On April 20, 2025, NASA’s Lucy mission will execute a close flyby of (152830) Donaldjohanson, a main belt asteroid thought to be a fragment of a larger body shattered in a collision over 150 million years ago. This encounter marks a key milestone in Lucy’s long journey toward the Jupiter Trojans, offering an early opportunity to test its instruments while capturing valuable data on a remnant of the early solar system.
Named after the paleoanthropologist co-discoverer of the Lucy hominin fossil, the asteroid holds symbolic significance for a mission that aims to trace the “fossils” of planetary formation. The upcoming observations may help constrain models of asteroid evolution, particularly in the context of collisional histories and surface regolith development. As detailed in a recent study published in The Planetary Science Journal, subtle spectral signatures and morphology seen in similar asteroids suggest long-term surface processing driven by micrometeorite impacts and thermal fracturing—factors that Donaldjohanson may also exhibit during this close inspection.
A Peculiar Asteroid With A Mysterious Past
Discovered in 1984, Donaldjohanson is named after the paleontologist who found the famous Lucy fossil, which reshaped our understanding of human evolution. Now, the asteroid is set to play a similar role in planetary science.
Recent studies suggest that Donaldjohanson originated from a larger asteroid that broke apart, forming the Erigone collisional asteroid family. Since then, its orbit and rotation have changed significantly, likely influenced by thermal torques that slowed its spin over time. Scientists believe its elongated shape and slow rotation make it a particularly interesting object for study.
Dr. Simone Marchi, deputy principal investigator of the Lucy mission, notes that this asteroid stands out among those observed from Earth.“Understanding the formation of Donaldjohanson could help explain its peculiarities,” he said.
A Key Step Before Reaching Jupiter’s Trojan Asteroids
The Lucy spacecraft, launched on October 16, 2021, is on a 12-year journey to explore 11 asteroids, including the Trojan asteroids that share Jupiter’s orbit. These ancient celestial bodies are thought to be leftovers from the early solar system.
Before reaching the Trojans, Lucy’s flyby of Donaldjohanson serves as both a scientific opportunity and a test for the spacecraft’s navigation system. Scientists hope that by analyzing the asteroid’s surface geology, shape, and cratering history, they can validate existing models and prepare for the more challenging encounters ahead.
Dr. Hal Levison, principal investigator of the mission, emphasizes the importance of this step: “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”.
Possible Links To Other Asteroids
Donaldjohanson’s location in the inner main belt places it near the regions where near-Earth asteroids Bennu and Ryugu originated. Both objects were visited by NASA’s OSIRIS-REx and JAXA’s Hayabusa2 missions, which returned samples that provided groundbreaking insights into the building blocks of planets.
However, Donaldjohanson appears to be significantly different from these asteroids. While Bennu and Ryugu are known for their loose, rubble-pile structures and carbon-rich compositions, Donaldjohanson is made mostly of silicate rocks, possibly containing clays and organic materials. The upcoming Lucy flyby will help scientists determine how these objects might be related, if at all.
What Scientists Hope To Discover?
So far, Earth-based observations and theoretical models have provided only limited information about Donaldjohanson’s exact shape, composition, and structure.
The Lucy spacecraft, equipped with high-resolution imaging and spectroscopic instruments, will provide the first close-up data on the asteroid.
Dr. Keith Noll, Lucy project scientist at NASA’s Goddard Space Flight Center, highlights the importance of this encounter: “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.”
