NASA demonstrates humanity may be able to stop an Earth-bound asteroid

A NASA spacecraft that smashed into an asteroid on purpose didn't just knock one rock off its course. It also nudged the orbit of the entire asteroid system it belongs to, a new study shows.

Researchers found that the impact from NASA's Double Asteroid Redirection Test mission altered the path of the asteroid pair Didymos and Dimorphos around the sun. The discovery further confirms the space agency's asteroid target practice as a valid planetary defense technique for moving a hazardous object off a collision course, should one ever barrel toward Earth in the future.

When the DART spacecraft deliberately crashed into Dimorphos in September 2022, astronomers quickly announced the success of the $330 million mission's primary goal. The crash changed the orbit of the 560-foot-wide rock, which orbits Didymos like a small moon. Dimorphos now circles its larger partner 33 minutes faster than it did before the strike.

But scientists really needed to know whether the impact had accomplished something larger in scope: Push the entire asteroid pair's orbit. Now they have evidence that it did. The findings show that hitting just one asteroid in the pair moved both. 

"This is a tiny change to the orbit," said Tom Statler, NASA's lead scientist for solar system small bodies, in a statement, "but given enough time, even a tiny change can grow to a significant deflection."

The newly detected change is tiny. The impact altered the asteroid system's speed along its solar orbit by less than two inches per hour. 

To spot that minuscule shift, researchers tracked Didymos for years using radar, telescopes, and a technique called a stellar occultation, which occurs when an asteroid briefly passes in front of a distant star. Those fleeting events can reveal an asteroid's position, even one as small as a half-mile across, like Didymos.

Didymos, a half-mile-wide binary asteroid, takes an elliptical orbit around the sun, traveling with a smaller asteroid that orbits it, called Dimorphos. Credit: NASA / JPL-Caltech diagram

Millions of asteroids orbit the sun, the majority of which travel between Mars and Jupiter. But those rocks can occasionally get bumped out of the belt into the inner solar system, closer to Earth.

Though no known asteroids pose an existential threat to the planet right now, scientists are on the lookout for any hazardous near-Earth objects. Even smaller rocks can be disastrous. In 2013, an undetected meteor exploded over Chelyabinsk, Russia, causing an airburst and shockwave that injured 1,600 people. The culprit was only 60 feet wide.

Boulders ejected deliver extra kick

The new DART mission measurements, reported in the journal Science Advances, show that the crash didn't just jolt Dimorphos. Some of the material blasted off the asteroid during the impact escaped the system entirely, carrying momentum with it and giving the whole pair an extra push.

In effect, the explosion of debris helped do the work. Scientists measure that extra boost using something called a momentum enhancement factor. The new analysis finds that the debris roughly doubled the kick delivered by the spacecraft.

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The observations also offer clues about the asteroids themselves. Researchers estimate that Didymos is relatively dense, while Dimorphos appears much lighter and more loosely packed. That difference supports the idea that Dimorphos may have formed from pieces of the larger asteroid long ago.

The size gap between them is enormous. Didymos is about 200 times heavier than its partner, which helps explain why the system's overall motion changed only slightly.

What's next for the DART study

The pair, millions of miles away, continues to pose no danger to Earth, and scientists say it will remain safely distant for at least the next century. But the experiment has already delivered something remarkable: A clear demonstration that humans can alter the motion of a natural object in space.

The story isn’t finished yet. Later this decade, the Hera mission from the European Space Agency will arrive at the asteroid system to inspect the crater left by the DART impact and map the asteroids in detail.

Those close-up observations should reveal exactly how the crash reshaped Dimorphos — and how effective similar missions might be if Earth ever needs to push a dangerous asteroid out of the way.