On the evening of September 26th, Elena Adams, the lead engineer for NASA’s asteroid-smashing DART spacecraft, peered at the data streaming to her computer console in mission control, at the Johns Hopkins University Applied Physics Laboratory, in Laurel, Maryland. Some forty other engineers were crammed into the room with her, sitting at rows of similar stations or gazing at large telemetry displays mounted on walls emblazoned with NASA heraldry. The DART spacecraft was seven million miles from Earth; after charting a ten-month, hundred-and-one-million-mile course around the sun, it had squared up for its terminal run against an asteroid called Dimorphos. If all went according to plan, DART would collide head on with Dimorphos at fourteen thousand miles an hour, fundamentally deforming the asteroid and changing its orbit. For the first time in more than sixty years of spaceflight, our species would be not just exploring the solar system but rearranging it.
Adams, who is forty-two, animated, and athletic, with a slight accent that reflects her Russian birth, had spent the run-up to impact day alongside her team, feeding the spacecraft commands, interpreting the numbers and images that it sent back, and making small course corrections. By this point in the mission, however, DART was essentially cruising on its own. Communications signals travel at the speed of light, but the asteroid was so far away that it was impossible for the engineers to manage the mission’s conclusion remotely. Now, as the spacecraft prepared for its final approach, an unsettling development was taking place. The mission planners had hoped that they might catch sight of Dimorphos as early as a hundred and twenty minutes before impact. But DART was just eighty minutes out, and Dimorphos still hadn’t appeared.
It was O.K., Adams reassured herself—they had rehearsed for this, considering a large number of ways in which things could go wrong. Dimorphos, the spacecraft’s target, is actually a moon, orbiting a larger, half-mile-wide asteroid called Didymos, which astronomers have deemed a “potentially hazardous object” because of its size and proximity to Earth. (DART is an acronym: the mission’s full name is the Double Asteroid Redirection Test.) Maybe Dimorphos—which, at a little more than five hundred feet across, was the smallest object that NASA had ever targeted—was, for some reason, concealed by Didymos. Maybe it was darker than expected. Maybe it was simply very small: the engineers anticipated that, when it first appeared, it would register only as a single pixel. Whatever the reason, if the moonlet did not materialize in the next ten minutes, they would have to find some way to save the mission. Adams didn’t want to panic the team; she approached a few colleagues discreetly, pulling them into a huddle at the back of the mission-operations center. “Let’s start thinking about our contingency plans,” she said. “Go back to your seats and pull them up, quietly. Be ready to go.”
DART, like all space missions, had experienced its share of glitches. The spacecraft incorporated several experimental technologies, including NEXT-C, a new kind of high-performance, fuel-efficient ion engine, and ROSA, or “roll-out solar arrays.” ROSA had worked flawlessly: during launch, just after DART separated from its rocket, two twenty-eight-foot-long panels scrolled outward into space on either side of the craft, like wings made of parchment. But NEXT-C gave everyone a fright. During its two-hour test firing, in December, the ion thruster emitted a sudden hundred-amp electrical surge. Fortunately, NEXT-C was not mission-essential—it had been added to the craft solely for testing purposes—and Adams made sure that the ion thruster stayed switched off from then on.
Later, though, the DART team wrestled with a more serious issue: nineteen days before impact, the spacecraft was fifty miles off course. The engineers suspected that the craft’s ordinary thrusters, which it was firing periodically to point its antenna at Earth, were to blame. The team ordered the craft to clean up its flight path by means of a trajectory-correction maneuver. But twelve days later—just a week before impact—they discovered that DART was now somehow seventy miles off target. Eventually, they determined that the spacecraft wasn’t reacting properly to the thruster-firing orders; sometimes the thrusters fired multiple times. The engineers implemented a software fix, and the issue disappeared.
The team had spent days addressing the drift issue, but now they would have only eighty minutes to implement changes if DART failed to locate Dimorphos. Of all the contingency plans that they’d drawn up, Adams dreaded one in particular: Contingency No. 21. This was the plan that they’d initiate if DART missed the target. It would mean turning on the troublesome NEXT-C ion engine and using it to circle the sun yet again, then trying to strike the object two years from now.
