Like many of the solar system’s rocky objects, Earth bears the scars of past asteroid impacts—including some wallops that shaped the arc of life itself. Some 66 million years ago, for instance, a six-mile-wide asteroid slammed into Earth near Mexico’s Yucatán Peninsula, triggering a mass extinction that wiped out the non-avian dinosaurs.
Now for the first time in our planet’s history, Earth is going to hit back.
At 10:21 p.m. Pacific Time on November 23, a NASA mission called DART (Double Asteroid Redirection Test) launched from Vandenberg Space Force Base, California, to embark on a nearly year-long voyage around the sun. If all goes well, DART’s journey will end on the evening of September 26, 2022, when the golf cart-size spacecraft will intentionally slam into a little, unsuspecting asteroid called Dimorphos.
Dimorphos is a harmless space potato, a 525-foot-wide “moonlet” that orbits a bigger asteroid called Didymos every 11 hours and 55 minutes. DART’s mission is to smash into Dimorphos at roughly 15,000 miles an hour, altering the moonlet’s orbit around its parent body. The name Dimorphos, Greek for “having two forms,” was chosen because the asteroid will have one form before DART and one form after.
DART’s collision with Dimorphos will destroy the spacecraft, but it should also cause the moonlet to settle into a tighter, shorter orbit around Didymos, which astronomers will measure with Earth-based telescopes. The $330-million mission is the first full test of technologies that could be used to avert a future asteroid impact—a natural hazard that, unlike earthquakes and volcanoes, humans can forecast many years ahead of time.
“This is a mission for planet Earth—all the peoples of the Earth—because we would all be threatened,” says NASA Administrator Bill Nelson. “I’m pretty charged about DART. … If they connect me to where I can watch this thing [collide], I guarantee you, I will be glued to a screen.”
Didymos and Dimorphos pose no threat to Earth, and no known asteroid is destined to collide with our planet for at least hundreds of years. But experts often say that it’s a matter of when, not if, Earth finds itself in the celestial shooting gallery.
“I tell people that planetary defense or near-Earth observations are not the highest-priority thing that NASA needs to be doing—but the day will come when it may be the most important thing that NASA does,” says the space agency’s planetary defense officer Lindley Johnson.
A DART-like deflector is only as effective as our surveys of the sky—the key to buying time. “DART is probably the high-profile end of planetary defense, but it’s only one part of planetary defense,” says DART coordination lead Nancy Chabot, a scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.
For decades NASA and other space agencies have been searching for asteroids whose paths cross the orbit of Earth and predicting their future movements. The goal is to understand the risks we face over centuries so we’re not caught unawares.
“The Hollywood movies make it very dramatic and entertaining, but in the real world, we don’t want to be in that situation,” Johnson says.
So far astronomers have found 890 near-Earth asteroids bigger than a kilometer (0.6 miles) wide, more than 95 percent of the expected total—and none of them will pose an impact risk for at least the next few centuries. However, asteroids as small as 140 meters (460 feet) wide could still devastate an area the size of some U.S. states, and many of these objects have yet to be discovered. Computer models suggest that there are roughly 25,000 near-Earth asteroids that are at least 140 meters wide, and as of late 2021, we’ve found only about 10,000 of them.
Upcoming observatories, including a planned NASA space telescope called the Near-Earth Object Surveyor, should accelerate the pace of discovery. If astronomers using these instruments find an asteroid whose orbit intersects with Earth’s, humankind’s response will depend on how much advance notice we have.
If a large asteroid were discovered only a few months before impact, one of our only options would be to detonate nuclear weapons next to the object. X-rays from the blast would vaporize parts of the asteroid’s surface, creating ejecta that would act like rocket thrust and nudge the asteroid, hopefully enough to get it off a collision course.
But if an asteroid that’s not too big were found well ahead of its forecasted impact, then the solution requires no nukes. A zippy spacecraft like DART—called a “kinetic impactor”—could be sent to collide with the asteroid and slightly tweak its orbit. Over many years, that small deviation would add up to a major change in the asteroid’s path, enough to render the object harmless.
