NASA's DART Mission Reveals Asteroids Throw "Cosmic Snowballs" at Each Other

Images from NASA's DART mission have revealed the first direct visual proof that asteroids are pelting their moons with slow-moving debris — "cosmic snowballs" drifting from parent asteroids to their smaller companions.

The discovery comes from analysis of images captured by NASA's Double Asteroid Redirection Test (DART) spacecraft in 2022 just before it intentionally collided with the asteroid moon Dimorphos. In those images, scientists noticed faint, fan-shaped streaks on Dimorphos' surface — evidence of material that had drifted from its parent asteroid, Didymos.

"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 lead author Jessica Sunshine, a professor at the University of Maryland. "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."

Roughly 15% of near-Earth asteroids have a smaller companion orbiting them. These binary systems are surprisingly common. The new findings show they are far more active than scientists once thought — not just orbiting each other, but actively exchanging rocks and dust through gentle impacts that gradually reshape their surfaces over millions of years.

The observations also provide the first visual confirmation of the YORP effect — a phenomenon where sunlight gradually accelerates the rotation of small asteroids. As spin increases, loose material can be flung off the surface and land on nearby companions.

The results were published March 6, 2026, in The Planetary Science Journal.

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