Peanut-Shaped Asteroid Reveals Its 150-Million-Year Origin

Picture a charcoal-grey rock about as long as two Central Parks laid end to end, drifting through the main asteroid belt with a wonky outline like a peanut shell. On April 20, 2025, NASA's Lucy spacecraft swept past it at roughly 600 miles (about 960 km) and sent home pictures sharp enough to read its life story. That story turned out to be 150 million years long, written in craters and a strangely pinched waist.

The object is asteroid Donaldjohanson, and the close-up did something rare in planetary science: it answered an old question and immediately raised stranger new ones.

A peanut with an impossible waist

From Earth, astronomers already suspected something odd. The asteroid's brightness swung wildly over a roughly 10-day cycle, the kind of flicker you get from a long, lumpy body turning in sunlight. But nobody could see the shape. Lucy could.

The images revealed a contact binary — two separate lobes fused together at a narrow neck. That part the team half-expected. What stopped them was the neck itself. Instead of a simple pinch, it looked layered and stepped, which one scientist likened to two nested ice cream cones jammed base to base. It's the sort of structure you don't get from a quick crunch. It hints at fragments that drifted together gently, under their own faint gravity, and stacked rather than splattered.

The whole rock is modest by asteroid standards, around 5 miles (8 km) at its longest. But the geometry is what makes scientists lean in. A shape like this is a frozen record of how it came to be.

The 150-million-year clue hiding in the craters

Here is the part worth texting a friend. Scientists figured out the asteroid's age by counting its scars.

Every pockmark on an airless world is a logged hit. The longer a surface sits exposed, the more craters it collects, and the size distribution acts like rings on a tree. When the Lucy team tallied the craters on Donaldjohanson, the math pointed to an age of roughly 150 to 155 million years. In other words, this rock took its current form back when long-necked sauropods were still lumbering across a much warmer Earth.

That number isn't a lonely guess. Donaldjohanson belongs to the Erigone family, a sprawling clan of nearly 2,000 carbonaceous asteroids that all trace back to a single parent body smashed apart in roughly the same era. The family's estimated birth and the asteroid's crater age land in the same window. Two independent clocks, one answer. That agreement is what lets researchers say the mystery of where this thing came from is, finally, mostly solved: it is shrapnel from an ancient demolition, slowly reassembled.

It tumbles, and it used to spin much faster

Most asteroids spin in a tidy way, like a thrown football. Donaldjohanson doesn't. Lucy caught it in a slow, awkward tumble — turning end over end about once every 10.5 days, while also rocking back and forth around its long axis on a separate roughly 26.5-day rhythm. It is, in plain terms, wobbling as it goes.

Even odder, this lazy spin appears to be a recent development. The evidence suggests the asteroid once rotated at least ten times faster and only slowed to today's crawl within the last 20 to 60 million years. The brake is sunlight itself. Tiny pushes from absorbed and re-radiated solar heat, acting over millions of years, can speed a small body up or grind it down. On Donaldjohanson, that whisper-soft force quietly rewrote how the asteroid turns.

A brief, ancient splash of water

Then there's the chemistry. Lucy's instruments picked up iron-rich clay minerals on the surface — and clays are a tell. They generally form when rock meets liquid water. So at some point in its deep past, this dry, frozen lump was wet, at least fleetingly.

The signal here is fainter than what missions found on asteroids Bennu and Ryugu, suggesting Donaldjohanson's encounter with water was shorter or less thorough. But it adds to a growing theme in solar-system science: water, and the watery chemistry that life leans on, was sloshing around the asteroid belt far earlier and more widely than the bone-dry textbook picture once allowed.

Why a small grey rock matters

It's fair to ask why a 5-mile pebble between Mars and Jupiter deserves the attention. A few reasons:

• It's a time capsule. Carbonaceous asteroids like this preserve some of the oldest, least-altered material from the solar system's formation. Reading one is closer to archaeology than astronomy.
• It rehearses the big show. Donaldjohanson was essentially a flight test. Lucy's cameras, tracking and timing all had to nail a fast flyby of a small, irregular target — practice for the harder encounters ahead.
• It connects to us. The asteroid is named for Donald Johanson, the paleoanthropologist who co-discovered the Lucy fossil in Ethiopia in 1974. The spacecraft borrowed that fossil's name on purpose. Just as those bones reshaped the human origin story, the mission aims to dig into the origin story of the planets. As one Lucy scientist put it, every subtle difference is another clue to that story.

That framing isn't marketing. The same collisions that build and break asteroids also delivered water and carbon-rich material to the early Earth. Studying the wreckage is, in a roundabout way, studying our own address.

The warm-up act before the main event

Donaldjohanson is only the second asteroid Lucy has visited. The first, in November 2023, was tiny Dinkinesh, which sprang its own surprise — a small moon, later named Selam, that itself turned out to be a contact binary. Two for two on weird.

Both were appetizers. Lucy launched in October 2021 on a roughly 12-year, billion-mile loop to reach its real prize: the Jupiter Trojans, two vast swarms of asteroids trapped in gravitationally stable points ahead of and behind Jupiter. These are pristine leftovers from the planet-building era, and no spacecraft has ever seen one up close. Lucy is slated to examine a record string of them, with its first Trojan flyby — past an asteroid called Eurybates — targeted for August 12, 2027.

If a humble peanut-shaped rock can hold a 150-million-year secret and a splash of long-gone water, the question almost asks itself. What are the Trojans, untouched since the dawn of the solar system, still keeping to themselves?