NASA's Lucy Finds Asteroid Donaldjohanson Wobbles on Two Axes and Holds Traces of Ancient Water

NASA's Lucy spacecraft has delivered its most detailed asteroid portrait yet, and the picture is stranger than models predicted.

A team led by Southwest Research Institute deputy principal investigator Simone Marchi published findings June 18 in the journal Science showing that the main-belt asteroid Donaldjohanson doesn't spin cleanly around a single axis. According to the EurekAlert release from SwRI, the asteroid turns on two axes simultaneously, rotating end-over-end once every 10.5 Earth days while also wobbling around its horizontal axis once every 26.5 days. That kind of non-principal axis rotation is unusual and tells scientists something about the forces that have acted on the rock over millions of years.

The spacecraft flew within roughly 961 kilometers of the asteroid on April 20, 2025, as reported in the paper reviewed at Science.org. That close pass let Lucy's instruments reconstruct the asteroid's three-dimensional shape and collect spectral data. What came back confirmed and extended Earth-based predictions: Donaldjohanson is a contact binary, two heavily cratered lobes connected by a narrower neck, with overall dimensions of roughly 8.8 by 4.4 by 3.1 kilometers.

The wobble almost certainly has a slow, subtle cause. The team attributes the altered rotation to the Yarkovsky-O'Keefe-Radzievskii-Paddack effect, a radiation pressure process in which sunlight absorbed and re-emitted by an irregular surface gradually torques the object's spin over millions of years. That mechanism has been documented on other asteroids, including Bennu and Ryugu, but Donaldjohanson's tumbling is a particularly clean illustration of how far it can go.

The surface chemistry adds another layer. According to the NASA Scientific Visualization Studio summary of the observations, Lucy's infrared spectrometer found evidence of iron-rich clays on the asteroid's surface. Clay minerals generally require liquid water to form, which means the parent body from which Donaldjohanson broke off was once wet. According to a ScienceDaily summary of the findings, the asteroid formed roughly 155 million years ago when fragments from a violent collision gradually came together, and since then traces of ancient water have remained preserved in its rocky surface.

The parent body breakup date comes from orbital and compositional analysis tying Donaldjohanson to the Erigone asteroid family. The Science paper puts that collision at somewhere between 115 million and 225 million years ago, with 155 million years as the most likely figure. That's consistent with the water-bearing mineral signature: the parent object would have needed to be large enough to retain internal heat and sustain liquid water before it was destroyed.

One methodological note worth flagging: Lucy captured Donaldjohanson only on its inbound leg, and observations were cut off 31 seconds before closest approach to keep instruments from pointing at the Sun, as described in the Science paper. That limits the coverage geometry, and some surface regions remain uncharacterized. The team is treating this flyby as a dress rehearsal, not a final account.

That framing matters because Donaldjohanson is not Lucy's primary science target. The mission is heading for the Jupiter Trojan asteroids, a population of ancient objects that share Jupiter's orbit and are thought to preserve material from the solar system's earliest period. Lucy's first Trojan encounter, with asteroid Eurybates, is scheduled for August 2027. The Donaldjohanson data will give the team a calibration baseline and, as Marchi noted in the SwRI release, a comparison point when the Trojan results come in. Every difference between Donaldjohanson and the Trojans will be data.

For now, what the flyby shows is that a five-mile-long rock born from a collision 155 million years ago can still hold a chemical record of water and a rotation shaped by nothing more than the steady push of sunlight. That's not a trivial result. It's a reminder that solar system history gets written into objects we haven't looked at closely yet.

Sources cited:
- Science (Marchi et al., June 18, 2026) (https://www.science.org/doi/10.1126/science.aec0503)
- SwRI / EurekAlert press release, June 18, 2026 (https://www.eurekalert.org/news-releases/1132250)
- NASA Scientific Visualization Studio, Lucy Observations of Asteroid Donaldjohanson (https://svs.gsfc.nasa.gov/5628)
- ScienceDaily summary, June 24, 2026 (https://www.sciencedaily.com/releases/2026/06/260624025455.htm)