A spacecraft slowed the orbit of a pair of asteroids around the sun by more than 10 micrometers per second — the first time human activity has altered the orbit of a celestial object, researchers report March 6 in Science Advances. The experiment could have implications for protecting Earth from future asteroid strikes.
NASA’s Double Asteroid Redirection Test, or DART, intentionally crashed a spacecraft into the small asteroid Dimorphos in 2022. The goal was to change Dimorphos’ orbit around its larger sibling, Didymos. Within a month, researchers showed that the impact shortened Dimorphos’ 12-hour orbit by 32 minutes.
Most of that change came from the impact itself. Some of it came from flying impact debris, which gave Dimorphos a little kick in the opposite direction of its motion.
Some of the rocks knocked off of Dimorphos fled the vicinity completely, escaping the gravitational influence of the Dimorphos–Didymos pair, says planetary defense researcher Rahil Makadia of the University of Illinois Urbana–Champaign. Those rocky runaways took some momentum away from the duo and changed their joint motion around the sun.
To figure out how much that motion was affected, astronomers watched the asteroids pass in front of distant stars, dimming some of the stars’ light like a tiny eclipse. These blinks, called stellar occultations, can be visible from anywhere on Earth and are predictable in advance.
“Oftentimes it’s amateur astronomers going out in the middle of nowhere to track Didymos based on predictions,” Makadia says. “There was an observer who drove two days each way into the Australian outback to get these measurements.”
Makadia and colleagues gathered 22 such measurements taken from October 2022 to March 2025. Calculating how far off occultation timings were from predictions revealed that the asteroids’ orbit around the sun was about 150 milliseconds slower than before the DART impact.
The result could be confirmed later this year, when the European Space Agency’s Hera spacecraft arrives at Didymos and Dimorphos for follow-up observations.
Didymos and Dimorphos are not a threat to Earth, Makadia says, and weren’t before DART. But knowing how a deliberate impact changes one asteroid’s orbit can help make defense plans against another, “in case we need to do a kinetic impact for real.”