That morning, Adams had placed specially made fortune cookies under the console seats in the mission-operations center. The fortunes read, “Today you will make an impact.” She glanced now at the team seated at their computers, at the telemetry data on the overhead displays, and at the digital clock on the wall, ticking away its bright red seconds in Universal Coördinated Time. She’d slept terribly the night before, and now felt every moment of the sleep she’d missed. She needed that pixel to appear.
As recently as a few decades ago, no one was very worried about killer asteroids. It wasn’t until the nineteen-nineties when the vast and largely submerged Chicxulub crater, near the Yucatán Peninsula, became widely recognized as a mark left by the asteroid that killed the dinosaurs. Fears increased in 2004, when astronomers discovered an asteroid that would speed past Earth in 2029, with a 2.7-per-cent chance of direct impact. Apophis, as the asteroid would later be named, is more than a thousand feet across—large enough to wipe out a large city or possibly trigger a tsunami. In response, Congress directed NASA to find and characterize at least ninety per cent of near Earth objects larger than four hundred and sixty feet in diameter within fifteen years.
Astronomers later determined that Apophis will not strike the Earth—at least not for a long time. But, in 2013, a sixty-six-foot-long rock punched through the atmosphere and exploded over Chelyabinsk, Russia, with the energy of roughly two dozen atomic bombs. The resultant shock wave damaged thousands of buildings and injured hundreds of people. Chelyabinsk galvanized NASA, which had been moving slowly in its asteroid-finding efforts; astronomers have now discovered just about every kilometre-size “dinosaur killer” asteroid out there, Tom Statler, the DART program scientist at NASA headquarters, told me. But the agency is still only forty per cent of the way to its congressionally mandated goal of finding smaller asteroids. Accordingly, it is currently developing the Near-Earth Object Surveyor, a space-based infrared telescope that will be focussed on the task. “The essence of planetary defense now is to find these potential hazards,” Statler said. “You can’t do anything about them unless you know they exist.” The surveyor will launch as early as 2026, depending on congressional appropriations.
In 2016, the agency established the Planetary Defense Coordination Office; its head, Lindley Johnson, is the nation’s first planetary-defense officer. “If we find an object on an impact trajectory, NASA is not going to be doing the response all on its own,” Johnson, who joined NASA after twenty-three years in the U.S. Air Force, where he worked on national-security space missions, told me. “We will be coördinating activities across U.S. agencies, and also coördinating with our international partners, because it’s very much an international problem.” Any response would probably include the Department of Defense, the Department of Energy, the Federal Emergency Management Agency, the International Asteroid Warning Network, the United Nations Office for Outer Space Affairs, and multiple European agencies. Johnson’s job is partly to manage the tumult that will erupt across the world if an asteroid threat is detected, but his office’s ultimate mission is to stop the asteroid before it strikes. DART is currently the centerpiece of that effort.
The DART mission began in the mind of Andy Cheng, a planetary scientist at the Applied Physics Laboratory. It was early 2011, and Cheng, who had started at A.P.L. in 1983, was in his basement, doing his morning stretches on a yoga mat. He got to thinking about asteroids. Cheng knew that, if you could spot an incoming asteroid when it was far enough from Earth, you could crash something into it, changing its velocity and therefore its course. A small change could add up, more than millions or billions of miles, to a big alteration, causing the asteroid to miss the Earth—which races around the sun at about a thousand and eighteen miles per minute—by tens of thousands of miles or more. The Earth travels the complete distance of its diameter every seven minutes; if would-be planetary defenders could delay the impact of a doomsday asteroid by just ten minutes—the extra three minutes account for Earth’s gravity—it would fly peacefully by.
But there were problems with this plan. To launch such a mission, we would need to know for sure that our assumptions about how asteroids react to collisions were correct. Ideally, we’d plow a practice spacecraft into an asteroid with the clear intent of changing its orbit, then measure that change. Yet any initial alteration in an asteroid’s flight path was likely to be small—measurable, maybe, in centimetres per second, a scale too minute for detection by Earth-based telescopes. A second probe, sent along with the first, could track the change. But a two-craft mission would cost the better part of a billion dollars—money that NASA might not be willing to spend.
