In a clean room in Building 23 at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, a spacecraft called DART was splayed open like a fractured, cubic egg. An instrument called a star tracker—which will, once DART is in deep space, ascertain which way is up—was mounted to the core, along with batteries and a variety of other sensors. The avionics system, DART’s central computer, was prominently attached to square, precision-machined panels that will form the sides, once the spacecraft is folded up. Wires ran from the computer to the radiosystem that DART will use to communicate with Earth. Gyroscopes and antennas were exposed. In a room next door, an experimental thruster system called NEXT-C was waiting its turn. Great bundles of thick tendrils wrapped in silver insulation hung down from the spacecraft and ran along the floor to the control room, where they connected to a towering battery of testbed computers operated by four engineers.
A clock over one of the computers read, “Days to DART Launch: 350:08:33.”
DART—the Double Asteroid Redirection Test—is designed to crash into an asteroid called Dimorphos. The impact will change Dimorphos’s speed by about one millimeter per second, or one five-hundredth of a mile per hour. Though Dimorphos is not about to collide with Earth, DART is intended to demonstrate the ability to deflect an asteroid like it that is headed our way, should one ever be discovered.
Since a Soviet probe called Luna 1 became the first spacecraft to escape Earth’s orbit on January 2, 1959, humanity has sent about 250 probes into the solar system. DART is unique among them. It is the first that sets out not to study the solar system, but to change it.
By 1980, astronomers had determined the orbits of about 10,000 asteroids, including 51 “near-Earth” asteroids (along with 44 near-Earth comets). Today, the numbers have swollen: the Minor Planet Center keeps track of about 800,000 asteroids in total, of which almost 24,000 have orbits that take them close to Earth. The vast majority of these have been discovered since 1998, when Congress gave NASA 10 years to identify every near-Earth object larger than one kilometer (0.6 miles) in diameter. Thanks to statistical analyses, astronomers believe they’ve found about 95% of the big near-Earth asteroids, the kind that would destroy civilization were they to hit our planet.
Earth moves the distance of its diameter every seven minutes. If the arrival time of an incoming object can be changed by more than about 10 minutes, it will miss us. (The details, of course, depend on the particular trajectory; the extra three minutes are to account for the effect of Earth’s gravitational pull.)
Didymos is about a half-mile across. Dimorphos is about 500 feet in diameter—about the size of a small sports stadium. Nobody yet knows what it looks like, because it is too small and far away for detailed observations from telescopes on or near