data collected could provide independent insights on such processes based on its shape, surface geology and cratering history.

"Based on ground-based observations, Donaldjohanson appears to be a peculiar object," said SwRI's Dr. Simone Marchi, deputy principal investigator of the SwRI-led Lucy mission and lead author of the paper, "A pre-flyby view on the origin of asteroid Donaldjohanson, a target of the NASA Lucy mission," in The Planetary Science Journal.

"Understanding the formation of Donaldjohanson could help explain its peculiarities."

"Data indicates that it could be quite elongated and a slow rotator, possibly due to thermal torques that have slowed its spin over time," added David Vokrouhlický, a professor at the Charles University, Prague, and co-author of the research.

Lucy's target is a common type of asteroid, composed of silicate rocks and perhaps containing clays and organic matter. The new paper indicates that Donaldjohanson is likely a member of the Erigone collisional asteroid family, a group of asteroids on similar orbits that was created when a larger parent asteroid broke apart.

The family originated in the inner main belt not very far from the source regions of near-Earth asteroids (101955) Bennu and (162173) Ryugu, recently visited respectively by NASA's OSIRIS-REx and JAXA's Hayabusa2 missions.

"We can hardly wait for the flyby because, as of now, Donaldjohanson's characteristics appear very distinct from Bennu and Ryugu. Yet, we may uncover unexpected connections," Marchi said.

Donaldjohanson is named for the paleontologist who discovered Lucy, the fossilized skeleton of an early hominin found in Ethiopia in 1974, which is how the Lucy mission got its name. Just as the Lucy fossil provided unique insights into the origin of humanity, the Lucy mission promises to revolutionize our knowledge of the origin of humanity's home world. Donaldjohanson is the only named asteroid yet to be visited while its namesake is still living.

"Lucy is an ambitious NASA mission, with plans to visit 11 asteroids in its 12-year mission to tour the Trojan asteroids that are located in two swarms leading and trailing Jupiter," said SwRI's Dr. Hal Levison, who is the principal investigator of the mission.

"Encounters with main belt asteroids not only provide a close-up view of those bodies but also allow us to perform engineering tests of the spacecraft's innovative navigation system before the main event to study the Trojans. These relics are effectively fossils of the planet formation process, holding vital clues to deciphering the history of our solar system."