Crash landings are rarely celebrated in mission control rooms. The collision of NASA’s DART spacecraft with an asteroid, on the other hand, was a resounding success. The spaceship collided with Dimorphos, an asteroid moonlet orbiting Didymos, a bigger space rock. The purpose of the mission was to get Dimorphos closer to its parent asteroid, shortening its 12-hour orbit around Didymos by few minutes.

The Double Asteroid Redirection Test, or DART, is the world’s first attempt to alter the motion of an asteroid by slamming a space probe into it (SN: 6/30/20). Dimorphos and Didymos pose no threat to Earth. However, evaluating how successfully DART’s move worked will demonstrate how simple it is to tamper with an asteroid’s trajectory – a tactic that could protect the planet if a huge asteroid on a crash course with Earth is ever identified.

“We don’t know of any huge asteroids that would be deemed a hazard to Earth in the next century,” says Angela Stickle, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “We’re doing something like DART because there are asteroids out there that we haven’t discovered yet.”

According to Jessica Sunshine, a planetary scientist at the University of Maryland in College Park who is also on the DART team, astronomers have discovered nearly all of the solar system’s kilometer-sized asteroids that may wipe out civilization if they collide with Earth. However, when it comes to space rocks 150 meters wide, such as Dimorphos, “we only know where roughly 40% of those are,” Sunshine explains. “And that is something that, if it did hit, would undoubtedly destroy a city.”

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According to Mark Boslough, a physicist at Los Alamos National Laboratory in New Mexico who has studied planetary defense but is not engaged with DART, Dimorphos is a safe asteroid to give an experimental nudge. He claims that “it’s not on a collision course” with Earth and that DART “can’t hit it hard enough to put it on a collision trajectory.” The DART spacecraft is around the size of two vending machines, whereas Dimorphos is about the size of Egypt’s Great Pyramid of Giza.

DART met Didymos and Dimorphos on their closest approach to Earth, roughly 11 million kilometers away, after a 10-month journey. DART could only observe the larger asteroid Didymos until the very end of its trip. However, DART detected Dimorphos in its field of vision about an hour before impact. The spacecraft used its onboard camera to direct itself toward the asteroid moonlet, slamming into it at 6.1 kilometers per second, or over 14,000 miles per hour.

After impact, DART’s camera feed went dark. However, another probe nearby is thought to have captured the collision on video. The Light Italian CubeSat for Asteroid Imaging rode DART to Dimorphos but disembarked a few weeks before impact to observe the event from a safe distance. Its purpose was to fly by Dimorphos about three minutes after DART’s impact to photograph the crash site and the plume of asteroid debris that was released into space. Within a few days, the probe is scheduled to broadcast photographs of DART’s destruction back to Earth.

“I was absolutely elated, especially when we watched the camera approaching closer and just understanding all the science that we’re going to discover,” NASA Deputy Administrator Pam Melroy said after the hit. “But the nicest part was realizing, at the end, that there was no doubt there would be an impact, and watching the team ecstatic with their victory.”

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Dimorphos is predicted to be pushed into a closer, shorter orbit around Didymos as a result of DART’s impact. Earth-based telescopes can time the orbit by observing how the amount of light from the binary asteroid system changes when Dimorphos passes in front of and behind Didymos. “It’s a pretty well-conceived experiment,” says Boslough. Hundreds of telescopes on every continent will be watching Dimorphos in the next weeks to discover how much DART affected its orbit. Images may also be obtained by the Hubble and James Webb space telescopes.

“It’ll be incredibly interesting to see what comes out,” Amy Mainzer, a planetary scientist at the University of Arizona in Tucson who isn’t engaged in DART, says. “Asteroids have a habit of surprising us,” she says, referring to how difficult it is to determine a space rock’s specific chemical makeup and internal structure based on observations from Earth. As a result, Dimorphos’ post-impact motion may differ from what researchers expected.

The DART team will compare data from Dimorphos’ new orbit to computer simulations to evaluate how close those models were to predicting the asteroid’s actual behavior, and they will make adjustments if needed. “If we can get our models to reproduce what actually happened,” says DART team member Wendy Caldwell, a mathematician and planetary scientist at Los Alamos National Laboratory, “then you can use those models to [plan for] other scenarios that might show up in the future,” such as the discovery of a real killer asteroid.

“Whatever happens,” she says, “we will obtain knowledge that will be useful to the scientific community and the planetary defense community.”

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