NASA's Lucy Reveals Wobbling Peanut-Shaped Asteroid with Signs of Ancient Water

When NASA's Lucy spacecraft flew past the small main-belt asteroid Donaldjohanson on April 20, 2025, mission scientists expected a useful rehearsal for the Jupiter Trojan encounters ahead. What they got was a compact world with a dramatic story of its own — a peanut-shaped, tumbling body that once held liquid water and is far younger than any other asteroid visited up close.

The findings, published on June 18, 2026, in the journal Science (DOI: 10.1126/science.aec0503), paint a detailed portrait of an asteroid that formed from the debris of a catastrophic collision and has been steadily slowing down and shaking itself apart ever since .

The Smoking Gun for Ancient Liquid Water

The most striking discovery came not from the visible-light images but from the infrared. Lucy's LEISA spectrometer, part of the L'Ralph instrument, detected iron-rich clay minerals (iron-bearing phyllosilicates) on the asteroid's surface . These clays are chemically similar to those found in carbon-rich meteorites such as QUE 97990 — and crucially, they can only form through the chemical alteration of rock by liquid water .

This is the first direct evidence that Donaldjohanson's parent body once contained liquid water. The alteration appears to have halted at an early stage, likely because the parent body lacked enough water or heat to drive the process further . The asteroid itself is a fragment from that larger, water-rich parent body, shattered in a collision long ago.

A Youngster in an Ancient Belt

Donaldjohanson is a member of the Erigone asteroid family, a group of objects created when a larger, carbon- and water-rich parent body was catastrophically disrupted . That collision happened about 155 million years ago — making Donaldjohanson a teenager compared to well-studied asteroids like Bennu and Ryugu, which are 1–2 billion years old .

Classified as a C-type (carbonaceous) asteroid, it is rich in carbon and water-bearing minerals, consistent with the composition of the Erigone family . Unlike Bennu and Ryugu, both of which migrated into near-Earth orbits, Donaldjohanson has stayed put in the main asteroid belt since it formed .

A Peanut That Wobbles

High-resolution images from Lucy's L'LORRI imager revealed that Donaldjohanson is not a simple, round rock. It has a bilobate, or "peanut," shape — two cratered lobes connected by a relatively smooth neck . This suggests that two fragments from the original parent-body collision gently coalesced under their own gravity to form the asteroid we see today.

But the shape is only half the story. Donaldjohanson does not spin like a typical asteroid. Instead, it undergoes a complex tumbling rotation: it rotates end-over-end once every 10.5 Earth days while simultaneously wobbling around its horizontal axis every 26.5 days . This is a non-principal-axis rotation — think of a wobbly top that has almost lost its spin.

This tumbling was likely induced gradually by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, where sunlight exerts tiny torques on an asteroid's irregular shape over millions of years . The asteroid probably rotated at least 10 times faster when it first formed and has since slowed to its current lethargic spin over the last 20–60 million years .

Another surprise: craters smaller than 0.4 km appear to have been preferentially erased from the surface, possibly by seismic shaking from a more recent impact event . The asteroid measures about 0.8 km (half a mile) in diameter .

A Dress Rehearsal for the Trojans

The Donaldjohanson flyby was never primarily about Donaldjohanson itself. It was explicitly designed as a dress rehearsal for Lucy's primary targets: the never-before-explored Jupiter Trojan asteroids . The encounter allowed the team to:

• Test all flyby instruments and procedures under real operating conditions before reaching the Trojans
• Practice the sequence of approach, closest-approach imaging, and post-flyby data transmission
• Demonstrate the spacecraft's ability to characterize a small, dark, main-belt asteroid, building confidence for the more distant and scientifically critical Trojan encounters later in Lucy's 12-year mission

As SwRI's Dr. Simone Marchi put it: "This encounter gave us an opportunity to test our instruments and our procedures to make sure we are ready when we get to Jupiter's Trojans."

In that sense, Donaldjohanson delivered everything NASA could have asked for — a scientifically rich target that also served as a perfect proving ground for the mission's most ambitious goals.