High above the main asteroid belt, NASA’s Lucy spacecraft raced past a strange object last April. The encounter lasted minutes. Yet the data streaming back has upended assumptions about how some of the solar system’s oddest rocks came to be.
On April 20, 2025, Lucy closed to within 600 miles of asteroid (52246) Donaldjohanson. Traveling at 30,000 miles per hour, the probe snapped images with its Long-Range Reconnaissance Imager. What emerged was a five-mile-long body shaped like a lumpy peanut. Two lobes. A narrow neck. Heavily cratered. Larger than pre-flyby models predicted. Gizmodo first detailed the raw visuals hours after downlink.
But the real story lay beneath the surface. Scientists now know Donaldjohanson is a remnant of a violent collision 155 million years ago. A parent body roughly 50 miles across slammed into a 12-mile intruder. The smash-up shattered the larger asteroid. Fragments slowly drew back together under their own gravity. The peanut shape formed as two main clumps gently merged. Not a single solid rock. A contact binary born from chaos.
That timeline makes Donaldjohanson one of the youngest major asteroids visited by spacecraft. Most primitives in the belt date back billions of years. This one carries scars from a relatively recent event in cosmic terms. And the evidence goes deeper.
New Analysis Reveals Tumbling Spin and Fleeting Water
Images alone told part of the tale. Full analysis, published June 18, 2026, in Science, adds rotation data, mineral signatures, and long-term evolution. The asteroid does not spin cleanly. It tumbles. End-over-end rotation every 10.5 days. A back-and-forth wobble around its long axis every 26.5 days. Non-principal axis motion. The kind that produces complex light curves visible from Earth but only fully explained up close.
“It’s helpful for scientists to compare Donaldjohanson with asteroids like Bennu and Ryugu, which are seemingly similar asteroids, because every subtle difference is another clue to our origin story,” said Simone Marchi, Lucy deputy principal investigator at Southwest Research Institute. He led the study. (NASA Science)
The wobble traces back to the YORP effect—sunlight heating one side, creating tiny torques that alter spin over millions of years. Donaldjohanson likely rotated ten times faster right after formation. Slowing occurred in the past 20 to 60 million years. That slowdown let material shift. Craters on the neck appear worn or erased. Smaller impacts may have triggered seismic shaking that smoothed features. A more recent collision around 40 million years ago could explain some of the erasure, according to Scientific American.
Surface ridges and craters tell a parallel story. Many look softened. Not from erosion in the usual sense—there is no air or water now—but from the changing spin and resulting landslides or reshaping. The neck shows fewer small craters than the lobes. Gravitational slopes and material movement wiped some away. Hal Levison, Lucy principal investigator, called the geology “strikingly complicated.” (Smithsonian Magazine)
Then there is the water. Or what remains of it. Spectral data reveal iron-rich clay minerals—phyllosilicates. These form only when liquid water alters rock. The parent body evidently carried water ice or hydrated minerals. Brief exposure to liquid water occurred early in the object’s history, before or soon after the big collision. Not long enough for magnesium to fully replace the iron. A fleeting aqueous chapter. Then it froze out or boiled away. Signs of that water survived the impact and the subsequent 155 million years in the harsh vacuum.
Compare that to Bennu and Ryugu. Those rubble piles also show hydrated minerals but formed one to two billion years ago and later migrated inward. Donaldjohanson stayed in the main belt. Its youth and different spin history offer contrast. Every difference sharpens the picture of how primitive material evolved. “Once we start learning more about the Trojans, a completely different population of space rocks with very different histories, our understanding of solar system formation is destined to be challenged,” Marchi added.
Lucy’s flyby served as dress rehearsal. The mission, launched in 2021, targets Jupiter’s Trojan asteroids beginning in 2027. Donaldjohanson, named for the discoverer of the Lucy fossil, provided the first high-resolution look at an Erigone-family member. Nearly 1,800 objects trace back to that same ancient breakup. One flyby constrains the whole family’s story.
But questions remain. Exactly how gently did the fragments coalesce? Did the neck form from a slow merger or later reshaping? How common are such young contact binaries? The Science paper tightens models. Yet it also highlights gaps. Seismic effects from later impacts. The precise role of sunlight in sculpting small bodies over time. These processes operate across the asteroid population.
And. The data keeps coming. More images and measurements continue downlink. Three-dimensional models will follow. Scientists will compare them against ground-based observations and simulations of the original collision. Each new detail refines the narrative of solar system assembly. Violent impacts. Gentle reaccumulation. Slow radiative forces. Brief water. All written on a single five-mile peanut tumbling through space.
The Lucy team sees this as more than one asteroid’s biography. It is a window into the conditions that delivered water and organics to the early Earth. The same family of processes that built the planets. Donaldjohanson does not orbit close to us. Yet its story touches ours. A reminder that even the weirdest rocks carry the solar system’s memory.
So the peanut spins on. Wobbling. Cratered. Marked by fire, water, and sunlight. And with each analysis, it yields another piece of the puzzle.
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