Asteroid ‘Snowballs’: DART Mission Reveals Asteroids Exchange Rock and Dust

NASA’s Double Asteroid Redirection Test (DART) mission continues to yield surprising insights into the dynamics of asteroid systems. Recent analysis of images captured during the mission, which deliberately impacted the asteroid Dimorphos in September 2022, has revealed the first visual evidence of little asteroids exchanging rocks and dust – a slow process that reshapes their surfaces over millions of years. This exchange, described by researchers as “cosmic snowballs,” offers a recent understanding of how these celestial bodies evolve and interact.

The DART mission, designed to test asteroid-deflection technology, wasn’t just about altering an asteroid’s course. The high-resolution images taken moments before impact revealed faint, fan-shaped streaks across Dimorphos’ rocky surface. These streaks, initially puzzling to the research team, have now been identified as material transferred from Dimorphos’ larger companion, Didymos.

How Asteroid ‘Snowballs’ Form

The process isn’t a rapid collision, but a gradual drift and accumulation of debris. Dimorphos and Didymos exist in a binary system, gravitationally bound to each other. Sunlight plays a key role in this exchange. As asteroids absorb sunlight, they re-emit energy as thermal radiation. This creates a tiny, continuous thrust – known as the YORP effect – that can slowly spin up the asteroid. As the asteroid spins faster, loose material can break free from its surface.

This ejected material doesn’t simply drift away into space. Instead, it slowly migrates between the two asteroids in the binary system, eventually landing on the other’s surface in low-speed impacts. “We had the first direct proof for recent material transport in a binary asteroid system,” explained Jessica Sunshine, of the University of Maryland, in a statement. The impacts are so gentle – roughly 30.7 centimeters (12.1 inches) per second – that they create deposits rather than craters.

Implications for Planetary Defense

Although the discovery of this material exchange is fascinating from a scientific perspective, it similarly has implications for planetary defense strategies. The DART mission not only demonstrated the feasibility of altering an asteroid’s trajectory, but also provided a unique opportunity to study the physical properties of these space rocks. Further analysis confirmed that DART altered not only Dimorphos’ orbit around Didymos, but also slightly changed the entire binary system’s orbit around the sun.

Researchers reported a shift in the system’s orbital speed of approximately 11.7 microns per second, or about 1.7 inches per hour, according to a study published in Science Advances. Rahil Makadia, a planetary defense researcher at the University of Illinois Urbana-Champaign, emphasized the significance of this finding: “Over time, such a small change in an asteroid’s motion can build the difference between a hazardous object hitting or missing our planet.”

The Role of Stellar Occultations and Future Missions

Identifying these subtle changes required sophisticated observational techniques. Scientists utilized stellar occultations – events where an asteroid passes in front of a distant star, blocking its light – to precisely determine the asteroid pair’s speed, shape and position. These events are difficult to predict and require astronomers to be in the right place at the right time to collect data.

Approximately 15% of near-Earth asteroids are binary systems, making this process potentially widespread. The YORP effect and subsequent material exchange can lead to the formation of ridges around the equators of asteroids, as observed by NASA’s Lucy spacecraft on the asteroid Dinkinesh and its moon Selam. Similar ridges are also visible on Dimorphos and Didymos, suggesting a common formation mechanism.

What’s Next for Dimorphos and Didymos?

The European Space Agency’s Hera spacecraft is scheduled to arrive at the Dimorphos-Didymos system in December 2026. This mission will conduct a detailed post-impact survey of Dimorphos, providing a closer look at the changes caused by the DART impact and potentially revealing whether the fan-shaped streaks identified in the initial images have persisted. Hera will also search for new ray-like patterns created by boulders dislodged during the collision, offering further insights into asteroid evolution.

The $398 million Hera mission will be crucial in assessing the long-term effects of the DART impact and refining our understanding of how to protect Earth from potential asteroid threats. As Sunshine noted, “These new details emerging from this research are crucial to our understanding of near-Earth asteroids and how they evolve. We now realize that they’re far more dynamic than previously believed, which will help us improve our models and our planetary defense measures.” The data gathered will contribute to more accurate risk assessments and potentially inform future asteroid deflection strategies.