Asteroid ‘Snowballs’: DART Images Reveal Material Exchange Between Binary Asteroids

Roughly 15% of asteroids near Earth aren’t solitary travelers; they have smaller companion moons orbiting them. These binary asteroid systems, as they’re known, are surprisingly common in our cosmic neighborhood. Now, new analysis of data from NASA’s Double Asteroid Redirection Test (DART) mission reveals these systems are far more dynamic than previously understood, actively exchanging rocks and dust in what researchers are calling “leisurely-motion collisions.” This exchange, occurring at a leisurely pace of roughly 30.7 centimeters per second – slower than a human walk – reshapes the asteroids over millions of years and offers new insights into planetary defense.

Unveiling the ‘Cosmic Snowballs’

The discovery stems from a detailed examination of images captured by the DART spacecraft in 2022, just before its intentional impact with the asteroid moon Dimorphos. Scientists, led by the University of Maryland, noticed bright, fan-shaped streaks across Dimorphos’ surface. These markings represent the first direct visual evidence of material naturally traveling between asteroids. The findings, published on March 6, 2026, in The Planetary Science Journal, could significantly improve our understanding of asteroids that pose a potential threat to 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,” explained Jessica Sunshine, lead author of the study and a professor at the University of Maryland, in a statement. “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.”

How Asteroids Exchange Debris

The process driving this material exchange is linked to the Yarkovsky-O’Keefe-Radzievskii-Paddak (YORP) effect. This phenomenon describes how sunlight gradually alters the rotation of small asteroids. As an asteroid spins faster, loose material can be ejected from its surface, and sometimes, this debris can form a smaller moon. In the case of the Didymos system – comprised of the larger asteroid Didymos and its moon Dimorphos – researchers believe material spun off Didymos eventually landed on Dimorphos, leaving the observed streaks. NASA’s DART mission images provided the crucial visual data to support this theory.

Correcting for Shadows and Lighting

Detecting these subtle streaks wasn’t straightforward. The initial images returned by DART didn’t clearly show the patterns. University of Maryland astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos developed specialized techniques to remove shadows cast by boulders and correct for lighting artifacts in the photos. Once these visual distortions were addressed, the fan-shaped streaks became apparent.

“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 given that it was subtle, and unique.” The spacecraft’s trajectory also presented a challenge. Because DART approached Dimorphos almost head-on, the lighting and viewing angle remained relatively constant, making it difficult to distinguish real features from lighting effects. To confirm the streaks were genuine, the researchers traced them back to a specific source region on Dimorphos, offset from the point directly overhead, ruling out sunlight as the sole cause.

Laboratory Confirmation and Simulations

To validate their findings, researchers conducted laboratory experiments at the University of Maryland’s Institute for Physical Science and Technology. They dropped marbles into sand containing scattered pieces of painted gravel, simulating boulders on Dimorphos. High-speed cameras recorded the results, revealing that the boulders blocked some particles while allowing others to pass through, creating ray-like patterns similar to those observed on the asteroid. These experiments corroborated the visual evidence from the DART mission.

Computer simulations performed at Lawrence Livermore National Laboratory further supported the team’s conclusions. Whether the incoming object was a solid rock or a looser clump of dust, the boulders on Dimorphos’ surface consistently shaped the material into the distinctive fan patterns.

Implications for Planetary Defense

Understanding how material moves within binary asteroid systems has implications for planetary defense. Knowing that asteroids can exchange debris helps scientists better model their evolution and predict their future trajectories. This is particularly important for asteroids that could potentially threaten Earth. The DART mission itself was a test of asteroid deflection technology, and the data gathered from the mission, including these new findings about material exchange, will inform future planetary defense strategies. The DART mission successfully altered the orbit of Didymos’ moon, demonstrating the feasibility of asteroid deflection.

What’s Next: The Hera Mission

The European Space Agency’s Hera mission, scheduled to reach Didymos in December 2026, will provide further insights into this dynamic system. Hera could determine whether the streak patterns observed by DART survived the impact. Researchers also anticipate that Hera might detect new ray patterns created by boulders dislodged when DART collided with Dimorphos. This continued observation will refine our understanding of asteroid behavior and improve our ability to assess and mitigate potential threats from near-Earth objects.

As Sunshine emphasized, “These new details emerging from this research are crucial to our understanding of near-Earth asteroids and how they evolve. We now know that they’re far more dynamic than previously believed, which will help us improve our models and our planetary defense measures.”