NASA IS USUALLY pretty careful with its space probes. But this time, with DART, it’s different. A team of scientists has now deliberately plowed a craft into a tumbling space rock at high speed. Mission accomplished.
It’s just a test, an effort to determine whether an asteroid can be nudged off its course—a strategy that could be used to divert a near-Earth object on a collision course with us if it’s spotted well enough in advance. This particular test subject is called Dimorphos, and it’s about 6.8 million miles from Earth. It’s actually the diminutive member of an asteroid pair: It’s a moon of its much larger sibling, Didymos.
The DART spacecraft is about the size of a vending machine, and it was hurtling at a ludicrous 14,000 miles per hour as it smashed into Dimorphos. As the craft sped along on its final approach, the DART team—watching from mission control—met each milestone with cheering and applause. “It went from a collection of individual pixels, and now you can see the shape and shading and texture of Didymos, and you can see the same thing with Dimorophos as we get closer and closer. This is so cool,” said Lori Glaze, NASA’s Planetary Science Division director, two minutes before impact.
The last shots from the craft’s camera revealed Didymos to be a slightly egg-shaped rock, littered with boulders and pockmarked with craters. The images quickly grew in size and then—the screen went blank. Loss of signal. That confirmed the spacecraft’s collision, and the room rang out with the shouts of researchers:
“Oh my goodness!”
“We got it!”
NASA scientists believe that the asteroid got dented but didn’t entirely break up, and they expect that the impact may have slightly shortened its orbit around Didymos. If true, that would demonstrate that a collision with a probe can alter an asteroid’s trajectory. As astronomers continue studying the asteroid pair in the coming weeks, the DART team will be able to assess exactly how well that worked.
Shortly after the crash, NASA administrator Bill Nelson congratulated and thanked the team, saying, “We are showing that planetary defense is a global endeavor, and it is very possible to save our planet.”
Dimorphos is on the small side, spanning 525 feet—which is about the size of the Great Pyramid. While it was never a threat to Earth, plenty more asteroids (and comets) of similar size proliferate in orbits closer than the asteroid belt, including some that NASA and its partners haven’t discovered yet. If a bigger space rock were to collide with us, humanity would likely go the way of the dinosaurs.
In 2005, Congress created a mandate for NASA to find asteroids larger than 460 feet in diameter, and so far the agency has detected and tracked almost all of the really huge near-Earth objects. (A privately-funded effort is also hunting for asteroids.) But NASA and its partners have found less than half of the asteroids that are smaller than that—maybe only 40 percent or so, says Tom Statler, program scientist at NASA’s Planetary Defense Coordination Office. Those are still big enough to demolish a whole city or even a country, if they were to hit the Earth.
“This is the first time we’ve actually attempted to move something in our solar system with the intent of preventing a [potential] natural disaster that has been part of our planet’s history from the beginning,” says Statler.
The DART probe—the name is short for the Double Asteroid Redirection Test—has been in the works since 2015. It was designed, built, and operated by Johns Hopkins University’s Applied Physics Laboratory, with support from many NASA centers, and launched last November. DART is a major part of AIDA, the Asteroid Impact and Deflection Assessment, a collaboration between NASA and the European Space Agency. The mission also depends on observatories in Arizona, New Mexico, Chile, and elsewhere; astronomers are keeping their telescopes focused on Dimorphos and Didymos to measure the post-impact deflection as precisely as possible.
Until the very end of DART’s flight, astronomers could see Dimorphos and Didymos only as a single dot of light. The smaller asteroid is so tiny it can’t be seen from Earth telescopes—but astronomers can track it by measuring how often it dims the already faint light from its bigger sibling as it orbits around it.
The craft’s final approach was captured by its optical camera, called DRACO, which is similar to the camera aboard New Horizons, which flew by Pluto. Even this much more close-up camera was able to see Dimorphos only as a separate object a few hours before impact.
