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 Asteroid Debris
Scientists had previously gathered indirect evidence suggesting that sunlight can increase the spin rate of small asteroids until surface material is ejected. However, the updated models created by the University of Maryland team provide the first visual confirmation of this process. The models also pinpoint where debris launched from Didymos eventually landed on Dimorphos.
Additional calculations led by UMD alum Harrison Agrusa (M.S. ’19, Ph.D. ’22, astronomy) determined that the debris left Didymos traveling at only 30.7 centimeters per second. That speed is slower than a typical human walking pace.
“That would explain the distinctive fan-shaped marks,” Sunshine said. “Instead of even spreading, these slow-moving impacts would create a deposit rather than a crater. And they are centered on the equator as predicted from modeling material spun off the primary.”
Laboratory Experiments Recreate “Cosmic Snowballs”
To test their explanation, researchers led by former UMD postdoctoral associate Esteban Wright conducted laboratory experiments at UMD’s Institute for Physical Science and Technology. In the tests, the team dropped marbles into sand that contained scattered pieces of painted gravel representing boulders on Dimorphos. High speed cameras recorded the results.
The experiments showed that the boulders blocked some particles while allowing others to pass through gaps between them. This produced ray like patterns similar to the streaks observed on Dimorphos.
Computer simulations performed at Lawrence Livermore National Laboratory reached the same conclusion. Whether the incoming object was a solid rock like the marble or a looser clump of dust, the boulders on the asteroid’s surface shaped the incoming material into the distinctive fan patterns.
“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.”
Hera Mission May Reveal More Clues
The European Space Agency’s Hera mission is scheduled to reach Didymos in December 2026. The spacecraft could determine whether the streak patterns survived the DART impact. Sunshine and her colleagues also expect Hera might detect new ray patterns created by boulders that were dislodged when DART struck Dimorphos.
“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.”
The paper, “Evidence of Recent Material Transport within a Binary Asteroid System,” was published on March 6, 2026 in The Planetary Science Journal.
This research was supported by NASA (Contract No. 80MSFC20D0004), the U.S. Department of Energy (Contracts DE-AC52-07NA27344 and LLNL-JRNL2002294) and the French National Research Agency (Project ANR-15-IDEX-01).