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Wednesday, July 24, 2024

DART Showed How to Smash an Asteroid. So Where Did the Space Shrapnel Go?

Nearly one year ago, NASA flung the DART spacecraft into the asteroid Dimorphos at 14,000 miles per hour. It was the first test to see whether they could slightly deflect a space rock’s trajectory using a high-speed collision, a technique that could be used to protect Earth from future killer asteroids. It worked. But now they’re trying to figure out the details of the crash. And if people have to defend earthly life from a potential asteroid impact, those details will surely matter.

Scientists are starting by studying the ejecta, boulders, and numerous smaller bits the strike cast off. They predicted there would be debris, but they didn’t know exactly what to expect. After all, compared to stars and galaxies, asteroids are tiny and dim, so it’s hard to ascertain their density and composition from afar. When you strike one, will it simply bounce? Will the probe thud into it and create a crater? Or if the asteroid is brittle, will slamming a craft into it risk creating space shrapnel that is still big enough to threaten Earth?

“This is exactly why we needed to do an in-space test of this technology. People had done laboratory experiments and models. But how would an actual asteroid, of the size we’re concerned about for planetary defense, react to a kinetic impactor?” says Nancy Chabot, the DART coordination lead and a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory, which developed the craft in partnership with NASA.

Many asteroids appear to be “rubble piles,” dirt, rocks, and ice loosely held together, rather than something hard and dense like a billiard ball. The asteroid Ryugu, visited by the Japanese space agency’s Hayabusa2 in June 2018, and the asteroid Bennu, which NASA’s OSIRIS-REx took samples from in 2020, both count as rubble piles. A new study published in July in Astrophysical Journal Letters shows that Dimorphos appears to be built like that too, which means that an impact is likely to create a crater and to fling off debris on or near the asteroid’s surface.

To figure out what happened after the crash, David Jewitt, a University of California, Los Angeles astronomer, and his colleagues used the Hubble Space Telescope to zoom in repeatedly on Dimorphos. The combined deep observations allowed them to discern objects that are otherwise too faint to see. A few months after the DART probe’s impact, they found a swarm of about three dozen boulders not seen before—the largest of which is 7 meters in diameter—slowly drifting away from the asteroid. “It’s a slow-speed cloud of shrapnel from the impact that’s carrying away a significant amount of mass: about 5,000 tons in boulders. That’s quite a lot, considering the impactor itself was only half a ton. So it blew out a tremendous mass in boulders,” Jewitt says.

Other researchers, including the DART team, have also been investigating the cloud of rocks thrown off by the spacecraft’s swift punch. Chabot and her colleagues published a study in Nature earlier this year, also using Hubble photos, imaging the ejecta. They showed that at first the pieces flew off in a cone-shaped cloud, but over time, that cone turned into a tail, not so different from a comet’s tail. That finding also means that models of the behavior of comets could be applied to impactors like DART, Chabot says.

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Dimorphos was never a threat to Earth, but details like these would matter in a real asteroid deflection scenario. Boulders and smaller ejecta would have to be knocked out of the way, along with the rest of the asteroid, in order to spare the planet. Or let’s say the asteroid wasn’t spotted until it was very close to Earth, and its trajectory couldn’t be altered enough to avoid a crash. Could it at least be pulverized into boulders small enough to burn up in Earth’s atmosphere? “Is it better to be shot by a high-velocity rifle bullet or a bunch of pellets from a shotgun?” asks Jewitt. “The answer is: The shotgun is better, because the smaller boulders are more likely to be cushioned or dissipated by the impact with the atmosphere.” 

A NASA-funded project is studying exactly that scenario. It sounds like something out of Deep Impact, Armageddon, or Don’t Look Up. But it’s possible that a small but dangerously sized asteroid could evade detection until it’s mere weeks away, rather than years or decades, says Philip Lubin, a University of California, Santa Barbara astrophysicist who leads the project. NASA and other organizations track as many near-Earth objects as possible to get a long warning time. But while they have spotted almost all the potential planet-killers in the solar system, they’ve found less than half of the ones 140 meters across or larger. Those are big enough to destroy a city, causing widespread devastation. 

In fact, just this summer, an asteroid called 2023 NT1 came from the direction of the sun, and no one spotted it until July 15, two days after it flew within 60,000 miles of the Earth. According to a new study Lubin and his team are completing, it’s probably around 30 to 60 meters across, a bit larger than the meteors that hit Tunguska in 1908 and Chelyabinsk in 2013, both sparsely populated parts of Russia. It would’ve been big enough to inflict large-scale damage if it had hit the Earth. 

Lubin and his colleagues are using computer simulations to explore the idea of throwing one or more large bullet-shaped interceptors, rather than a boxy DART-like spacecraft, into an asteroid. The vending-machine-sized DART ultimately achieved a minor deflection of the 160-meter Dimorphos, shortening its 11-hour, 55-minute orbit around the larger asteroid Didymos by 32 minutes. Lubin’s team suggests that instead of merely putting a ding into an asteroid and nudging its trajectory, they could instead penetrate its heart so that the ensuing shockwave pulverizes it, like a jackhammer breaking up concrete into manageable chunks. “We found that we could, in theory, take apart Dimorphos completely—which probably people would be bummed about—with a modest interceptor. Instead of making a dent in it, we could destroy it,” Lubin says. 

Lubin’s team’s work suggests a short warning time might not mean the end of the world. Their simulations show that a SpaceX Falcon 9, like the one that propelled DART into space last year, or a larger rocket could launch such an interceptor and blow a 160-meter asteroid apart. They think that any resulting rocks would be small enough to not be dangerous if they continued on their Earthward trajectory. 

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In the meantime, scientists are working on getting a closer look at what DART wreaked. As Dimorphos and Didymos continue on their path around the sun, by the spring of 2024 they’ll be close enough that they’ll be easier for Hubble and ground-based telescopes to spot. The European Space Agency is also sending a follow-up mission, called HERA, to inspect the aftermath of the impact. HERA is planned to launch in October 2024 and reach Dimorphos in late 2026. 

Then, in mid-2028, NASA plans to launch NEO Surveyor, which is designed to find at least two-thirds of near-Earth objects 140 meters or larger—potentially hazardous Dimorphos-sized asteroids. It will use infrared sensors, which must be deployed in space since Earth’s atmosphere blocks most infrared light.

Chabot hopes to see more planetary defense-related missions after that. Last year, in a once-in-a-decade report, planetary scientists endorsed investing in a variety of asteroid defense techniques, not just kinetic impactors like DART. These include using ion beams to deflect them, or using the “gravity tractor” technique to pull one onto a slightly different course by flying a spacecraft beside it for years. It’s important to have more than one tool available, Chabot says. “We’re proud of DART, and it brought a lot of attention to planetary defense,” she says. “But there’s a lot more to do and test out to be in a place where we could protect our planet in the future.”

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