Binary asteroid systems are not uncommon in our cosmic neighborhood with about 15 percent of asteroids around the Earth having small moons around them.

A team of astronomers (headed by the University of Maryland) has since found that these binary asteroid systems are much more dynamic than they thought- involving active exchange of rocks and dust in slow, slow-motion collisions that reform them over millions of years.

Upon the analysis of the images captured by the NASA Double Asteroid Redirection Test (DART) spacecraft just before deliberately colliding with the asteroid moon Dimorphos in 2022, the team observed bright, fan-shaped streaks across the surface of the moon, which is the first direct evidence of the material naturally traveling between two asteroids. The implications of the findings given by the researchers in The Planetary Science Journal on March 6, 2026, regarding the information about asteroids that may pose a threat to the earth are far reaching.

Initially, we assumed that it must have been a problem with the camera, then we assumed it must have been a problem with our processing of the images, said the lead author of the paper, Jessica Sunshine, a professor with joint appointments in both the Department of Astronomy and Department of Geological, Environmental, and Planetary Sciences of UMD. However, once we cleared it up we found the marks we were observing were quite regular with respect to low velocity collisions, such as tossing cosmic snowballs. We possessed the first direct evidence of material movement within the recent past in a binary asteroid system.

The results of the team were also the first, visual confirmation of the Yarkovsky-O Keefe Radzievskii Paddak (YORP) effect wherein small asteroids rapidly rotate due to the presence of sunlight, causing material to be thrown off their surfaces to form moons. This was probably true of Didymos and its smaller satellite Dimorphos in the case of Sunshine reported the remnants of the so-called cosmic snowballs which had been deposited on the surface of Dimorphos.

How they found these traces?

They took months of investigative efforts to find these traces. The original images captured by the DART spacecraft could not see the fan-shaped streaks yet, UMD astronomy research scientist Tony Farnham and former postdoctoral researcher Juan Rizos developed more intricate methods to eliminate the boulder shadow and lightning effects in the images and exposed the eye-opening streaks that were left behind by the ‘cosmic snowballs’.

We finally saw these rays wrapping round Dimorphos, something no one has ever seen, you see, Farnham said. At the initial stages, it could not be believed because it was gentle and distinct.

To the researchers, the path of the DART mission provided a peculiar challenge. The space ship flew directly into the target with only slight distinctions in lighting and viewpoint that made it hard to differentiate actual features and any potential lighting possibilities. To demonstrate the authenticity of the streaks the team traced them to the source in one of the areas near the edge of Dimorphos- clearly out of phase with where the sun was overhead. Having done this, the team came to the conclusion that the traces left by the so-called cosmic snowballs were not really a light illusion.

Not fainter as we smoothed out the 3D image of the moon the fan-shaped streaks became more distinct, Farnham said. “It made us sure that we were dealing with a reality.

Earlier researchers noted an indirect evidence of the sunlight causing small asteroids to spin faster triggering the expulsion of material off their surfaces. However, the recently perfected models of the asteroid moon Dimorphos created by the UMD team give the first graphic assurance of the process and the precise sites of the shed material of its original asteroid, Didymos. Additional calculations by UMD alumnus Harrison Agrusa (M.S. ’19, Ph.D. ’22, astronomy) also indicated that the material moved Didymos at 30.7 centimeters per second, which is slower than the typical pace of a human walking.

Fan-shaped marks

“That would be why it had the fan-shaped marks,” Sunshine said. “These slow moving effects would not cause a crater as they would cause a deposit instead of being evenly distributed. And they are focused on the equator as theorized on modeling material ripped off the primary.”

The researchers headed by the former UMD postdoctoral associate Esteban Wright conducted a battery of experiments in their laboratories to test their hypotheses at the UMD Institute of Physical Science and Technology. To replicate boulders on Dimorphos, they tossed marbles into a sand filled with painted gravel. The experiment was recorded with high-speed cameras, and it was found that boulders filtered some material and allowed other particles to stream in-between the boulders- forming ray-like patterns similar to those found on Dimorphos.

The results were verified in computer simulations of effects of loose clumps of dust done at Lawrence Livermore National Laboratory. The shape of the fan-shaped rays on the surface of the asteroid was naturally formed by boulders that formed the cosmic snowballs on the surface of the asteroid whether the impactor was a compact rock such as the marble or a loose clump of material.

These marks could be seen on Dimorphos in that film taken by the DART spacecraft immediately before the large collision, evidence that there was an exchange of material between it and Didymos, said Sunshine. The fan line deposit must stretch up to the side of the moon that we did not strike and there is a chance that it was not smashed in by the blow.

These features could be found to be still present on Didymos as the Hera mission of the European Space Agency will possibly arrive in December 2026 and see them. Sunshine and her colleagues give an estimate as to how Hera will also witness new ray patterns formed when boulders are struck by the DART spacecraft, knocking them loose, which gives them a different perspective of the asteroids that have the potential to threaten the earth.

According to Sunshine, these new findings which arise out of this research play a critical part in our knowledge about the near-Earth asteroids and their evolutionary patterns. It has been discovered that they are much more dynamic than we thought before and this will assist us in streamlining our models and our planetary defense efforts.

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