🪐 A Wobbling “Peanut” Asteroid May Still Carry Traces of Ancient Water

NASA’s Lucy mission has revealed one of the strangest small worlds ever seen up close: asteroid Donaldjohanson — a young, peanut-shaped rock in the main asteroid belt that tumbles through space and still preserves chemical hints of liquid water from its distant past.

Lucy flew past Donaldjohanson on April 20, 2025, at about 30,000 mph, coming within just 650 miles of the asteroid. The encounter was meant partly as a rehearsal before Lucy reaches Jupiter’s Trojan asteroids in 2027. Instead, it became a science story of its own.

Donaldjohanson does not rotate like a simple spinning rock. Data from Lucy show that it tumbles end-over-end once every 10.5 Earth days, while also wobbling around its long axis every 26.5 days — more like an unstable top than a normal asteroid.

Its shape is just as unusual. Donaldjohanson is a contact binary: two lobes joined by a narrow neck, giving it a cosmic peanut-like form. Scientists think it formed about 155 million years ago, when fragments from a violent collision gently came back together under their own gravity.

Since then, sunlight has been slowly reshaping it. Through the YORP effect — a tiny torque caused when sun-warmed surfaces radiate heat back into space — Donaldjohanson’s spin appears to have slowed by at least a factor of 10 over the last 20–60 million years. As the rotation changed, loose material likely slid down its slopes, softening craters and reshaping the surface.

But the most intriguing clue came from Lucy’s infrared data: iron-rich clay minerals on the surface. These minerals form in the presence of liquid water, meaning Donaldjohanson’s parent body once experienced aqueous alteration. But unlike Bennu and Ryugu, which contain magnesium-rich clays suggesting longer exposure to water, Donaldjohanson’s chemistry points to a much shorter episode.

🔹 Donaldjohanson is a bilobed “contact binary” asteroid
🔹 It tumbles on two axes, with rotation periods of 10.5 and 26.5 days
🔹 Its current body likely formed around 155 million years ago
🔹 Sunlight gradually slowed its spin through the YORP effect
🔹 Iron-rich clays suggest liquid water was present — but only briefly
🔹 The flyby was also a successful rehearsal for Lucy’s Trojan asteroid encounters, beginning with Eurybates in August 2027

Important caveat: this was a fast flyby, not an orbital mission or a sample return. Lucy measured the surface remotely; the asteroid’s interior remains unknown.

Still, Donaldjohanson matters because it gives scientists a rare comparison point. Bennu and Ryugu are near-Earth asteroids with long migration histories. Donaldjohanson is a much younger main-belt object that stayed closer to its birthplace. Its strange shape, unstable spin, and brief water history offer a fresh clue to how small bodies evolved — and how water-rich material may have moved through the early Solar System.

📄 Sources: https://www.science.org/doi/10.1126/science.aec0503

#NASA #LucyMission #Asteroid #PlanetaryScience #SolarSystem