A planned collision with a small asteroid moonlet ended up shifting the orbit of an entire binary system around the Sun, scientists say the surprise result strengthens the case for planetary defense
When NASA deliberately crashed a spacecraft into the asteroid moonlet Dimorphos in September 2022, the goal was straightforward. Test whether humanity could shove a space rock off course. The mission worked. But new research published in Science Advances reveals the collision did something nobody predicted. It slightly altered the orbit of the entire asteroid system around the Sun.
The DART mission’s original success
The Double Asteroid Redirection Test, or DART, targeted Dimorphos, a 560-foot-wide body orbiting a larger companion called Didymos. Before impact, Dimorphos circled Didymos roughly every 12 hours. After the spacecraft slammed into it at roughly 14,000 miles per hour, that orbital period shortened by 33 minutes.
That alone counted as a landmark. For the first time, humans had deliberately changed the motion of a natural object in space. NASA celebrated. Headlines moved on. But scientists kept watching.
A surprise written in fractions of a second
Researchers tracking the Didymos system in the months and years following the collision noticed something subtle. The 770-day orbit the two asteroids share around the Sun had shifted by about 0.15 seconds. The system’s orbital speed changed by roughly 11.7 microns per second, or about 1.7 inches per hour.
Those numbers sound almost laughably small. They are not. Over decades or centuries, even a fractional change in velocity can push an asteroid thousands of miles from where it would have been. That margin could mean the difference between a catastrophic impact and a clean miss.
Why the push was bigger than the punch
DART weighed about 1,200 pounds at the time of impact. Dimorphos weighs billions of tons. So how did a relatively tiny spacecraft budge an entire binary system?
The answer is debris. When the probe struck Dimorphos, it blasted a massive plume of rock and dust into space. That ejected material carried momentum away from the asteroid, functioning almost like exhaust from a rocket engine. Scientists calculated what they call the momentum enhancement factor at roughly two, meaning the debris approximately doubled the force the spacecraft delivered on its own.
Some of that material escaped the gravitational grip of the binary system entirely, and that loss of mass nudged the shared orbit of Didymos and Dimorphos around the Sun.
How scientists caught a fraction of a second
Measuring such a tiny orbital change required extraordinary precision. Researchers combined radar data and ground-based telescope observations with 22 stellar occultations recorded between October 2022 and March 2025. A stellar occultation occurs when an asteroid passes in front of a distant star, briefly blocking its light and allowing scientists to calculate position and speed with pinpoint accuracy.
Volunteer astronomers stationed across the globe made those observations possible, sometimes traveling to remote locations with no guarantee of clear skies.
What this means for DART and planetary defense
Didymos was never headed toward Earth, and the DART collision could not have sent it our way. But the findings validate a critical idea. A kinetic impactor launched early enough could meaningfully redirect a threatening asteroid, and the debris effect could amplify the deflection well beyond what the spacecraft alone would achieve.
NASA is already building on that foundation. The Near-Earth Object Surveyor, a space telescope managed by the Jet Propulsion Laboratory, will be the first observatory designed specifically for planetary defense. Its infrared instruments will scan for dark asteroids and comets that reflect too little visible light for ground-based telescopes to spot easily.
Clues about how Dimorphos was born
The research also offered a bonus. By tracking changes in the system’s motion, scientists estimated the densities of both asteroids. Their calculations suggest Dimorphos is slightly less dense than earlier models indicated, supporting the theory that it formed from loose rubble shed by a rapidly spinning Didymos and gradually clumped together under gravity.
Three years after a small spacecraft met a violent end on the surface of a distant rock, the aftershocks are still teaching scientists new things about how to protect this planet and how the solar system’s smallest residents came to be.