Why a NASA spacecraft will crash into an asteroid Monday

An illustration depicts NASA's DART probe and the Italian Space Agency's LICIACube before impact with an asteroid.
This illustration depicts NASA’s DART probe, foreground right, and Italian Space Agency’s LICIACube, bottom right, before impact with the asteroid Dimorphos, left.
(Steve Gribben / NASA)

In the first-of-its kind, save-the-world experiment, NASA is about to clobber a small, harmless asteroid millions of miles away.

A spacecraft named DART will zero in on the asteroid Monday, intent on slamming it head-on while traveling 14,000 mph. The impact should be just enough to nudge the asteroid into a slightly tighter orbit around its companion space rock, demonstrating that if a killer asteroid ever heads our way, we’d stand a fighting chance of diverting it.

Cameras and telescopes will watch the crash, but it will take months to learn whether it actually changed the orbit.


The $325-million planetary defense test began with DART’s launch last fall.

Asteroid target

The asteroid with the bull’s-eye on it is Dimorphos, about 7 million miles from Earth. It is actually the puny sidekick of a 2,500-foot asteroid named Didymos, Greek for twin. Discovered in 1996, Didymos is spinning so fast that scientists believe it flung off material that eventually formed a moonlet. Dimorphos — roughly 525 feet across — orbits its parent body at a distance of less than a mile.

“This really is about asteroid deflection, not disruption,” said Nancy Chabot, a planetary scientist and mission team leader at Johns Hopkins University, which is managing the mission. “This isn’t going to blow up the asteroid. It isn’t going to put it into lots of pieces.” Rather, the impact will dig out a crater tens of yards in size and hurl some 2 million pounds of rocks and dirt into space.

NASA insists there’s a zero chance either asteroid will threaten Earth — now or in the future. That’s why the pair was picked.

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DART, the impactor

Johns Hopkins took a minimalist approach in developing DART — short for Double Asteroid Redirection Test — given that it’s essentially a battering ram and faces sure destruction. It has a single instrument: a camera used for navigating, targeting and chronicling the final action.

Believed to be essentially a rubble pile, Dimorphos will emerge as a point of light an hour before impact, looming larger and larger in the camera images beamed back to Earth.

An illustration depicts NASA's DART probe on course to impact the asteroid Dimorphos, near another asteroid.
An illustration depicts NASA’s DART probe on course to impact the asteroid Dimorphos, left, which orbits Didymos.
(Steve Gribben/Johns Hopkins APL/NASA via Associated Press)

Managers are confident DART won’t smash into the larger Didymos by mistake. The spacecraft’s navigation is designed to distinguish between the two asteroids and, in the final 50 minutes, target the smaller one.

The size of a small vending machine at 1,260 pounds, the spacecraft will slam into roughly 11 billion pounds of asteroid. “Sometimes we describe it as running a golf cart into a Great Pyramid,” Chabot said.

Unless DART misses — NASA puts the odds of that happening at less than 10% — it will be the end of the road for DART. If it goes screaming past both space rocks, it will encounter them again in a couple years for a second try.

Saving Earth

Little Dimorphos completes a lap around big Didymos every 11 hours and 55 minutes. The impact by DART should shave about 10 minutes off that.

Although the strike itself should be immediately apparent, it will take months to verify the moonlet’s tweaked orbit. Cameras on DART and a mini tagalong satellite will capture the collision up close. Telescopes on all seven continents, along with the Hubble and Webb space telescopes and NASA’s asteroid-hunting Lucy spacecraft, may see a bright flash as DART smacks Dimorphos and sends streams of rock and dirt cascading into space.

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The observatories will track the pair of asteroids as they circle the sun, to see if DART altered Dimorphos’ orbit. In 2024, a European spacecraft named Hera will retrace DART’s journey to measure the impact results.

Although the intended nudge should change the moonlet’s position only slightly, that will add up to a major shift over time, according to Chabot.

“So if you were going to do this for planetary defense, you would do it five, 10, 15, 20 years in advance in order for this technique to work,” she said.

Even if DART misses, the experiment still will provide valuable insight, said NASA program executive Andrea Riley.

“This is why we test,” she said. “We want to do it now rather than when there’s an actual need.”

Asteroid missions galore

Planet Earth is on an asteroid-chasing roll. NASA has close to a pound of rubble collected from asteroid Bennu headed to Earth. The stash should arrive next September.

Japan was the first to retrieve asteroid samples, accomplishing the feat twice. China hopes to follow suit with a mission launching in 2025.

NASA’s Lucy spacecraft, meanwhile, is headed to asteroids near Jupiter, after launching last year. Another spacecraft, Near-Earth Asteroid Scout, is loaded into NASA’s new moon rocket awaiting liftoff; it will use a solar sail to fly past a space rock that’s less than 60 feet next year.

In 2026, NASA will launch a census-taking telescope to identify hard-to-find asteroids that could pose risks. One asteroid mission is grounded while an independent review board weighs its future.

Hollywood’s take

Hollywood has churned out dozens of killer-space-rock movies over the decades, including 1998′s “Armageddon” which brought Bruce Willis to Cape Canaveral for filming, and last year’s “Don’t Look Up” with Leonardo DiCaprio and Jennifer Lawrence leading an all-star cast. NASA’s planetary defense officer, Lindley Johnson, figures he’s seen them all since 1979′s “Meteor,” his personal favorite “since Sean Connery played me.”

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While some of the sci-fi films are more accurate than others, he noted, entertainment always wins out. The good news is that the coast seems clear for the next century, with no known threats.

What’s worrisome, though, are the unknown threats. Fewer than half of the 460-foot objects have been confirmed, with millions of smaller but still-dangerous objects zooming around.

“These threats are real, and what makes this time special, is we can do something about it,” said NASA’s science mission chief Thomas Zurbuchen. Not by blowing up an asteroid as Willis’ character did — that would be a last, last-minute resort — or by begging government leaders to take action as DiCaprio’s character did in vain. If time allows, the best tactic could be to nudge the menacing asteroid out of our way, like DART.