Asteroid ‘Cosmic Snowballs’: DART Mission Reveals Material Exchange Between Space Rocks

New data from NASA’s Double Asteroid Redirection Test (DART) mission reveals that asteroids aren’t isolated bodies in space, but actively exchange material – what researchers are calling “cosmic snowballs” – with their orbiting moons. The findings, published March 6, 2026, in The Planetary Science Journal, offer a new understanding of how near-Earth asteroids evolve and could refine planetary defense strategies. This exchange isn’t a violent collision, but a gradual, gentle process reshaping these celestial bodies over millions of years.

Binary Asteroid Systems: More Dynamic Than Previously Thought

Roughly 15% of asteroids near Earth are part of binary systems, meaning they have a smaller moon orbiting them. These systems are common in our cosmic neighborhood, but until recently, scientists assumed they were relatively static. The DART mission, which intentionally collided with the asteroid moon Dimorphos in 2022, provided a unique opportunity to observe these systems in action. Analysis of images captured by the DART spacecraft just before impact revealed bright, fan-shaped streaks across Dimorphos’ surface – the first direct visual evidence of material traveling between asteroids.

“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 Jessica Sunshine, lead author of the study and a professor at the University of Maryland. “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.”

The YORP Effect and Material Exchange

The observed material exchange is linked to a phenomenon known as the Yarkovsky-O’Keefe-Radzievskii-Paddak (YORP) effect. This effect describes how sunlight can gradually increase the rotation speed of minor asteroids. As an asteroid spins faster, loose material can be ejected from its surface, potentially forming a smaller moon or, as now observed, drifting towards a companion asteroid. Researchers believe this is precisely what happened in the Didymos system, with debris spun off the larger asteroid Didymos eventually landing on Dimorphos.

The team, led by Sunshine and UMD astronomy research scientist Tony Farnham, developed specialized techniques to remove shadows and lighting artifacts from the DART images, revealing the subtle streaks left by the “cosmic snowballs.” Farnham explained, “We ended up seeing these rays that wrapped around Dimorphos, something nobody’s ever seen before. We couldn’t believe it at first because it was subtle and unique.”

Slow-Motion Impacts and Experimental Confirmation

The debris traveling between Didymos and Dimorphos isn’t moving at high speeds. Calculations led by UMD alum Harrison Agrusa determined the debris travels at a mere 30.7 centimeters per second – slower than a typical human walking pace. This slow speed explains the distinctive fan-shaped marks observed on Dimorphos. Instead of creating craters, these impacts deposit material in a spread-out pattern.

To validate their findings, researchers at the University of Maryland’s Institute for Physical Science and Technology conducted laboratory experiments. They dropped marbles into sand containing painted gravel representing boulders on Dimorphos. High-speed cameras recorded the results, showing that boulders blocked some particles while allowing others to pass through, creating ray-like patterns similar to those observed on the asteroid moon. Further supporting this, computer simulations at Lawrence Livermore National Laboratory confirmed that the boulder-strewn surface shapes incoming material into the observed fan patterns.

Implications for Planetary Defense

Understanding how asteroids exchange material is crucial for refining planetary defense models. The DART mission itself was a test of asteroid deflection technology, and the new findings provide valuable insights into the physical properties and evolution of asteroids that could potentially threaten Earth. Knowing that these asteroids are dynamic and constantly reshaping themselves helps scientists better assess the risks and develop more effective mitigation strategies.

The discovery also highlights the importance of continued observation. The European Space Agency’s Hera mission, scheduled to reach Didymos in December 2026, will provide further data. Hera could determine whether the streak patterns survived the DART impact and potentially detect new patterns created by boulders dislodged during the collision. NASA’s JPL provides updates on the DART mission and its ongoing impact on our understanding of asteroid dynamics.

What Comes Next: Continued Observation and Modeling

The research team plans to continue analyzing the DART images and incorporating the new findings into their models of asteroid evolution. The Hera mission’s data will be critical for validating these models and refining our understanding of the YORP effect and material exchange processes. ScienceDaily reports that this discovery shows near-Earth asteroids are much more active than previously believed. Further research will focus on characterizing the size and composition of the debris being exchanged and determining the frequency of these events across different binary asteroid systems. This ongoing investigation promises to reveal even more about the complex and dynamic nature of asteroids in our solar system.