To practice saving the world, NASA just crashed a spacecraft into an asteroid

Laurel, MarylandNearly seven million miles from Earth, a spacecraft traveling more than 14,000 miles an hour smashed into a small, unsuspecting asteroid that had been floating through space undisturbed for eons.

The collision between NASA’s Double Asteroid Redirection Test (DART) spacecraft and a 500-foot-wide rock called Dimorphos, which took place at 7: 14 p.m. ET marks the first time humans have deliberately changed the course of a celestial object. It is also the first time that humans have tried a bold strategy to deflect future asteroids from colliding with Earth.

While scientists are confident that a large enough asteroid to cause planet-wide extinction will not threaten Earth for at least a hundred years–after which time it is difficult to predict their orbits–it is still possible that we could be surprised by a smaller, potentially city-destroying rock from space. It is almost certain that Earth’s existence will be threatened by an asteroid in the future, whether it happens in thousands, hundreds, or millions of years.

“I don’t really lose sleep about the Earth getting destroyed by asteroids, but I am excited about living in a world where we might be able to potentially prevent this in the future,” says Nancy Chabot of Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland, which manages the DART mission. “This is just the first step, but isn’t it exciting that we’re going from science fiction to science reality?”

To practice shaping that reality, NASA sent DART hurtling toward its demise. The spacecraft snapped images of Dimorphos as it approached. It quickly grew from a tiny bit of light to fill its field until the moment of impact when everything went dark.

Dimorphos orbits a larger asteroid called Didymos, and the two asteroids are not considered threats to Earth–which is one of the reasons NASA put them in the DART spacecraft’s crosshairs for this first planetary defense test.

Discovered in 1996, Didymos, which means “twin” in Greek, is roughly a half-mile across and relatively well studied. It was only just before DART crashed into it that anyone had seen the tiny moonlet. The space rock was recently named Dimorphos by teams. This is Greek for “having 2 forms”, one before and one after the impact.

The collision resembles the desperate, last-minute attempts to save Earth from cosmic annihilation in Hollywood blockbusters. The DART impact was not an attempt to destroy Dimorphos, unlike the movie plots to explode asteroids before they hit Earth. It was more like a flick, a nudge that packs enough punch to change the orbit of the moonlet without destroying it.

Making sure this strategy works will require some careful follow-up observations, and an impressive array of instruments have swiveled to monitor the Didymos system.

Three minutes after the collision, a cubesat called LICIACube arrived to surveil the wreckage. The little spacecraft will beam images from the crash site back to Earth over the next few days. Scientists will then study them to find out more about the structure of Dimorphos. NASA’s James Webb Space Telescope and Hubble Space Telescope were also trained on the Didymos system at the moment of impact, and Lucy, a spacecraft on its way to a strange population of asteroids that orbit near Jupiter, was close enough to observe the impact as well.

The goal of these observations is to look for any hints of brightening in the Didymos system, which will provide crucial information about how much dust and pulverized rock the impact kicked up. Three dozen ground-based telescopes will now begin to measure the new orbit of the moonlet. And in 2026, a spacecraft called Hera built by the European Space Agency (ESA), will arrive to monitor the collision’s aftermath.

“This inaugural planetary defense test mission marks a major moment in human history,” Bobby Braun, head of APL’s space exploration sector, said during a pre-impact briefing on September 12. “For the first time ever, we will measurably change the orbit of a celestial body in the universe.”

Our first look at Dimorphos

DART launched atop a SpaceX Falcon 9 rocket on November 23, 2021, and set sail for its rocky destination.

Didymos is what’s called an S-type asteroid, one of the more common types of near-Earth asteroids. Didymos, like many asteroids is a relic from the infant solar system. It contains preserved records of how our neighborhood was 4.5 billion years ago.

Dimorphos, however, was a mystery until tonight. Scientists suspected that it was made from similar materials to Didymos. They speculated that it might be a loosely assemble rubble heap, rather than one shard. However, its mass, shape and composition remained a mystery until tonight. Team members began to speculate about the shape of the impact in the weeks before it happened. For example, a dog bone or a donut would be more difficult to hit. A rounder blob would be a better target.

As DART streaked toward its rendezvous with destiny, the spacecraft’s onboard camera DRACO got its first good look at the moonlet: a lumpy egg-shaped moonlet with discernable boulders on its surface. Two minutes before impact, Dimorphos flooded the spacecraft’s view. DRACO took images every second, creating close encounter sequences that contain a wealth of science data and also revealed that the mission was successful. The team knew that the spacecraft had hit its target when DART transmissions ended.

“These images will continue until they don’t, so that’ll be a pretty definitive look into the final moments of the DART spacecraft,” Chabot said before the impact.

