Science News from research organizations NASA DART mission reveals asteroids throw “cosmic snowballs” at each other Asteroids with moons may be quietly pelting each other with slow-motion “cosmic snowballs,” reshaping their surfaces over time. Date: March 8, 2026 Source: University of Maryland Summary: Asteroids with tiny moons may be quietly trading material across space. Images from NASA’s DART mission revealed faint streaks on the moon Dimorphos—evidence of slow “cosmic snowballs” drifting from its parent asteroid, Didymos. The discovery provides the first direct visual proof that sunlight can spin asteroids fast enough to shed debris that lands on nearby companions. It also shows that near-Earth asteroids are much more active and constantly reshaped than scientists once believed. Share: Facebook Twitter Pinterest LinkedIN Email FULL STORY [Left] The boulder-covered moon Dimorphos as seen 8.55 seconds before the impact of the DART spacecraft. [Right] The same image after correcting for lighting conditions across the surface and shadows cast by boulders, revealing a fan-shaped pattern of streaks (highlighted in color for emphasis). Credit: NASA/JHU-APL/UMD Roughly 15% of asteroids that pass near Earth have a smaller companion orbiting them. These paired objects are known as binary asteroid systems, and they are surprisingly common in our region of the solar system. A research team led by the University of Maryland has now found that these systems are far more active than scientists once thought. Instead of simply orbiting one another, the two bodies can exchange rocks and dust through gentle, slow moving impacts that gradually reshape their surfaces over millions of years. The discovery comes from a close analysis of images captured by NASA's Double Asteroid Redirection Test (DART) spacecraft in 2022 just before it intentionally collided with the asteroid moon Dimorphos. In those images, the scientists noticed bright, fan shaped streaks on Dimorphos' surface. These markings provide the first direct visual proof that material can naturally travel from one asteroid to another. The results were published March 6, 2026 in The Planetary Science Journal and could help scientists better understand asteroids that might one day threaten Earth. "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 the paper's lead author Jessica Sunshine, a professor with joint appointments in the Department of Astronomy and Department of Geological, Environmental, and Planetary Sciences at UMD. "But after we cleaned things up, we realized the patterns we were seeing were very consistent with low velocity impacts, like throwing 'cosmic snowballs.' We had the first direct proof for recent material transport in a binary asteroid system." Evidence of the YORP Effect on Asteroids The observations also provide the first visual confirmation of a process known as the Yarkovsky-O'Keefe-Radzievskii-Paddak (YORP) effect. In this phenomenon, sunlight gradually accelerates the rotation of small asteroids. As the spin increases, loose material can be flung off the surface and sometimes form a small moon. Sunshine explained that this likely occurred in the Didymos system, which includes the larger asteroid Didymos and its smaller moon Dimorphos. The marks left by the so called "cosmic snowballs" on Dimorphos suggest that debris spun off Didymos and later landed on its companion. Detecting Hidden Streaks in DART Images Finding this evidence took months of careful analysis. The streak patterns were not visible in the original images returned by the DART spacecraft. UMD astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos developed specialized techniques to remove shadows cast by boulders and lighting artifacts from the photos. Once those visual effects were corrected, the subtle streaks left by the "cosmic snowballs" began to appear. "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." The spacecraft's flight path added another complication. Because DART approached Dimorphos almost directly, the lighting and viewing angle changed very little during the encounter. That made it difficult to determine whether certain features were real or simply the result of lighting conditions. To confirm the streaks were genuine, the researchers traced them back to a specific source region near the edge of Dimorphos. That location was offset from the point where the Sun was directly overhead, which showed that the patterns were not caused by sunlight alone. "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." Slow Moving