- March 09, 2026
- By Georgia Jiang
Binary asteroid systems near Earth are far more dynamic than anyone realized—with main bodies that actively exchange rocks and dust with their moons in gentle, slow-motion collisions that reshape them over millions of years, according to new University of Maryland-led research.
The team’s findings, published Friday in The Planetary Science Journal, have significant implications for understanding asteroids that could one day threaten Earth.
After analyzing images taken by NASA’s Double Asteroid Redirection Test (DART) spacecraft in 2022, just before its deliberate collision with asteroid moon Dimorphos, the team identified bright, fan-shaped streaks across the moon’s surface—the first direct visual evidence of material naturally traveling from one asteroid to another.
“At first, we thought something was wrong with the camera, and then we thought it could’ve been something wrong with our image processing,” said lead author Jessica Sunshine, a UMD professor with joint appointments in the Department of Astronomy and Department of Geological, Environmental, and Planetary Sciences. “But after we cleaned things up, we realized the patterns we were seeing were very consistent with low-velocity impacts, like throwing ‘cosmic snowballs.’”
The research was supported by NASA, the U.S. Department of Energy and the French National Research Agency.
The team’s findings also provided the first visual confirmation of the Yarkovsky-O’Keefe-Radzievskii-Paddak effect, in which sunlight makes small asteroids spin faster until material flies off their surfaces, sometimes creating moons. Sunshine noted that this was likely the case for Didymos and its moon Dimorphos, as evidenced by the traces of cosmic snowballs left on Dimorphos’ surface.
Finding these traces required months of detective work. The fan-shaped streaks were invisible in the DART spacecraft’s original images, but UMD astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos helped develop sophisticated techniques to remove boulder shadows and lightning effects from the pictures, revealing the surprising streaks that cosmic snowballs left behind.
“We ended up seeing these rays that wrapped around Dimorphos, something nobody’s ever seen before,” Farnham said. “We couldn’t believe it at first because it was subtle and unique.”
For the researchers, the DART mission’s trajectory created an unusual challenge. The spacecraft hurtled straight toward its target with barely any change in lighting or perspective, making it difficult to distinguish real features from possible lighting artifacts. To prove the legitimacy of the streaks, the team mapped them back to their origin in a single region near Dimorphos’ edge—distinctly offset from where the sun was directly overhead. By taking this approach, the team concluded that the marks left by cosmic snowballs weren’t just a trick of the lighting.
“As we refined our 3D model of the moon the fan-shaped streaks became clearer, not fainter,” Farnham said. “It confirmed to us that we were working with something real.”
To test their theories, the researchers led by former UMD postdoctoral associate Esteban Wright performed a series of laboratory experiments at UMD’s Institute for Physical Science and Technology. They dropped marbles into sand scattered with painted gravel to simulate boulders on Dimorphos. High-speed cameras captured the experiment, revealing that boulders blocked some material while letting other particles stream between them—creating ray-like patterns matching those on Dimorphos.
Computer simulations of impacts of loose clumps of dust carried out at Lawrence Livermore National Laboratory confirmed the results. Whether the impactor was a compact rock, like the marble, or a looser clump of material, boulders on the asteroid’s surface naturally sculpted the cosmic snowballs into fan-like rays on the ground.
“We could see these marks on Dimorphos from that footage captured by the DART spacecraft right before the big collision, proof that there was material exchange between it and Didymos,” Sunshine said. “The fan line deposit should extend to side of the moon we did not hit, and there is a possibility it was not destroyed by the impact.”
The European Space Agency’s mission, set to arrive at Didymos in December, may reveal whether these features survived DART’s collision. Sunshine and her team predict Hera might also observe new ray patterns created by boulders that the DART spacecraft knocked loose, shedding new light on asteroids that could pose a threat to Earth.
“These new details emerging from this research are crucial to our understanding of near-Earth asteroids and how they evolve,” Sunshine said. “We now know that they’re far more dynamic than previously believed, which will help us improve our models and our planetary defense measures.”