Bull’s-eye in space

For most of its 10-month journey, the DART spacecraft couldn’t even see its target–instead, it was autonomously guided to its rendezvous with Dimorphos by onboard navigation software. After the asteroid pair came into view as a few small pixels, DART’s SMART Nav system locked on to Didymos. On final approach, about 50 minutes before impact, the system switched over to the moonlet and guided the spacecraft to its crash site. It was vital that DART was correctly positioned on the correct object.

“That’s a very sweaty time for us,” Evan Smith, DART’s deputy mission systems engineer said at a briefing before the impact. “We’re going to be watching the telemetry like hawks, very scared, but excited.” If DART missed its target, the team wouldn’t have another opportunity until 2024.

“This is a par one course, so we’re going in for the hit this time,” Smith said. “We do have an opportunity coming up in two years, but we don’t want to play that round of golf.”

To test the crucial guidance system, DART’s DRACO camera swiveled to stare at Jupiter and its four largest moons in July and August of this year. DART was able practice locking on to a small object that emerged from behind a larger object to test the guidance system.

Lining up the shot was kind of like playing darts, mission systems engineer Elena Adams said, except you’re throwing the dart from John F. Kennedy International Airport in New York City to a target at Washington Dulles International Airport outside Washington, D.C.

“The dart itself is only 2.5 millimeters–it’s tiny–and you take the dart at JFK and you throw it to Dulles, and you hit the center of the bull’s-eye, except you don’t know where the dartboard is at,” Adams told reporters.

The search for deadly asteroids

Asteroid deflection technology will only be useful if there is something to deflect, which is why NASA and other institutions are focused on finding and tracking all the space rocks that could be on Earth-crossing orbits.

“We’re not aware of a single object right now, within the next hundred years or so, that’s really threatening the Earth. I also guarantee that there will be an object if you wait long enough,” Thomas Zurbuchen (NASA’s associate administrator of science) said at the preimpact briefing. How do I know this? These objects have had a significant impact on our history, and we have a geologic record to prove it. We have a geologic record to prove that.”

Earth’s evolution has been molded by impacts from the very start. Since Earth was a molten planetary embryo, comets and asteroids have ravaged the planet. Some of these objects provided the water that now flows through Earth’s rivers, lakes, and oceans. Others caused mass extinctions that were cataclysmic.

Scientists suspect we’ve located the bulk of potentially harmful asteroids, defined as bodies more than 450 feet across that come within five million miles of Earth. The largest of these asteroids, which measure more than six miles in length and could trigger a global extinction, have been discovered. And the space agency estimates that it has discovered roughly 95 percent of asteroids that are about the size of Didymos. It is easier to track smaller bodies, such as Dimorphos. NASA estimates that less than half of the objects we have found are that large. Large rocks could wipe out major cities.

“The most important thing we have to do first is find the population of hazardous asteroids out there,” Lindley Johnson, NASA’s planetary defense officer, said at the briefing. “We have the technology now to do this and to find these objects years, decades, even a century before they pose an impact threat to the Earth.”

Finding the remainder of these asteroids, Johnson says, will need to be done from space. In the coming years, NASA plans to launch a telescope called the Near-Earth Object Surveyor, which will be able to pick up the infrared signatures of space rocks that are hidden by the glare of the sun, helping to make sure humanity doesn’t get blindsided by an unseen asteroid in the future.

After the dust settles

In the days and weeks after the DART impact, ground-based telescopes across all seven continents–including the Green Bank Telescope in West Virginia, the Goldstone Deep Space Antenna in California, and the Very Large Telescope in Chile–will precisely measure the moonlet’s new orbit.

For DART to have achieved its goal, the impact needs to have shortened Dimorphos’s 11-hour, 55-minute orbit by just 73 seconds. But Chabot expects to see a much larger shift, closer to 10 minutes or so.

“It might be 20 minutes; it might be five minutes,” she said. “That’s going to be related to how much ejecta is thrown off, how much pulverized rock and material is in this energetic collision … That’s one of the main reasons for doing this full-scale test in space.”

How much that orbit changes will help scientists learn more about the moonlet, but more importantly, it will tell us whether this type of kinetic impactor is a viable option for deflecting hazardous asteroids. DART information could prove crucial in saving the world if an asteroid is ever thrown off course.

“If you were going to do this for planetary defense, you would do it five, 10, 20 years in advance,” Chabot said. “Give it a small nudge … You’re not trying to make yourself a problem where you destroy an asteroid, and you make a lot of different pieces.”

If the DART mission is successful, we’ll have at least one tool in the shed to ward off incoming dangers in the sky.

“Doing so has clear benefits in ensuring humanity’s ability to deflect a potential threatening asteroid in the future,” Braun said. “It also speaks volumes to how far our space program has come in just the last 60 years and how important the space program can be to all of us here on the Earth.”

If the dinosaurs had a space program, the joke goes, they’d still roam the Earth.