Asteroids Caught Pelting Each Other With Rocky Debris in First Direct Evidence of Binary System Material Transfer

Jessica Sunshine knew something was off. The images from NASA’s DART spacecraft looked clean enough in the raw data, but once her team at the University of Maryland began correcting for shadows and uneven lighting across the boulder-covered surface of asteroid moon Dimorphos, strange streaks kept appearing. Fan-shaped, faint, converging on a single region just off the visible limb. “At first, we thought something was wrong with the camera, and then we thought it could’ve been something wrong with our image processing,” she says.

The team spent months trying to make the streaks disappear. They tested different photometric models. They rebuilt their 3D shape model. They checked whether the patterns matched known lighting artefacts from the Earth’s Moon, or whether they correlated with incidence and emission angles in a way that would betray a computational glitch. Nothing. The streaks held. In fact, as the 3D model improved, so did the features. “As we refined our 3D model of the moon the fan-shaped streaks became clearer, not fainter,” says Tony Farnham, a research scientist in UMD’s astronomy department who led the image processing work. “It confirmed to us that we were working with something real.”

What they were looking at, the team now argues, is the first direct visual evidence that rocks physically travel between paired asteroids in a binary system — shed from the larger body and landing, in slow motion, on its smaller companion. The findings, published in The Planetary Science Journal on 6 March 2026, revise what we thought we knew about how near-Earth asteroid pairs evolve, with implications for planetary defence. About 15% of near-Earth asteroids have small moons in orbit around them, making binary systems a common feature of our cosmic neighbourhood. Until now, nobody had seen direct proof of the two bodies actually exchanging material.

The physics behind it is, in a sense, elegant. Sunlight does not merely warm asteroids; it exerts torque on them, slowly spinning them faster over millions of years through a mechanism known as the Yarkovsky-O’Keefe-Radzievskii-Paddak effect (YORP, mercifully). At some threshold, the spin becomes fast enough that loose surface material is flung outward. In larger asteroid systems, this process is thought to be how moons form in the first place: material shed from the primary accumulates into a secondary body over time. The evidence for this has previously been indirect, inferred from the shapes of primaries and the orbits of their moons. Here, the UMD team believes they can see the aftermath of that transport process preserved on Dimorphos’s surface.

The numbers are almost comically gentle for a cosmic event. Harrison Agrusa, an alumnus of UMD’s astronomy programme, calculated that material leaving Didymos (the primary, roughly 730 metres across) needs to travel at only about 30.7 centimetres per second to escape — barely faster than a slow walk, and only about 2.5 cm/s above the equatorial rotation speed of Didymos itself. By the time it reaches Dimorphos, it arrives at around 6 cm/s, well below the escape velocity of the smaller body. No crater forms. Instead, the impacting material drapes across the surface in a thin, bright deposit, interacting with the pervasive boulders that cover Dimorphos like a rubble field. “Instead of even spreading, these slow-moving impacts would create a deposit rather than a crater,” Sunshine says. “And they are centered on the equator as predicted from modeling material spun off the primary.”

The fan shape is where the boulders come in. To test whether boulder fields could plausibly produce the kind of raylike deposits seen on Dimorphos, Esteban Wright, a former postdoctoral associate at UMD, ran a rather low-tech experiment in the university’s Institute for Physical Science and Technology: dropping a glass marble into a tray of sand scattered with painted gravel. High-speed cameras at 1000 frames per second captured what happened. The boulders blocked some of the ejecta while letting streams of material flow around them, producing exactly the kind of divergent, filamentary rays visible on Dimorphos’s albedo-corrected surface. Numerical simulations at Lawrence Livermore National Laboratory confirmed the same result for loose, unconsolidated impactors too — whether the incoming material was a coherent rock or a soft clod, boulders sculpted its deposit into fans.

