NASA's asteroid deflection test for planetary defence even more successful than we thought

One day, we could face the threat of a potentially hazardous asteroid, and scientists say humanity may need the ability to deflect it if it is set on a collision course with Earth. 

Asteroids vary widely in size, which means impacts can have very different effects — from small objects that streak across the sky as brilliant fireballs with powerful sonic booms, to massive space rocks large enough to cause global devastation and even mass extinction.

In 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft deliberately smashed into the small moon Dimorphos — which orbits the larger asteroid Didymos — after travelling 10 months to reach the binary asteroid system. 

The mission to the region about 11 million kilometres from Earth was a success: scientists found the hit from the spacecraft shortened Dimorphos’s orbit around Didymos by around 32 minutes. 

And a new study published in Science Advances reports an additional effect: the collision slightly altered the pair’s trajectory around the sun. 

Rahil Makadia, the study’s lead author and a planetary defence scientist from University of Illinois Urbana-Champaign, said the findings are a major step forward in protecting our planet.

“We don’t need to blow the asteroids up,” he said. “If we just give it a tiny shove well in advance, then we can potentially push a threatening asteroid clear of the Earth.” 

While some asteroids exist alone, many have small companion moons, while some form binary or triple systems, where two or more rocky bodies orbit each other in space. 

Derek Richardson, emeritus professor of astronomy at University of Maryland, says the DART team intentionally targeted a binary system, because it is “much easier to measure the change in the orbit of a little guy going around a slightly bigger guy.” 

In this system, he says, Dimorphos orbits Didymos roughly every 12 hours, while the full pair takes more than two years to circle the sun. That short cycle allowed scientists to detect orbital changes far more quickly than they could in the system’s much longer heliocentric orbit, which refers to circling around the sun.

The test also served as a small-scale demonstration of planetary defence: if an asteroid orbiting the sun were ever on a collision course with Earth, the same approach could be used to nudge it.

“Our task was, ‘Let’s hit a small version of the solar system,’” said Richardson. 

During the test, Makadia says the impact instantly blasted off “a bunch of material, like rocks, pebbles, even boulders that were sitting happily on the surface of Dimorphos.”

While the change in Dimorphos’s orbit around Didymos was detected quickly, measuring the shift in the system’s orbit around the sun required long-term data collected between October 2022 and March 2025.