In a first, NASA changed an asteroid’s orbit around the sun—by accident

Sooner or later, humanity will spot an asteroid heading our way—one capable of destroying an entire city or even a whole country. That’s why, back in 2022, NASA ran a dress rehearsal for defending the planet: They deliberately smashed an uncrewed spacecraft into an innocuous asteroid to change its path.

The target was a 525-foot-long moonlet named Dimorphos that orbits a 2,550-foot-long asteroid named Didymos. Neither poses a threat to Earth, and altering the smaller asteroid’s orbit around its larger companion was extremely unlikely to change that. The mission, known as the Double Asteroid Redirection Test (DART), was a success, demonstrating that swatting an asteroid away from Earth is possible.

But painstaking telescope observations of the pair now reveal that DART’s kamikaze rendezvous with Dimorphos was so forceful that the recoiling moonlet gave Didymos a gravitational jostle, shifting both asteroids onto a different orbital racetrack around the sun.

In other words, “by hitting the moon as hard as we did, we also moved the giant thing next to it a little bit,” says Andy Rivkin, a planetary astronomer at the Johns Hopkins University’s Applied Physics Laboratory and one of the authors of the new study, published March 6 in the journal Science Advances. It marks the first time that humanity has changed the solar orbit of an asteroid.

DART’s overachieving sucker punch 

Binary asteroids, like Didymos and Dimorphos, share a gravitational center. That center is dominated by the heftier object—in this case, Didymos, which is 200 times more massive than its moonlet. But if you give the smaller object a thwack, its larger sibling will also ‘feel’ it.

Prior to DART’s 2022 collision with Dimorphos, scientists had to think through every possible outcome to the mission, including several truly grim scenarios. “What if this experiment puts the Didymos system on a collision course with the Earth?” says Rahil Makadia, another study couthor and a planetary defense researcher at the University of Illinois at Urbana-Champaign. “That’s obviously not desirable. So, we looked into it.”

They found there would be no detectable effect on Didymos. It would notice the impact on Dimorphos, but Didymos itself wouldn’t budge. 

At the time, NASA said that for the mission to be considered a success, DART would have to change Dimorphos’s orbit around Didymos by 73 seconds. Instead, the small, van-sized spacecraft ended up shrinking the asteroid’s orbit by 33 minutes—thanks to the force of DART’s sucker punch and the eruption of rocky debris that blasted off Dimorphos upon impact.

Prior to the mission, astronomers suspected that Dimorphos was what’s known as a rubble pile: instead of a behemothic singular rock, it’s more like a fleet of boulders barely held together by the asteroid’s weak gravity. Hit it with a spacecraft at 14,000 miles per hour, and some of it will inevitably be shorn off into space.

(Read more about the swarm of debris that DART left in its wake.)

But DART’s impact liberated far more debris than anyone was expecting. That spray acted like a rocket’s jet vigorously propelling the asteroid backward, far more so than most had predicted.

Everyone who witnessed this drama unfold all thought the same thing: “This must have unexpected consequences,” says Federica Spoto, an asteroid dynamics researcher at the Center for Astrophysics, Harvard and Smithsonian, and who was not involved with the new study. If Dimorphos was affected in such a profound way by the spacecraft, then what might have happened to Didymos?

Garden snails versus asteroids

Since DART’s demise in 2022, Makadia and his team tracked Didymos and Dimorphos using a technique called stellar occultation. It’s a troublesome affair that requires being at the right telescope, at the right time, to see a celestial object pass in front of a distant star. Based on how the object temporarily blocks out the starlight, astronomers can ascertain how quickly, and in what direction, it’s moving through space.

Thanks to nearly two-dozen stellar occultations, Makadia’s team determined that the asteroid pair had slowed down—but only by 22 millionths of a mile per hour. For context, a typical garden snail moves about 1000 times faster.

Unlike the shift in Dimorphos’s orbit around Didymos, the alteration to Didymos’ circumsolar voyage was miniscule—“equivalent to moving the Didymos system the length of the Eiffel Tower over the course of a year,” says Cristina Thomas, a planetary astronomer at Northern Arizona University and who was not involved with the study.

“This is a tiny, tiny, tiny change,” says Makadia. But over time, subtle changes can stack up to drastically transform the orbits of asteroids. Just to be safe, they ran the numbers to see where Didymos and its moonlet might end up in the long run.

Don’t worry. “We’re safe from Didymos impacting the Earth,” says Makadia.

(These MIT students drafted a blueprint for saving Earth.)

DART’s lasting legacy

The measurements show that astronomers can detect unfathomably precise asteroid orbit changes in the name of planetary defense. “This is fantastically impressive,” says Thomas. 

As a bonus, the interlinked jiggling of both Dimorphos and Didymos allowed the team to accurately determine how dense both asteroids were. Dimorphos has a density not too much higher than water, which helps explain why it changed shape like a fluid when DART plunged into it. Didymos is considerably denser, more mountain-like. 

Knowing the densities of different asteroids matters if you’re hoping to save the world. If you recklessly try to deflect a rubble pile like Dimorphos by hitting it too enthusiastically with a DART-like spacecraft, it could fragment into multiple earthbound shards. Conversely, a rigid asteroid like Didymos won’t jettison huge amounts of debris, meaning you’ll need multiple DART-like interceptors—or a single nuclear bomb-armed spacecraft—to get the deflection you desire.

Later this year, the European Space Agency’s Hera spacecraft will arrive at Dimorphos to forensically survey the wreckage left by DART. It will no doubt offer new revelations about humanity’s first-ever planetary defense experiment. The spacecraft may have exploded into a million pieces back in 2022, but it left oodles of interesting science behind that could help defend Earth if (or when) humanity detects an inbound space rock.