Scientists have discovered that asteroids in binary systems aren’t just orbiting each other – they’re actively exchanging material, a process likened to throwing “cosmic snowballs.” This finding, stemming from analysis of images captured by NASA’s DART spacecraft, provides the first direct visual evidence of material transfer between asteroids and offers new insights into the evolution of these celestial bodies and potential planetary defense strategies. Roughly 15% of asteroids that pass near Earth are part of these binary systems, making this a surprisingly common phenomenon.
The research, published March 6, 2026, in The Planetary Science Journal, centers on observations of the asteroid Dimorphos, which was intentionally impacted by the Double Asteroid Redirection Test (DART) mission in 2022. Analysis of images taken just before the impact revealed bright, fan-shaped streaks on Dimorphos’ surface, indicating the slow accumulation of debris from its companion asteroid, Didymos. This discovery challenges previous assumptions about the relative inactivity of these systems and suggests a more dynamic interplay than previously understood.
Evidence of Material Exchange
The team, led by Jessica Sunshine, a professor at the University of Maryland, initially suspected an issue with the DART spacecraft’s camera or image processing. “At first, we thought something was wrong with the camera, and then we thought it could’ve been something wrong with our image processing,” Sunshine said. “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.”
These “cosmic snowballs” aren’t high-speed projectiles. Calculations by University of Maryland alum Harrison Agrusa determined the debris travels at a remarkably slow 30.7 centimeters per second – slower than a typical human walking pace. This slow speed explains the distinctive fan-shaped patterns observed on Dimorphos, as opposed to impact craters. The debris appears to be settling and accumulating, rather than explosively impacting the surface.
The YORP Effect and Asteroid Evolution
The observations also provide the first visual confirmation of the Yarkovsky-O’Keefe-Radzievskii-Paddak (YORP) effect, a phenomenon where sunlight gradually alters an asteroid’s rotation. As an asteroid spins faster, loose material can be ejected, potentially forming a smaller moon. Sunshine explained that the Didymos system likely formed this way, with debris spun off Didymos eventually landing on Dimorphos. This process highlights how sunlight can actively reshape asteroids over millions of years.
Uncovering Hidden Details in DART Images
Detecting these subtle streaks wasn’t easy. The patterns weren’t immediately visible in the raw images returned by DART. University of Maryland astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos developed specialized techniques to remove shadows and lighting artifacts, revealing the faint streaks. “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 team confirmed the streaks were genuine by tracing them back to a specific source region on Dimorphos, offset from the point directly overhead, ruling out sunlight as the sole cause. Refining a 3D model of the moon further clarified the patterns, solidifying the evidence of material transfer. Researchers also conducted laboratory experiments at the University of Maryland’s Institute for Physical Science and Technology, dropping marbles into sand containing gravel to simulate the process, and computer simulations at Lawrence Livermore National Laboratory, which corroborated their findings. These experiments demonstrated how boulders on the asteroid’s surface shape incoming material into the observed fan patterns.
“We could notice 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.”
What’s Next: The Hera Mission
The European Space Agency’s Hera mission, scheduled to arrive at Didymos in December 2026, will provide further insights. Hera will assess whether the streak patterns survived the DART impact and potentially detect new patterns created by boulders dislodged during the collision. These new details are crucial for improving our understanding of near-Earth asteroids and refining planetary defense models.
This research underscores the dynamic nature of asteroids and the importance of continued observation. As Sunshine noted, “We now grasp that they’re far more dynamic than previously believed, which will help us improve our models and our planetary defense measures.”
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