NASA probe reveals a tumbling, peanut-shaped asteroid

Brain City Berlin

Even small asteroids can lead surprisingly complex lives. That is the finding of new research in which the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) in Brain City Berlin played a central role. NASA's Lucy spacecraft visited the asteroid Donaldjohanson in April 2025 and uncovered a remarkably layered history.

The journey of the Lucy spacecraft

NASA's Lucy spacecraft has been travelling through space since 2021, gathering data in the vicinity of Jupiter. On 20 April 2025, it captured images and data of the asteroid Donaldjohanson from a distance of just over a thousand kilometres, revealing something unusual: rather than rotating stably around a fixed axis like most celestial bodies, the asteroid tumbles simultaneously around two axes. On 18 June 2026, the scientific team published their findings in the journal Science. Dr Stefano Mottola and Frank Preusker from the DLR Institute of Planetary Research contributed calculations of the rotation parameters, a three-dimensional shape model and the gravitational field.

The encounter with the asteroid was planned as a dress rehearsal for the main phase of the Lucy mission. From August 2027 onwards, Lucy will explore the so-called Trojan asteroids on Jupiter's orbital path, well-preserved rocky bodies from the early history of our solar system. The instruments and procedures are working as expected, and the team gained the rare opportunity to study a previously unexplored asteroid at close range.

Tumbling in peanut form

There were already indications before the flyby that Donaldjohanson was tumbling. It was only the image data from the Lucy spacecraft that made it possible to understand this rotation in precise detail. The asteroid is 8.8 kilometres long and up to 3.5 kilometres wide. It rotates around its shortest axis every 26.4 Earth days, while that axis itself moves around a higher-order axis in space every 7.5 days. This creates two overlapping periods and a total rotation of more than 26 days, longer than originally assumed.

The peanut-shaped body is a so-called contact binary: it formed from fragments that merged after a violent collision around 155 million years ago. At the time of its formation, it was probably rotating considerably faster. Over time it slowed down, loose rocky material slid down its slopes and left behind the many craters visible today.

Traces of ancient water

As Lucy flew past the asteroid at nearly 50,000 kilometres per hour, its spectroscopic sensors detected signs of iron-bearing phyllosilicates on the surface: clay minerals of the kind also found on Earth. These minerals can only form in the presence of liquid water. The fact that iron in the clay minerals has not been replaced by magnesium suggests that water was present for only a relatively short period.

"Comparing Donaldjohanson with apparently similar asteroids such as Bennu and Ryugu is enormously helpful for asteroid research, because every subtle difference is another clue to our own origins," said Simone Marchi, deputy head of the Lucy science team and lead author of the Science study. With the insights gained from Donaldjohanson, the mission is well prepared for its next step: the exploration of the Trojan asteroids, which could fundamentally challenge our understanding of how the solar system formed.

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