The researchers at APL who built the spacecraft have spent more than a decade thinking through how binary asteroid systems like Didymos and Dimorphos could provide a useful, safe place to test a kinetic impactor. Adjusting an asteroid’s orbit around the sun could have unforeseen consequences, such as unintentionally putting it on a far-future collision course with Earth. Instead, DART will tweak the orbit of a smaller asteroid around a bigger asteroid, with practically no effect on the binary pair’s overall path.
The most daunting technical challenge DART faces: pinpointing Dimorphos’s position as the spacecraft careens toward it at nearly 15,000 miles an hour. “We have no clue what Dimorphos actually looks like,” says APL’s Elena Adams, the mission system engineer for DART. “It could be a dog bone; it could be a doughnut.”
To ensure that DART hits its mark, mission engineers developed a guidance system called SMART Nav that will let the spacecraft autonomously home in on Dimorphos using an onboard telescope.
For most of its journey, DART will have remarkably little to go on. The spacecraft won’t be able to see the larger asteroid Didymos until four hours before impact, and Dimorphos itself won’t pop into view until an hour before showtime. By the time DART finishes its final trajectory corrections—with two minutes and 500 miles until oblivion—Dimorphos will be just 41 pixels across in DART’s field of view.
As it screams toward the target, DART will send back as many images of Dimorphos as it can, possibly as many as one every 2.5 seconds before impact. The terrain captured in these final images will be crucial to understanding the blow that DART deals to its target because the amount of ejecta thrown off the asteroid will depend on where the spacecraft hits.
“There’s a lot of sensitivity to the details of where it lands: if it happens to land on a boulder, or if it happens to land in finer materials,” says Megan Bruck Syal, a scientist at Lawrence Livermore National Laboratory who studies asteroid deflection simulations.
Exactly how much DART will nudge Dimorphos is unclear, but the spacecraft’s creators are confident that it will pack plenty of punch. For NASA to consider DART a success, the impact will need to shorten Dimorphos’s orbit around Didymos by at least 73 seconds. DART’s team predicts that the spacecraft could shave off as much as 10 minutes.
DART won’t be alone in its final moments. About 10 days before impact, the spacecraft will eject a small CubeSat called LICIACube. Built and operated by the Italian Space Agency, LICIACube will fly past Dimorphos 165 seconds after DART makes contact.
Along the way, the little spacecraft will take pictures of Dimorphos’s newly marred surface and the impact’s ballooning plume of debris. LICIACube could even capture the flash of light from the impact. “We are the real-time witness,” says Simone Pirrotta, the Italian Space Agency’s project manager for LICIACube.
The CubeSat’s vantage point is crucial to DART’s mission: Scientists need to have a precise accounting of how much momentum DART transferred to Dimorphos, which means they must watch for the growing shroud of ejecta that the collision will spray out.
“It’s going to blast many tons of material off—maybe thousands of tons—and we need to know how much material there is, how fast it’s going, and where it’s headed,” APL’s Andy Cheng, a DART investigation team lead, said in a November 4 press briefing.
For several weeks, LICIACube will transmit data back to Earth, and then it will continue to drift through the solar system, its purpose fulfilled. But it won’t be the last spacecraft to gaze upon the surface of Dimorphos. The European Space Agency is working on a follow-up mission called Hera, which will launch in 2024. Hera will perform a more thorough survey of Dimorphos, poring over the moonlet’s surface like a crime scene investigator.
Beyond advancing planetary defense, DART, LICIACube, and Hera will fill a major scientific gap by visiting a binary asteroid system. Such a visit has happened only once before: In 1993 NASA’s Galileo spacecraft flew by the asteroid Ida on its way to Jupiter, where it discovered the asteroid had a moonlet, now called Dactyl. “Science-wise, we have never visited a binary asteroid on purpose,” says APL scientist and DART investigation team lead Andy Rivkin.
Understanding how moonlets like Dimorphos form will give scientists insight into the formation mechanisms at play in the early solar system, when the planets themselves accreted from smaller bits of material. Rivkin adds that since roughly 10 to 15 percent of all asteroids are binaries, there is a reasonable chance that Earth’s next major asteroid threat could be a binary—making study of these celestial pairs all the more important.
For Chabot, DART’s biggest promise lies in being the first of many missions focused on averting a possible asteroid apocalypse. “It’s not just an end in itself,” she says. “It’s opening up a whole new beginning.”
Editor's note: This article was updated to reflect DART's successful launch.