“Because you’re coming in so fast, it’s only within the last few minutes that we’ll get to see what Dimorphos looks like: What is the shape of this asteroid we’ve never seen before?” said Nancy Chabot, planetary scientist at Johns Hopkins University and DART’s coordination lead, in an interview a few days before the impact. “It’s really only within the last 30 seconds that we’ll resolve surface features on the asteroid.”
In fact, until today, scientists weren’t really sure whether the asteroid would be more like a billiard ball or a dust ball. “Is this moon a single giant rock, or is it a collection of pebbles or particles? We don’t know,” said Carolyn Ernst, a JHU researcher and DRACO instrument scientist, speaking before the impact. Its makeup could affect a number of variables scientists want to study: How much the crash will alter the asteroid’s trajectory, if it’ll leave an impact crater, rotate the asteroid, or eject rock fragments.
Unlike most space probes, DART didn’t slow down before reaching its target. As it approached, its camera continually took images of the asteroid as it grew in the frame, sending them to Earth via the Deep Space Network, an international system of antennas managed by NASA’s Jet Propulsion Laboratory.
Those images aren’t just important for research; they’re key for navigation. It takes 38 seconds for human operators to send signals to DART—or for the probe to send images back to Earth. When the timing was critical, it was necessary for the probe to pilot itself. Within the last 20 minutes, its SMART Nav automated system made a “precision lock” on the target and used these images to adjust the spacecraft’s course with thruster engines.
But Ernst points out that there’s one piece of data they won’t get from the spacecraft: “We can’t image the crater, because we are the crater.”
Dimorphos’ special orbit around its more massive partner will be key in aiding astronomers’ measurements of DART’s deflection. Most asteroids simply go around the sun, so a tiny tweak in their orbits could take years to notice. But the DART collision altered Dimorphos’s orbit around its partner, not the asteroids’ solar orbit. Since it takes Dimorphos 11 hours and 55 minutes to circle its neighbor, scientists might need only a few weeks to measure multiple orbits and assess the change—the journey might shorten by a few minutes, for example. It’s akin to having a watch that’s slightly off, Chabot says: After a week, you notice you’re a little behind.
In addition to these observations from Earth-based telescopes, plus the images from DRACO, Chabot and her team are also looking forward to photos from the Italian Space Agency’s LICIACube, a small, briefcase-sized spacecraft deployed by DART 15 days ago. It flew by the asteroid three minutes after the impact in an attempt to provide “after” images of the crash site—although a cloud of dust and rock bits could block a clear photo. LICIACube has the data stored onboard, and those images will be sent back in the coming days and weeks, Chabot says.
That’s why the AIDA collaboration includes the European Space Agency’s upcoming Hera mission, which is planned for launch in October 2024 and will rendezvous with the asteroid pair in late 2026. With ground-penetrating radar and other instruments, the spacecraft will probe the aftermath of DART’s crash and measure the mass and composition of the asteroid, its post-impact internal structure, and the shape of the crater.
“To understand how efficient this technique is and whether we can even use it for much larger asteroids, like the dinosaur-killer asteroid, we really depend on getting this additional information from Hera,” says Ian Carnelli, Hera’s project manager. While researchers have run plenty of models and simulations of various kinds of deflections, this experiment will finally provide real-universe data for them.
Since a collision with a probe only nudges an asteroid, the DART technique will work only if there’s enough warning time that a dangerous asteroid or comet is heading toward Earth. Scientists would need to know a decade or so in advance in order to position the probe to meet the space rock before it’s too close to be deflected with a little push. (It won’t be like the movie Don’t Look Up, where there was only a six-month warning time.)
Recent public opinion polls have consistently shown that Americans rank planetary defense and climate science as top priorities for the US space program, higher than planned crewed missions to the moon and Mars. Considering that near-Earth asteroids and climate change could threaten everyone on the planet, that’s understandable, and it’s why scientists have to attempt tests like the DART mission. “DART is really the first demonstration of a technique we might use to defend the planet,” Ernst says. “You can theorize, you can run laboratory experiments at small scales. But this data point is really critical for us to understand what we could actually do should we see a hazard.”