“We ended up seeing these rays that wrapped around Dimorphos, something nobody’s ever seen before,” Farnham says. “We couldn’t believe it at first because it was subtle and unique.” The subtlety is worth dwelling on a moment. The deposit is only about 25% brighter than the surrounding surface in the normalised albedo images, and invisible entirely without the photometric corrections the team spent months developing. The original DART images showed none of it. The marks weren’t made by the spacecraft’s collision — that came later — but pre-existed it, a record of natural material exchange that had been sitting in the data, invisible, for years.

The YORP confirmation matters beyond this single system. It provides the first visual corroboration of a process that planetary scientists have long argued shapes the structure of small-body populations across the solar system. The spin-up and mass-shedding cycle is thought to explain not only binary asteroid formation but also the equatorial ridges and bulges seen on other near-Earth objects like Ryugu and Bennu.

There is also a practical dimension. Dimorphos was DART’s target precisely because binary asteroids, with their measurable orbital periods, are useful test subjects for planetary defence experiments. Understanding that these systems are dynamically active — continuously reshaping themselves through material exchange — matters for any future hazard assessment that relies on knowing what an asteroid looks like, how it is structured, and how it might respond to an impactor.

The story has one more chapter still to come. The European Space Agency’s Hera mission is on course to arrive at the Didymos system in December 2026. When it does, it will find a surface transformed by DART’s impact. Whether the pre-existing deposit survived the collision is unknown. “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,” Sunshine says. Hera’s colour and compositional instruments may be able to distinguish the ancient, gentle deposit from the material disturbed by DART’s hypervelocity strike, giving scientists two contrasting impact records to compare on the same small body.

If the deposit is gone, that too is information. The DART ejecta — boulders lifted and redeposited at low velocity — may have left their own raylike marks. Either way, Dimorphos is no longer just a test target. It is a palimpsest, its surface layered with the slow quiet physics of a solar system in constant, if glacial, motion.

Study link: https://iopscience.iop.org/article/10.3847/PSJ/ae3f27

Frequently Asked Questions

How does material actually get from one asteroid to another without a collision? Sunlight exerts a tiny rotational torque on small asteroids over millions of years, gradually spinning them faster until loose surface material is flung off. In a binary system, some of that shed material has just enough velocity to cross to the companion body and land — at walking pace, essentially, which is why it leaves a thin deposit rather than a crater. The UMD team calculated the transfer velocity at roughly 30 centimetres per second, only fractionally above the equatorial spin speed of the primary.

Why couldn’t scientists see these streaks in the original DART images? The surface of Dimorphos is carpeted in boulders that cast complex shadows and scatter light unevenly, masking subtle albedo differences. The fan-shaped deposits are only about 25% brighter than the surrounding rock, and they only become visible once a detailed photometric model is used to strip out the lighting variation. The team spent months building progressively better 3D models of the asteroid before the streaks emerged cleanly — and notably, the features became sharper as the model improved, which is what convinced them they weren’t looking at an artefact.

Does this change how we should think about asteroid threats to Earth? Somewhat, yes. Planetary defence models depend on understanding the structure and evolution of near-Earth asteroids, and this finding suggests binary systems are considerably more dynamic than previously assumed. An asteroid that is actively shedding and redepositing surface material is reshaping itself over time, which affects assumptions about surface cohesion, internal structure, and how it might respond to a deflection attempt.

Will the Hera mission be able to confirm what the DART data showed? Hera arrives at the Didymos system in December 2026, and the team expects it to find either the pre-existing deposit — which may have survived on the side of Dimorphos that DART didn’t strike — or new ray patterns created by DART ejecta raining back down at low velocity. Distinguishing between the two would give scientists a rare side-by-side comparison of a natural slow-motion deposit and a hypervelocity impact aftermath on the same 150-metre body.

What is the YORP effect and why does it matter here? YORP (Yarkovsky-O’Keefe-Radzievskii-Paddak) is the process by which uneven thermal emission from sunlight gradually alters the spin rate of small asteroids. It has been inferred indirectly from asteroid shapes and binary system architecture for years, but this study provides the first direct visual evidence of material it shed actually landing somewhere. That confirmation closes a gap between theory and observation that has been open for decades.