“At First, We Thought Something Was Wrong” – NASA DART Mission Reveals a Cosmic Snowball Fight

Images from NASA’s DART mission revealed the first direct evidence that asteroids in a binary system can exchange rocks and dust. Slow moving debris from the asteroid Didymos appears to have struck its moon Dimorphos, leaving distinctive streaks scientists describe as “cosmic snowballs.”

Around 15% of asteroids that pass near Earth have a smaller moon orbiting them. These pairs, known as binary asteroid systems, are surprisingly common in our part of the solar system.

New research led by astronomers at the University of Maryland shows that these asteroid pairs are far more active than scientists once thought. Instead of simply orbiting quietly, the two bodies can exchange rocks and dust through slow, gentle collisions that gradually reshape their surfaces over long periods of time.

The discovery comes from a detailed analysis of images captured in 2022 by NASA’s Double Asteroid Redirection Test (DART) spacecraft shortly before it intentionally collided with the asteroid moon Dimorphos. Researchers identified bright, fan shaped streaks across Dimorphos’ surface. These markings provide the first direct visual evidence that material can naturally travel from one asteroid to another. The findings, published March 6, 2026, in The Planetary Science Journal, could help scientists better understand the behavior of asteroids that might pose a risk 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,” 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.”

This is footage of a marble experiment conducted on Earth simulating ‘cosmic snowballs’ thrown by Didymos and their impacts on Dimorphos. Credit: NASA/JHU-APL/UMD

Evidence of the YORP Effect

The observations also provide the first visual confirmation of the Yarkovsky-O’Keefe-Radzievskii-Paddak (YORP) effect. In this process, sunlight slowly increases the spin rate of small asteroids. As their rotation speeds up, loose material can be flung off the surface and in some cases form a moon.

Sunshine explained that this process likely shaped the Didymos system, which consists of the larger asteroid Didymos and its smaller companion Dimorphos. The marks left by the so-called ‘cosmic snowballs’ on Dimorphos suggest that debris spun off Didymos and later landed on its moon.

Hidden Streaks Revealed in DART Images

Identifying the streaks took months of careful analysis. The patterns were not visible in the original DART images. UMD astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos developed specialized techniques to remove shadows from boulders and lighting effects from the photographs. Once those distortions were corrected, the subtle streaks created by the ‘cosmic snowballs’ became visible.

“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 approach also created challenges for the research team. DART traveled almost directly toward its target, meaning there was little change in lighting or viewing angle during the encounter. That made it difficult to separate genuine surface features from visual artifacts caused by lighting.

To confirm the streaks were real, the team traced them back to a single source area near the edge of Dimorphos. That location was offset from the point where the Sun was directly overhead. This analysis showed that the features were not simply lighting effects.

“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 Debris From Didymos

Scientists had previously gathered indirect evidence that sunlight can spin small asteroids faster over time, causing material to escape from their surfaces. The updated models produced by the University of Maryland team provide the first visual confirmation of this process. They also reveal exactly where debris from Didymos landed on Dimorphos.

Further calculations led by UMD alum Harrison Agrusa (M.S. ’19, Ph.D. ’22, astronomy) showed that the material left Didymos traveling at just 30.7 centimeters per second. That speed is slower than the average 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 the Patterns

To test their explanation, researchers led by former UMD postdoctoral associate Esteban Wright carried out experiments at UMD’s Institute for Physical Science and Technology. In the laboratory, the team dropped marbles into sand containing scattered pieces of painted gravel meant to represent boulders on Dimorphos. High-speed cameras recorded the results.

The experiments showed that the simulated boulders blocked some of the incoming material while allowing other particles to pass between them. This created streak-like patterns similar to the rays observed on Dimorphos.

Computer simulations performed at Lawrence Livermore National Laboratory confirmed the same outcome. Whether the incoming material was a compact rock like the marble or a looser clump of dust, boulders on the asteroid surface naturally shaped the debris into fan-like rays.

“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 Provide More Answers

The European Space Agency’s Hera mission is scheduled to reach Didymos in December 2026. The spacecraft could reveal whether these streak patterns survived the DART collision. Sunshine and her colleagues also expect Hera may detect new ray patterns formed by boulders that were knocked loose when DART struck Dimorphos. Such observations could offer further insights into how asteroids evolve and how they might behave if they ever threatened Earth.

“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.”

Reference: “Evidence of Recent Material Transport within a Binary Asteroid System” by J. M. Sunshine, J. L. Rizos, O. S. Barnouin, R. T. Daly, C. M. Ernst, T. L. Farnham, H. F. Agrusa, E. Wright, S. E. Wiggins, M. Bruck Syal, A. M. Stickle, J. M. Pearl, C. D. Raskin and K. M. Kumamoto, 6 March 2026, The Planetary Science Journal.
DOI: 10.3847/PSJ/ae3f27

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).

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