Asteroid defense mission shifted the orbit of more than its target
Between October 2022 and March 2025, we captured 22 stellar occultations from the Didymos system. Combined with a massive publicly available dataset at the Minor Planet Data Center, including nearly 6,000 ground-based astrometric measurements taken over 29 years, optical navigation data from the DART probe approach, and ground-based radar measurements, the researchers finally had everything they needed.
“Once we had enough measurements before and after the DART impact, we were able to discern how Didymos’ orbit changed,” Makadia said.
When the vending machine-sized DART probe crashed into Dimorphos at more than 22,000 kilometers per hour, it reduced the speed along the track of the entire Didymos system by about 11.7 micrometers per second. But the team thinks it’s still important. “When you do it early enough, even a small push can build up over years and cause significant change,” Makadia explained.
Additionally, the DART impact itself is not the only force that changed Didymos’ orbit.
The engine ejected
The sheer kinetic energy of a 500-kilogram spacecraft hitting at hypersonic speeds is impressive, but alone it wouldn’t slow down a huge asteroid that much. When DART hit Dimorphos, it sent pulverized rock and dust flying into the void. “The material projected from the surface of an asteroid acts like an additional rocket plume,” Makadia said.
Scientists call this effect the momentum enhancement factor, denoted by the Greek letter beta. If the spacecraft impact transferred exactly its own momentum and no debris was thrown, the beta would be exactly that.
Because Dimorphos orbits Didymos, some of the ejecta became trapped in the system, where they altered the mutual orbit between the two rocks. But a crucial fraction of the ejecta reached the escape velocity of the entire binary system. The momentum carried by debris escaping from the system is what ultimately helped move the center of mass of the entire Didymos-Dimorphos pair. “In our case, we found that the beta parameter due to the impact of DART was around two,” Makadia explained.
