In September 2022, NASA slammed a spacecraft into an asteroid moonlet called Dimorphos at more than 14,000 miles per hour (22,530.8 km/h). The impact was deliberate. The mission, known as the Double Asteroid Redirection Test (DART), was designed to test whether a high-speed collision could shift an asteroid’s orbit. It was the first real-world demonstration of a planetary defense strategy.
By initial measurements, the mission succeeded. The orbit of Dimorphos around its larger companion, Didymos, shortened by over 30 minutes. It proved that humans can move a space rock with a kinetic hit. That alone was historic.
But recent peer-reviewed analysis published in The Planetary Science Journal revealed a far more complex outcome than expected. The impact blasted more than 100 large fragments off the asteroid, many moving faster and in directions that didn’t match DART’s trajectory. Some of that ejected material may have delivered more momentum than the spacecraft itself.
Debris Clouds Carried Unexpected Force
Minutes after the collision, a companion probe called LICIACube, developed by the Italian Space Agency, flew past the asteroid and captured images of the expanding debris plume. These images became critical in a new University of Maryland-led study that tracked 104 large boulders dislodged by the impact.
Some fragments measured more than 11 feet across, moving at speeds of up to 52 meters per second. Far from dispersing evenly, more than 70 percent of the debris clustered in a concentrated southern plume.
“We saw that the boulders weren’t scattered randomly in space,” said Tony Farnham, lead author of the study and a scientist in Maryland’s Department of Astronomy. “They were clustered in two pretty distinct groups, with an absence of material elsewhere, which means that something unknown is at work here.”
The team believes part of the spacecraft, likely its solar panels, collided with two prominent surface boulders, Atabaque and Bodhran, just before the main body hit. These secondary impacts may have shattered the rocks and contributed to the directional debris jets.
Secondary Momentum Changed the Physics
Calculations in the same 2025 Planetary Science Journal paper show that the boulders launched from Dimorphos carried more than three times the momentum of the DART spacecraft itself. Even more significantly, much of that energy moved perpendicular to the original impact path.
Rather than simply altering the orbit, the debris may have caused the asteroid’s orbital plane to tilt by up to one degree. Small in appearance, but substantial in terms of long-term motion.
“The boulders ejected gave an additional kick that was almost as big” as the spacecraft’s, Farnham explained. This complicates assumptions that an asteroid’s course can be reliably altered with a single directional push.
If a threatening asteroid is headed for Earth, and the objective is to shift it with high precision, these secondary effects could reduce accuracy or trigger unintended motion.
Deep Impact Told a Different Story
Nearly two decades earlier, NASA tested a similar concept with a different target. The Deep Impact mission struck Comet Tempel 1 in 2005 to study its interior. That collision produced a predictable, symmetrical plume that closely followed models.
What changed with DART? The surface.
Unlike Tempel 1’s relatively smooth composition, Dimorphos is a rubble-pile asteroid, a loosely bound collection of rocks and voids. This kind of terrain introduced chaotic physics when hit.
“Deep Impact hit a surface that was essentially very small, uniform particles, so its ejecta was relatively smooth and continuous,” said Jessica Sunshine, professor of astronomy and geology at Maryland and co-author of both missions. “And here, we see that DART hit a surface that was rocky and full of large boulders, resulting in chaotic and filamentary structures.”
That difference explains why earlier impact models didn’t fully capture DART’s aftermath. It also raises a critical concern for planetary defense: surface composition matters more than previously accounted for.
More Answers Expected from Hera
While much has been learned, the long-term effects of the DART mission are still unfolding. The European Space Agency’s Hera mission, scheduled to reach Dimorphos in 2026, will offer a second look. Hera is expected to map the asteroid’s new shape and rotation, study the crater left by DART, and refine estimates of momentum transfer.
Hera will play a key role in confirming how much of the energy from the spacecraft and from the debris ultimately changed the asteroid’s behavior.
Data from Hera will also help improve international planetary defense protocols, as coordinated by agencies such as ESA and NASA’s Planetary Defense Coordination Office. This includes refining how surface properties are assessed and how energy transfer models are updated for real-world application.
Farnham emphasized the broader stakes in his team’s findings. “We succeeded in deflecting an asteroid, moving it from its orbit. But that additional factor changes the physics we need to consider when planning these types of missions.”