Cheng’s eureka moment came when he thought about how astronomers find the far-off worlds that circle distant stars. They don’t do so optically, by snapping photos of planets, but through a process called light-curve analysis—the measurement of luminosity across time. When we look at the stars through Earth’s wobbly atmosphere, they seem to twinkle; when we look at them in space, their light is largely fixed and unblinking. When space-based telescopes notice a nearly imperceptible dip in a star’s light, this is sometimes because a planet has briefly swept in front of it. In 1952, an astronomer named Otto Struve proposed that astronomers could search for planets by studying these oscillations in starlight; researchers have since used Struve’s methods to discover more than five thousand exoplanets orbiting stars all across the Milky Way.
Cheng realized that we could use the same technique closer to home. Suppose that an asteroid had its own tiny asteroid moon. If the pair were oriented just so, and if sunlight hit both asteroids in just the right way, then each time the moonlet eclipsed the asteroid relative to Earth the main asteroid’s light would dim. Through light-curve analysis, astronomers could measure precisely the speed at which an asteroid moonlet was circling its parent; they could also measure changes in that speed. To test the effects of a spacecraft-to-asteroid impact, therefore, you wouldn’t need two probes—all you’d need was a “binary asteroid” in the near Earth region of space, plus a telescope on Earth. The mission would be cheaper by hundreds of millions of dollars, and the data collected could apply to future impacts with any asteroid, big or small, with or without a moon.
After selecting the Didymos system, the scientists and engineers at A.P.L.—which had designed the New Horizons mission to Pluto and the MESSENGER mission to Mercury, and developed missile technology for the Department of Defense—devised a nonintuitive approach to changing the speed of its moonlet. (They changed its name from Didymos B to Dimorphos—a Greek word meaning “having two forms.”) The craft’s head-on collision would initially slow Dimorphos, and the brief reduction in its speed would cause it to fall slightly closer to its parent. As a result of the law of conservation of angular momentum, Dimorphos would then begin orbiting faster than it had previously, in more or less the same way that a slowly twirling ice skater who pulls her arms inward will speed up. Their goal was to increase the speed of Dimorphos by at least ten per cent. If their models were right, this could ultimately alter the trajectory of both asteroids. Dimorphos acts as a “gravity tractor” for Didymos, Cheng told me. “Even though we changed the orbit of the small moon, it is tugging on the main guy as well, and it changes the orbit of the entire system,” he said.
When I visited A.P.L., fourteen months before launch, in September, 2020, the DART spacecraft lay in a clean room. When complete, DART would be a cube, about six feet on a side. But at that time its outer panels were disassembled and clamped onto work benches; engineers had bolted various pieces of equipment—star trackers, gyroscopes, antennas—onto the panels, and cables connected some of it to a bank of computers on the other side of a plexiglass window. The engineers were feeding the spacecraft commands and scenarios, and it believed that it was firing its thrusters and flying through space.
Adams, wearing a head-to-toe white protective suit that matched my own, met me in the clean room to show me around. I marvelled at the spacecraft’s home-brew quality. Spaceship parts can’t be bought at Best Buy; if engineers need an avionics system, they must build it themselves. The same goes for thrusters, wiring, cameras, and solar arrays. Nearly every part in the spacecraft had been handmade. I told Adams that I was struck by its utilitarian design—there were no swept-back fins, and the craft didn’t have a pointed cone to sharpen its impact. “It really is just ½mv2,” Adams said, using the equation that describes how mass and velocity transform the momentum of objects. She widened her eyes behind her mask. “It’s just a kinetic transfer.”
On DART’s impact day, as Dimorphos failed to materialize on the screen, Adams contemplated her list of contingency plans. In planning the mission, the engineers had considered a seemingly endless list of possible mishaps. SMART Nav, the spacecraft’s autonomous targeting computer, might fail to lock onto Dimorphos, and instead remain on course for the larger Didymos. The star tracker, which the spacecraft used for navigation, might become confused or disengaged. DART’s maneuvering thrusters might push the spacecraft too far, or not far enough; its solar arrays might tilt slightly too far from the sun, reducing the amount of power available.
