NASA’s DART Impact Permanently Changed the Shape, Orbit of Asteroid Moon

When NASA’s Double Asteroid Redirection Test (DART) spacecraft smashed into an asteroid moon called Dimorphos as part of an experiment in 2022, the resulting crater and other changes dramatically reshaped the moon. A new study led by a University of Maryland astronomer suggests the impact was so strong it knocked Dimorphos off-kilter, derailing the moon from its original evolutionary progression.

Now, Dimorphos may start to “tumble” chaotically as it moves back toward gravitational equilibrium with its parent asteroid named Didymos, according to the paper published Friday in Planetary Science Journal.

The overall DART mission was meant to explore the possibility of changing trajectories of asteroids that might be headed toward a collision with Earth. Pre-impact predictions about how DART would change how Didymos and its moon move in space were correct, said Derek Richardson, a UMD professor of astronomy and a working group lead on DART.

“But there are some unexpected findings that help provide a better picture of how asteroids and other small bodies form and evolve over time,” he said.

One of the biggest surprises was how much the impact with DART changed the shape of Dimorphos. According to Richardson, the asteroid moon was originally oblate (shaped like a hamburger) but became more prolate (stretched out like a football) after the DART spacecraft collided with it.

Asteroids like Dimorphos typically become prolate—“an elongated body that would always point its long axis toward the main body,” he said.“But this result contradicts that idea and indicates that something more complex is at work here.”

Furthermore, the impact-induced change in Dimorphos’ shape likely changed how it interacts with Didymos, he said. Although DART struck only the moon, the moon and the main body are connected through gravity. The debris scattered by the spacecraft on impact also played a role in the disturbed equilibrium between the moon and its asteroid, shortening Dimorphos’ orbit around Didymos.

Interestingly, Didymos’ shape remained the same—a finding that indicates that the larger asteroid’s body is firm and rigid enough to maintain its form even after losing mass to create its moon.

According to Richardson, Dimorphos’ changes have important implications for future exploration efforts, including the European Space Agency’s follow-up mission to the Didymos system, known as Hera, slated for October.

The research team is now waiting to find out when the ejected debris from the impact will clear from the system, whether Dimorphos is still tumbling in space and when it will eventually regain its previous stability.

“One of our biggest questions now is if Dimorphos is stable enough for spacecraft to land and install more research equipment on it,” Richardson said. “It could take a hundred years to see noticeable changes in the system, but it’s only been a few years since the impact. Learning about how long it takes Dimorphos to regain its stability tells us important things about its internal structure, which in turn informs future attempts to deflect hazardous asteroids.”

Following its late 2026 arrival at the binary asteroid system containing Dimorphos and Didymos, the Hera mission will assess the internal properties of both asteroids for the first time, providing a more detailed analysis of the DART mission and its implications for the future.

“DART gave us insight into complicated gravitational physics that you can’t do in a lab, and all of this research helps us calibrate our efforts to defend Earth in the event of an actual threat,” Richardson said. “There’s a nonzero chance that an asteroid or comet will approach and endanger the planet. Now, we have an additional line of defense against these kinds of external threats.”