American Scientist

In September 2022, NASA conducted the Double Asteroid Redirection Test (DART) mission. It was the first-ever mission to focus on purposely crashing a spacecraft into an asteroid—in this case, Dimorphos—in order to change its flight path. The mission was a success, and the new book by award-winning science journalist Robin George Andrews, How to Kill an Asteroid: The Real Science of Planetary Defense, takes readers behind the scenes of DART and the larger planetary defense movement.

Inside DART’s mission control room, [mission systems engineer] Elena Adams was holding court, standing in front of a handful of reporters, and flanked at some distance by a series of stuffed animals. I didn’t know the story behind these furry friends. But it instinctively made sense to me, mixing the grandiose with the silly—even when you’re talking about a mission that’s going to practice deflecting the sort of asteroid that could destroy a city.

Someone asked Adams how hard it would be to hit Dimorphos. Deep Impact hitting Comet Tempel 1 had been far from easy, but at least the target was almost four miles long. Dimorphos is just 520 feet across, moving through space at tens of thousands of miles per hour. “Imagine you’re at the JFK airport, and you know where the Dallas airport is. And you have a dart. The dart itself is only two and a half millimeters at the tip; it’s tiny. And you take that dart from JFK, and you throw it to Dallas, and you hit the center of a bullseye on the actual dartboard. Except you don’t know where the dartboard is within the Dallas airport,” Adams explained. “So that’s how hard it is to do it.”

Mission control rooms in the real world look like you’d imagine: There are consoles and seats, and people staring at screens. It’s not too different from how they’re depicted in movies. But the monitors on the walls immediately reminded me of the dashboard of an X-Wing fighter—you know, the workhorse of the Rebel Alliance, the one that dared to do the trench run and destroy the Death Star? If you don’t know what I’m talking about, then firstly—how have you not seen Star Wars? Secondly, imagine the sorts of graphics you would have seen in an arcade game in the 1980s, with lines and circles and dots representing space stuff. That’s what those screens looked like, some of which were tracking DART’s movement through space toward a target.

Despite some anxiety about whether the impact would happen, Adams was having plenty of fun previewing it. Dimorphos, lest we forget, was a largely unknown entity; apart from a vague size estimate, nobody knew much else about it. Someone asked Adams what she thought it would look like, from DART’s unique perspective. “It’ll get bigger and bigger, and then, boom,” she said. Fair enough.

I knew that various team members had made bets among themselves about how much Dimorphos would be deflected. The minimum threshold was to reduce the asteroid’s 11 hours 55 minutes-long orbit around Didymos by just 73 seconds, but some scientists were optimistically hoping for an adjustment closer to 10 minutes. A greater time change would imply that most asteroids of this size could be knocked back by a relatively small mass, something that would bode well for defending the planet—especially if we have little warning time. But I was curious: Had bets been made about the appearance of Dimorphos?

“We did talk about starting a pool,” said Adams. They hoped it wasn’t something too weird, like a dog bone shape; that would make aiming at the center more troublesome for SMART Nav [DART’s algorithmic guidance system]. “These are kind of the pathological shapes . . . that and the donut, which we’ve trained ourselves against.” Ah yes, the donut: a ringed asteroid with a hole in the middle. Improbable, perhaps—but not impossible, for the universe is a demented sculptor.

Adams explained that they simulated a donut scenario with DART to test its mettle. Just because they could, they transformed Didymos into the Death Star, with the Dimorphos donut emerging from behind it. Once again, Star Wars had found its way into the planetary defense saga. Adams thanked Evan Smith, one of her engineers, who came up with the idea. “We flew right through the middle of the donut,” she said. “We hit, but we didn’t.” A donut-shaped Dimorphos would be a bit of an issue, it seemed.

Suddenly, one of the fluffy creatures became thrillingly relevant. Adams explained that DART has its own mascot. “And it is DART Vader.” She held up one of the plushies: It looked like a very adorable, pint-size Darth Vader, with two white eyes of slightly different shapes gleaming on his helmet-covered face. The asymmetric eyes represented Didymos and Dimorphos. Smith “was dressed as DART Vader too, for some of this,” said Adams. That may have been the greatest thing I had ever heard. Who says you can’t have fun while trying to save the world?

What about other scenarios in which the mission does not go according to plan? They had practiced nearly two dozen of them, Adams explained. What would happen if the communications relay between Earth and the spacecraft was severed at an especially awkward moment? What if the asteroid was so dimly lit that DART had difficulty tracking it? These were relatively easy hurdles to leap over. Others were more problematic, including the possibility that DART would maintain its initial lock on the larger Didymos, and refuse to switch its flight path toward Dimorphos.

What if they missed? For a moment, Adams lost some of her seemingly irrepressible zeal. “We need to make sure the spacecraft is safe,” she said. They would switch DART out of its kamikaze run, make sure it was saving what it had left of its propellant, and then hopefully command it to swing around the inner solar system in preparation for another impact attempt. “It will take us another two years to get back and try [to hit] Dimorphos again.” October 31, 2024: “Because we like to hit all the major holidays.”

An impact in 2024 would still be a success—but a lesser kind of victory, and clearly a letdown for everyone hoping to make a needle-shifting splash on the first try. Just a few weeks before my tour of mission control, this contingency seemed highly unlikely; the whip-smart algorithmic brain and nimbleness of the spacecraft meant that September 26, 2022, appeared destined to be DART’s death day. But an impact felt all but impossible when, on September 7, NASA announced that DART had gotten its first good look at Didymos—and its target asteroids looked so ridiculously tiny they may as well have been invisible. By this point, the spacecraft had traveled many millions of miles on a circuitous, circumsolar racetrack toward its goal. A composite shot made of 243 separate images, taken by DART’s eye, DRACO, depicted Didymos as nothing more than an extremely faint dot 20 million miles away, a snowflake lost in a diamond-suffused ocean. That the binary asteroid could be seen at all was good news: DRACO was operating as it should be. But that shot reinforced Adams’s analogy—hitting this small asteroid was like throwing a dart across America to impact a minute target that nobody could see.

At the press conference just before the mission control tour, I clocked Smith, the Star Wars–loving engineer—and he looked almost distressed, which was perfectly understandable considering how close we now were to impact day. Adams was a little nervous, too, but she held her extremely talented team of engineers together so well it looked deceptively breezy. Having fun with the mission certainly played a role in quashing some of the anxiety, and she wasn’t the only one to be in that mindset.