Or maybe Dimorphos itself—which had never before been imaged optically—might have had some surprise in store. Most asteroids are shaped like potatoes. What if Dimorphos was like a doughnut—round with a big hole in the center, right where DART was aiming? The spacecraft would fly clear through it. (No doughnut-shaped asteroids have been discovered, but A.P.L.’s scientists had still considered the possibility.)
Twenty-one contingency plans had been drawn up to address many of these scenarios. Now one of them—Contingency No. 21—might have to be implemented in the next ten minutes. For the past five years, DART had been the single organizing force of Adams’s professional life; now its fate might come down to a split-second decision.
SMART Nav remained locked onto Didymos, searching desperately for the Dimorphos pixel. Then Mark Jensenius, one of the developers of SMART Nav, raced over with news. The spacecraft had found a white dot on a field of black. It was Dimorphos. DART soon started the first of several “burns,” its thrusters firing as it maneuvered toward its new target.
Thirty minutes before impact, Adams spoke into her headset, polling the rest of the room for a systems check. “This is DART M.S.E.,” she said, identifying herself as the mission-systems engineer. “It is time for the last status poll.” She paused and then celebrated: “Yes!”
Applause filled the room.
“Image quality?” Adams asked.
“Still looking very good,” an engineer replied. “Dimorphos still tracking along that same brightness predict as Didymos.”
“That’s great. . . . SMART Nav?”
“SMART Nav is looking nominal,” Jensenius said. “We are at under thirty metres of projected miss distance right now.”
Adams looked at her screen. “That’s looking fantastic.”
Soon, a soft-spoken voice sounded over the headsets. “M.S.E., this is S.N.5,” it said. It was the SMART Nav console.
“Go ahead, S.N.5,” Adams said.
“We are precision-locked and still tracking Dimorphos.”
“Yes!” Adams said again. Relief was audible in her voice. They had crossed the final milestone before the impact event; the spacecraft had figured out everything that it needed to know in order to hit the asteroid squarely. “We’re about forty-five hundred miles away from Didymos and Dimorphos,” she said. “Let’s see what happens.”
On the screen, Dimorphos had grown to a few fuzzy pixels. The camera remained centered on the moonlet as the spacecraft fired its thrusters to stay lined up. Now only five minutes remained until impact.
“Contingencies done!” Adams said into her headset, laughing. The window had closed for sending any further commands to the spacecraft. DART was on its own. As the team applauded, she stood and pushed her rolling chair out of the way. The scientists and engineers talked excitedly as the surface of Dimorphos became visible. Boulders were scattered across a gray landscape, and the moonlet soon supplanted Didymos entirely in the camera’s field of view. Thirty seconds before impact, Dimorphos seemed to balloon in size, filling the screen as the spacecraft approached. At 7:14 P.M.—exactly as predicted—the DART spacecraft, coursing through space at four miles per second, collided with the asteroid moon. The final image it sent back to the Earth was that of a rock-strewn field—a geologist’s dream, forever altered. Faced with the last thing that DART ever saw—the terminal second of so many long years and late nights crystallized on every console screen and overhead display—some team members embraced, some clapped and cheered, and some just stared, agog at what they had done.
The DART mission is not finished. Astronomers and planetary scientists will study the effect of the impact for months and years, applying their findings to models of other small bodies in the solar system. A small CubeSat called LICIACube, which was built by the Italian space agency, recorded the impact and will continue to return images of DART’s final moments, and of the huge cloud of debris that was ejected from Dimorphos, over the next several months. The European Space Agency plans to launch a spacecraft called Hera, in 2024; it will fly to Dimorphos and Didymos and survey their mass and composition, and the fallout from the impact. This will further constrain our models, better preparing us for danger from above. Astronomical observatories in space and around the world also watched the impact and will attempt to tease some science from the data collected.
To Adams, it all seemed a little unreal. But, that night, a network of telescopes called ATLAS—the Asteroid Terrestrial-Impact Last Alert System—sent over the first images that it had captured of the impact. Through the years, Adams had always seen the asteroid as the same hazy blob in space; now, in the ATLAS images, she could see a huge, bright plume—an explosion millions of miles from Earth. “I sent it to the whole team,” she said. “We did that—look at that.” They had, in fact, made an impact. ♦
