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.
