NASA's Lucy Cracks a 155-Million-Year Asteroid Mystery

A spacecraft named after a 3.18-million-year-old fossil just flew within 650 miles of a lumpy space rock and, in doing so, read 155 million years of violent history off its surface. On 20 April 2025, NASA's Lucy probe shot past asteroid Donaldjohanson at roughly 30,000 mph and sent home portraits of an object shaped uncannily like a peanut. The full analysis, published this month, turns that odd silhouette into one of the more revealing asteroid stories in years.

The headline is simple and strange: this peanut-shaped asteroid is not a single boulder but two lobes fused at a narrow neck, the wreckage of an ancient smash-up that scientists can now date with surprising confidence. The detail is where it gets genuinely fascinating.

A peanut built from a 155-million-year-old wreck

Donaldjohanson is about 5 miles (8 km) across, drifting in the main asteroid belt between Mars and Jupiter. When Lucy's cameras resolved it, researchers saw a classic bilobate form — two rounded chunks joined at a slim waist, the same basic outline seen in some comets and other small bodies.

The leading explanation is a collision around 155 million years ago, back when dinosaurs walked the Earth. A much larger parent asteroid is thought to have been struck by a rock roughly 12 miles (20 km) wide and shattered. Some of those fragments didn't scatter for good. Pulled by their own faint gravity, two of them crept back together and settled into a soft contact, neck to neck. The peanut is, in effect, a slow-motion reunion of debris.

That origin story matters because it lets scientists treat the asteroid as a kind of frozen crime scene. Its shape, its spin and its chemistry all carry fingerprints from that single event.

The asteroid that can't spin straight

Most asteroids turn tidily around one axis, like a top. Donaldjohanson refuses. Lucy's data show it tumbling on two axes at once — a genuine cosmic wobble.

The numbers are oddly specific. It completes one full rotation roughly every 10.5 Earth days, while also rocking back and forth around its long axis about once every 26.5 days. Picture a poorly thrown rugby ball that never quite settles, stretched across weeks instead of seconds.

It wasn't always so sluggish. The team calculates that the asteroid once spun at least 10 times faster, then gradually braked over the last 20 to 60 million years. The culprit is sunlight. Tiny amounts of heat re-radiated from a rotating, irregular body produce a faint but relentless push — enough, over tens of millions of years, to reshape how a small asteroid turns. As Donaldjohanson slowed, the tug-of-war between gravity and the outward force of its own spin shifted, and loose rubble appears to have slid down its slopes. The peanut, in other words, is still settling.

A brief brush with water

Then there is the chemistry, which adds a quietly remarkable twist. Lucy picked up the signature of iron-rich clay minerals on the surface. Clays don't form in dry rock. They need liquid water, which means this asteroid was wet at some point in its history.

What makes it interesting is the contrast. The asteroids Bennu and Ryugu, both visited by sample-return missions and both thought to be 1 to 2 billion years old, carry magnesium-rich clays that hint at long, sustained contact with water lasting millions of years. Donaldjohanson's iron-rich clays suggest something briefer — a short, sharp encounter with liquid rather than a long soak. Each asteroid, it turns out, had its own private relationship with water in the early solar system.

Why a fossil named the spacecraft

The names here are not a coincidence, and the chain behind them is delightful.

1. In 1974, anthropologist Donald Johanson and his team uncovered a remarkably complete early-human skeleton in Ethiopia. Around the camp that night, a Beatles song was playing on repeat, and the fossil picked up a nickname: Lucy.
2. That fossil, an Australopithecus afarensis specimen about 3.18 million years old, became one of the most famous finds in the study of human origins.
3. NASA named its asteroid-hopping spacecraft Lucy after the fossil, hoping the mission would do for the solar system what the skeleton did for human evolution — reveal deep history.
4. The asteroid it practised on was then named Donaldjohanson, closing the loop back to the man who started it all.

The spacecraft leans into the theme. It launched from Cape Canaveral on 16 October 2021 carrying a plaque with messages and lyrics from figures including Carl Sagan, Martin Luther King Jr. and the Beatles, plus a disc of lab-grown diamonds inside one of its instruments. A fossil, a pop song and a diamond-bearing probe, all sharing one name.

Donaldjohanson was only the warm-up

Here is the part that should make space fans sit up: this jaw-dropping encounter was essentially a dress rehearsal. Donaldjohanson was a convenient target to test Lucy's cameras and tracking before the real work begins.

Lucy's true destination is the Trojan asteroids — two vast swarms of primitive rock that share Jupiter's orbit, one running ahead of the planet and one trailing behind. These bodies are thought to be leftover building blocks from the era when the giant planets formed, and no spacecraft has ever visited them. They are, in a real sense, fossils of the solar system itself.

The schedule reads like an explorer's logbook:

• August 2027 — Lucy reaches its first Trojan, Eurybates, and its tiny moon.
• September 2027 — a flyby of Polymele.
• 2028 — encounters with Leucus and then Orus.

Across its full tour, Lucy is set to study a record-breaking string of asteroids, more individual bodies than any previous mission.

Why this small rock matters

It is easy to shrug at a 5-mile asteroid in a belt full of millions of them. But Donaldjohanson is a reminder that even an unremarkable rock can hold a remarkably detailed diary — a collision date, a slowing spin, a vanished splash of water, all legible to a passing spacecraft.

More than that, it tells us the main event could rewrite textbooks. The Trojans have sat largely untouched since the planets were assembling, which is exactly why researchers expect them to surprise us. The lead scientist on the new study has suggested that as we learn more about that population, our picture of how the solar system came together may have to change.

For now, the peanut has given up its secrets. A rock that formed when dinosaurs roamed, that tumbles instead of spins, and that once held water, was decoded by a probe named for a fossil named for a song. The best part is that the mission has barely started.