Images from NASA's DART mission have provided the first direct visual evidence that asteroids in binary systems exchange material through slow-moving debris. Faint streaks on the asteroid moon Dimorphos suggest debris from its parent asteroid Didymos landed there, driven by sunlight-induced spin. This discovery highlights the dynamic nature of near-Earth asteroids.
In 2022, NASA's Double Asteroid Redirection Test (DART) spacecraft intentionally collided with Dimorphos, the moon of the asteroid Didymos. A detailed analysis of images captured just before the impact has uncovered bright, fan-shaped streaks on Dimorphos' surface. These markings indicate recent material transport from Didymos to its companion.
The research, led by a team from the University of Maryland, was published on March 6, 2026, in The Planetary Science Journal. Lead author Jessica Sunshine, a professor in the Department of Astronomy and Department of Geological, Environmental, and Planetary Sciences at the university, explained the initial surprise: "At first, we thought something was wrong with the camera, and then we thought it could've been something wrong with our image processing. 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 streaks emerged after researchers Tony Farnham and Juan Rizos applied specialized techniques to remove shadows from boulders and lighting artifacts. Farnham noted, "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."
This evidence confirms the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, where sunlight accelerates an asteroid's rotation, ejecting loose material. Models by Harrison Agrusa show the debris traveled at 30.7 centimeters per second, slower than a human walking pace, creating fan-shaped deposits rather than craters.
Laboratory experiments at the University of Maryland's Institute for Physical Science and Technology, led by Esteban Wright, recreated these patterns by dropping marbles into sand with gravel obstacles. Computer simulations at Lawrence Livermore National Laboratory supported the findings, showing boulders shaping incoming material into rays.
Sunshine added, "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."
The European Space Agency's Hera mission, arriving at Didymos in December 2026, may assess if these streaks survived the DART impact or reveal new patterns from dislodged boulders. Sunshine emphasized the implications: "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."
Binary asteroid systems make up about 15% of near-Earth asteroids, underscoring the relevance of this active reshaping process over millions of years.