NASA spent slightly more than $1 million for every day its DART spacecraft lived. From the time the ship launched on Nov. 13, 2021, to the time it deliberately smashed into the asteroid Dimorphos at 7:14 p.m. ET on Monday, Sept. 26, exactly 317 days elapsed. In the fullness of time, the $325 million the spacecraft cost might turn out to be one of the best investments NASA has ever made.
DART—short for Double Asteroid Redirection Test—was a trial run of the technology that could one day be used to protect Earth from the kind of collision with incoming space debris that wiped out the dinosaurs 65 million years ago. The deliberate cosmic crack-up NASA staged was proof that humanity indeed has the wherewithal to target a piece of interplanetary rubble, intercept it on the fly, and—potentially—redirect its trajectory enough to keep Earth safe from harm. Though it will take some time for astronomers to determine if the mission achieved its asteroid-deflection goals, NASA officials are optimistic.
“As far as we can tell, our first planetary defense test was a success,” said Elena Adams, Program Manager and DART Mission Systems Engineer, following the impact. “I think Earthlings should sleep better. Definitely I will.”
Dimorphos was a good choice for that first test. As asteroids go, it is not much to speak of. Measuring just 160 m (525 ft.) across, it is a moonlet of the larger 780-m (2,560-ft.) asteroid Didymos. The Didymos-Dimorphos duo are currently 11 million km (7 million mi.) from Earth, orbiting the sun in a path that never brings them anywhere within the vicinity of our planet. It is both that safe remove from Earth, and the particularities of Dimorphos’s orbit that made it a prime target for an initial test of deflection technology.
It takes the Dimorphos moonlet precisely 11 hours and 55 minutes to complete one orbit around Didymos. The goal of the mission was to see if a collision with a spacecraft could change that orbital period, slowing it down by about 10 minutes—or just 1%. Just such a slight pumping of the brakes when an actual piece of incoming ordnance is approaching Earth might be enough to allow our planet to fly past the point in space at which the two bodies might have collided—essentially dodging a cosmic bullet.
The match-up between DART and Dimorphos did not, initially, seem like a terribly even one. Dimorphos is estimated to weigh roughly 5 billion kg (11 billion lbs.) and is close to the length of two football fields. DART weighed just 570 kg (1,260 lbs.) and measured only 2.6 m (8.5 ft.) across. But when it comes to space physics, size is less important than speed. The faster one object is moving when it collides with another, the more of an energetic wallop it packs—and DART was moving fast, blazing toward Dimorphos at 22,530 k/h (14,000 mph). That, in theory at least, should have imparted enough energy to slow the orbit by the desired 1%.
So small is the Dimorphos-Didymos system that it was only an hour before DART’s kamikaze impact that the spacecraft’s onboard camera was able to spot the two bodies. But so fast was DART approaching that with each one-shot-per-second image the camera sent back, the two asteroids grew considerably, with the final pictures in the final seconds before impact showing Dimorphos’s rubble-and-boulder strewn surface. Then, all at once, the pictures stopped and all telemetry streaming back from the spacecraft vanished, indicating that the planned collision had indeed been achieved.
In Mission Control, at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md., Adams leapt in the air and shouted, “We have impact!”
In a later statement to the Associated Press, NASA program scientist Tom Statler said, “Normally, losing signal from a spacecraft is a very bad thing. But in this case, it was the ideal outcome.”
DART likely vaporized on impact, but some of the mission’s hardware remains. Carried aboard the ship was a toaster-sized spacecraft built by the Italian Space Agency (ISA), dubbed the Light Italian CubeSat for Imaging Asteroids (LICIACube). The little free-flier separated from DART on Sept. 11, and its job was to follow along and swoop close enough to the impact point on Dimorphos to take images of the crater and the plume of ejecta the impact would cause, but not so close that it would be damaged by the flying debris. The pictures the LICIACube took will take weeks to be beamed back to Earth, given the tiny spacecraft’s even tinier antenna, but they could yield additional information about the kind of scar DART left in Dimorphos, which could tell NASA more about not just the make-up of Dimorphos, but about how how big an impactor spacecraft needs to be in order to have a significant effect on an asteroid.
The real measure of the mission’s success, of course, will be whether DART indeed shaved the expected 10 minutes off of Dimorphos’s orbit, and that will not be known for a few weeks at least. Dozens of ground- and space-based telescopes around the world are already training their focus on Dimorphos, tracking its path as it circles Didymos, timing its orbit and comparing their findings.
What the telescopes discover will matter—a lot. NASA’s Center for Near Earth Object Studies (CNEOS) keeps a running tally of asteroids that fly within 45 million km (28 million mi.) of Earth, considered close enough that even a slight change in their trajectory—perhaps caused by a collision with another piece of space debris—could send them hurtling our way. According to CNEOS’s census, there are 855 such asteroids measuring at least 1 km (0.62 mi.), and more than 10,000 that are at least 140 m (460 ft.) across. Overall, there are 29,801 known near-Earth asteroids of all sizes in the CNEOS database.
“Planetary Defense is a globally unifying effort that affects everyone living on Earth,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate, in a statement. “Now we know we can aim a spacecraft with the precision needed to impact even a small body in space. Just a small change in its speed is all we need to make a significant difference in the path an asteroid travels.”
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