Jason DavisJun 11, 2024

What DART has taught us so far

In September 2022, a spacecraft named DART sped through space toward its destination. Sporting long, winged solar arrays, the mini-fridge-sized probe was on a mission to test whether humans could alter an asteroid’s path, validating a technique that could one day be used to save lives on Earth.

A pair of asteroids suddenly emerged from the darkness. Strewn with boulders and shards of rock, they were essentially rubble piles floating through space. Didymos, the larger asteroid, measured about 780 meters (a half-mile) wide, while the smaller asteroid, Dimorphos, had a span of about 170 meters (560 feet).

DART’s camera locked on to Dimorphos, and the spacecraft methodically steered toward its surface. Images beamed back to Earth showed the egg-shaped world getting bigger and bigger, until it filled the frame completely.

And then the transmissions stopped. DART smashed into Dimorphos at a staggering 6.6 kilometers (4 miles) per second, releasing 11 gigajoules of energy. The asteroid’s loose surface shattered and sprayed into space, forming a long tail that was easily visible through ground and space-based telescopes.

“One of the first things that we learned just from those telescopic observations on the ground was that the DART impact had a major effect,” said Terik Daly, who was a member of the DART investigation team at the Johns Hopkins University Applied Physics Laboratory. “The moment people started seeing those plumes coming off the asteroid, we knew this was going to be big.”

Dimorphos and boulders from Hubble
Dimorphos and boulders from Hubble This image from the Hubble Space Telescope shows boulders that were ejected from the asteroid Dimorphos after the DART spacecraft slammed into it in September 2022. The bright object with a tail is Dimorphos, and the tiny white dots clustered around it are boulders ranging in size from 1 to 6.7 meters (3 to 22 feet) in diameter.Image: NASA, ESA, D. Jewitt (UCLA)

Scoring a bulls-eye

Just two weeks after impact, NASA confirmed that DART changed the orbit of Dimorphos around Didymos. Whereas it once took Didymos 11 hours and 55 minutes to make an orbit, it now takes 11 hours and 32 minutes. This far exceeded the mission’s minimum success criteria of shortening the orbital period by 73 seconds.

The conclusion was clear: Humans can indeed move an asteroid. If we detect a dangerous space rock headed toward Earth, knocking it off course with a spacecraft is a potential option.

“You have lots of theories and ideas about ways that might potentially prevent an asteroid impact,” said Daly. “And for decades, they've just been that: theories and ideas. But now we can check the box and say that we know how to do this for real.”

DART demonstrated that a spacecraft can track down an asteroid and steer into it on its own, without ever having seen it before. At the moment of DART’s impact, transmissions traveling at the speed of light took 38 seconds to reach Earth, ruling out any real-time interventions from the ground. DART’s camera system, DRACO, fed images to a series of navigation algorithms that were able to identify Dimorphos and aim for the center of its lighted surface. The spacecraft scored a bulls-eye, smashing into Dimorphos just 25 meters (80 feet) away from where it was aiming.

“It's really a testament to the engineers, to the folks who built the autonomous navigation here at APL,” said Andy Rivkin, who was a DART investigation team lead at the Johns Hopkins University Applied Physics Laboratory.

Rivkin also lauded the work that astronomers did to pinpoint where the asteroids would be 11 months after DART launched. “It's getting DART to the right place, and knowing where the right place is,” he said.

From squished ball to watermelon

Although scientists heavily observed Dimorphos after DART’s impact, they don’t know exactly what happened on the asteroid’s surface. The answers will come in 2025, when the European Space Agency’s Hera mission arrives for a comprehensive follow-up survey. Scheduled to launch later this year, Hera will survey the aftermath of what happened in 2022.

One fact is already known: DART did so much damage that it actually changed Dimorphos’ shape. Around 1% of the asteroid’s mass was flung into space, forming the tail that scientists saw from the ground. The material in the tail is believed to have weighed around 10 million kilograms (22 million pounds), enough to fill about 60 rail cars.

All of that material streaming away from Dimorphos gave it a powerful shove that was 3.6 times stronger than the impact of DART itself.

“That is a key result from the DART mission,” said Daly. “The momentum enhancement contributed by the ejecta really does give you an extra push beyond what the spacecraft itself provides.”

Still more material resettled elsewhere on Dimorphos. The result of all this mayhem actually reshaped the asteroid: Before impact, it was shaped like a symmetrical, squished ball. After impact, it resembles an oblong watermelon.

Dimorphos shape change
Dimorphos shape change The impact of NASA's DART spacecraft changed the shape of asteroid Dimorphos. Before impact, the asteroid was shaped like a squashed ball. After impact it took on a more elongated shape, like a watermelon.Image: NASA/JPL-Caltech

Dimorphos also used to be tidally locked. Like our own Moon, the same side always faced Didymos, and the asteroid made a complete rotation once every orbit.

DART disrupted that. While the impact changed the asteroid’s orbital period around Didymos, it did not change its rotation period. This has created a tug-of-war between the two asteroids that is trying to force Dimorphos back into its tidally locked state.

“We think that has led to a situation where Dimorphos is now trying to point at Didymos, but sometimes it kind of flips, so the other side is pointing at Didymos,” Rivkin said.

Dimorphos with north up
Dimorphos with north up NASA's DART spacecraft captured this image of asteroid Dimorphos, which has been oriented to place its north pole at the top, using the DRACO imager.Image: NASA/Johns Hopkins APL

A global response

Just as an asteroid heading toward our planet should merit a global response, the DART team partnered with observers on all seven continents to capture the results of the spacecraft’s impact. When the big moment arrived, one of the best viewing angles came from telescopes operated by the Las Cumbres Observatory at the South African Astronomical Observatory.

“Right around the time of impact, it was telescopes in South Africa that were taking some of the initial observations that showed this rapid plume expanding,” said Daly. “If we only had U.S. people observing, we would've missed those observations entirely because it was the wrong time of day.”

Following DART’s fateful impact, scientists published their findings in two special issues of The Planetary Science Journal and Nature. All of the papers are open access for anyone to read.

Didymos and Dimorphos’ orbit around the Sun slowly carried them away from Earth, but they are now back in our neighborhood, offering new opportunities for observations.

The next planetary defense mission will be the follow-on Hera mission. After that, NASA plans to launch NEO Surveyor in 2028. The asteroid-hunting telescope has a goal of finding 90% of near-Earth objects with diameters of at least 140 meters (160 feet).

Daly said that ideally, countries would work together to develop rapid-response spacecraft that would visit Earth-bound asteroids for reconnaissance. The data they collect would help determine the best deflection technique, or in the worst-case scenario help with disaster preparedness.

“Asteroids don’t care which country you’re in,” he said. “It’s up to us to try to unite across the world to have an effective response, and move from a situation where we have this one-off test with DART toward more of an operational planetary defense capability.”

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