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.” 

Why scientists targeted a two-asteroid system

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.

To track the effects of the impact, scientists combined radar measurements — which are extremely precise and can pinpoint an asteroid to within 10–15 meters of its location, even across millions of kilometers — with stellar occultations, where an asteroid passing in front of a star allows for extremely precise position measurements.

Using these methods, Makadia says the spacecraft collision slowed the asteroid system’s orbit by 11.7 microns per second — about one-tenth the width of a human hair — which then adds up to about 360 meters per year.

“This was the first time a heliocentric orbit change has been observed, ever,” he said. 

What the breakthrough means for Earth’s safety

While there’s no known object threatening Earth, Richardson says the technology demonstrated it has the ability to intervene if one ever appears. 

If an asteroid were to strike, he says, the odds are it would first hit the oceans, which cover around 70 per cent of the planet’s surface. 

But the technology is also relevant for protecting large land masses. 

Over the past century, two major impacts struck Russia: the 1908 Tunguska event and the 2013 Chelyabinsk meteor, which exploded in the atmosphere, causing damage and injuries to people on the ground. 

Russia is often hit first, says Richardson, because it is the largest land mass on the planet, with Canada ranking closely behind.

“It’s like a dartboard basically, and Canada is taking up a good portion of the dartboard,” he said. 

The DART mission also provides a critical data point for planning future asteroid-deflection efforts, according to Makadia. 

When the spacecraft struck Dimorphos, the collision blasted a cloud of rocky debris into space, reshaping the asteroid. 

The escaping material carried its own momentum, he says, giving Dimorphos an extra push — known as the momentum-enhancement factor. The research found this effectively “doubled the total push” delivered by the spacecraft alone.

These findings can be applied to plan future kinetic impact missions, whether targeting single asteroids or binary systems, providing a reliable way to predict how much an asteroid can be nudged.

“We should be resting easier tonight because we know how to push asteroids away from the sun and away from the Earth if we need to,” Makadia said.