This artist’s concept compares the approximate size of Lucy’s next asteroid target, Donaldjohanson, to the smallest main belt asteroids previously visited by spacecraft — Dinkinesh, visited by Lucy in November 2023, and Steins — as well as two recently explored near-Earth asteroids, Bennu and Ryugu. Lucy, an SwRI-led NASA mission, plans to visit 11 asteroids in 12 years, culminating in the first encounters with Jupiter’s Trojan asteroids. Credit: SwRI/ ESA/ OSIRIS/ NASA/ Goddard/ Johns Hopkins APL/ NOIRLab/ University of Arizona/ JAXA/ University of Tokyo & Collaborators.
New modelling has assessed the age of asteroid Donaldjohanson, which NASA’s Lucy mission will encounter on April 20, 2025.
New Southwest Research Institute-led modelling 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.
SwRI’s Dr Simone Marchi, deputy principal investigator of the SwRI-led Lucy mission and lead author of research, said: “Based on ground-based observations, Donaldjohanson appears to be a peculiar object. Understanding the formation of Donaldjohanson could help explain its peculiarities.”
Co-author David Vokrouhlický, a professor at Charles University, Prague, added: “Data indicates that it could be quite elongated and a slow rotator, possibly due to thermal torques that have slowed its spin over time.”
Erigone asteroid family
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.
Marchi said: “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.”
Donaldjohanson is named for the palaeontologist who discovered Lucy, the fossilised 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 revolutionise 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.
SwRI’s Dr Hal Levison, who is the principal investigator of the mission, said: “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.
“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.”
The research is published in the Planetary Science Journal
