NASA’s DART probe impact permanently altered the shape and orbit of the asteroid’s moon, new study finds

According to a new study, when NASA’s Double Asteroid Redirection Test (DART) spacecraft collided with an asteroid moon called Dimorphos in 2022, the moon was significantly deformed, creating a large crater and reshaping it so dramatically that the moon was derailed from its original evolutionary progression. The study’s researchers believe Dimorphos may have begun to “rock” chaotically as it attempts to return to gravitational equilibrium with its parent asteroid named Didymos.

“Overall, our initial predictions about how DART would change the way Didymos and its moon move through space were correct,” said Derek Richardson, a professor of astronomy at the University of Maryland and leader of the DART study working group. “But some unexpected findings shed light on how asteroids and other small bodies form and evolve over time.”

The article published in Journal of Planetary Sciences On August 23, 2024, a team led by Richardson detailed notable post-impact observations and described possible implications for future asteroid research.

One of the biggest surprises was how much the DART impact changed Dimorphos’ shape. According to Richardson, the asteroid’s moon was originally flattened (shaped like a hamburger) but became more elongated (stretched like a football) after the collision with the DART spacecraft.

“We expected Dimorphos to be elongated before impact, simply because we generally thought that’s how the central body of a moon gradually accumulates material that has broken off from a main body like Didymos. It would naturally tend to form an elongated body that would always point its long axis toward the main body,” Richardson explained. “But this result contradicts that idea and indicates that something more complex is at work here. Furthermore, the impact-induced shape change of Dimorphos likely changed the way it interacts with Didymos.”

Richardson noted that even though DART only hit the Moon, the Moon and the main body are connected by gravity. Debris scattered by the spacecraft during the impact also played a role in disrupting the balance between the Moon and its asteroid, shortening Dimorphos’ orbit around Didymos. Interestingly, Didymos’ shape remained the same, indicating that the larger asteroid body is firm and rigid enough to maintain its shape even after losing mass to create its moon.

Richardson said the changes in Dimorphos have important implications for future exploration efforts, including the European Space Agency’s follow-up mission to the Didymos system planned for October 2024.

“Dimorphos was probably originally in a very relaxed state and had one side pointed toward the main body, Didymos, just as Earth’s moon always has one side pointed toward our planet,” Richardson explained. “Now it’s out of alignment, meaning it can wobble back and forth in its orientation. Dimorphos could also be ‘rocking,’ meaning we may have caused it to spin in a chaotic and unpredictable way.”

The team is now waiting to find out when the ejected debris will be removed from the system, whether Dimorphos continues to orbit in space, and when it will finally regain its previous stability.

“One of the most important questions today is whether Dimorphos is stable enough for space probes to land there and install more research equipment,” he said. “It may take a hundred years to see noticeable changes in the system, but it’s only been a few years since the impact. Learning more about how long it takes for Dimorphos to regain stability tells us important things about its internal structure, which in turn will inform future attempts to deflect dangerous asteroids.”

Richardson and his team hope that Hera will provide more information about DART’s impact. By the end of 2026, Hera will reach the binary asteroid system containing Dimorphos and Didymos to 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 has given us a better understanding of complex gravitational physics that is impossible to do in a laboratory. All of this research helps us better tailor our efforts to defend Earth in the event of a real threat,” Richardson said. “There is a non-zero probability that an asteroid or comet will approach and endanger the planet. We now have an additional line of defense against these types of external threats.”