Between October 2022 and March 2025, we captured 22 such stellar mysteries of the Didymos system. Combined with a huge dataset publicly available at the Minor Planet Data Center, which included nearly 6,000 ground-based astrometric measurements taken over 29 years, optical navigation data from the DART probe’s approach, and ground-based radar measurements, the researchers finally had everything they needed.
“Once we have enough measurements before and after the DART impact, we can understand how Didymos’ orbit changed,” Makadia said.
When the vending-machine-sized DART probe crashed into Dimorphos at a speed of more than 22,000 kilometers per hour, it reduced the track velocity of the entire Didymos system by about 11.7 micrometres per second. But the team feels it is still important. “When you do it early enough, even a small impulse can accumulate over years and cause a meaningful change,” Makadia explained.
Also, the DART effect was not the only force that changed Didymos’ orbit.
ejecta engine
The net kinetic energy of a 500-kilogram spacecraft hitting a rocket at hypersonic speed is impressive, but by itself, it wouldn’t be able to slow a giant asteroid that much. When the dart attacked Dimorphos, it threw pulverized rock and dust into the void. “The material ejected from the asteroid’s surface acts like an additional rocket plume,” Makadia said.
Scientists call this effect the speed increase factor, which is represented by the Greek letter beta. If the impact spacecraft moved at exactly the same speed and no debris came up, the beta would be exactly one.
Because Dimorphos orbits Didymos, some of the ejecta remained trapped in the system, where it changed the mutual orbit between the two rocks. But a significant fraction of the ejecta achieved the escape velocity of the entire binary system. The momentum carried away by the debris leaving the system ultimately contributes to pushing the center of mass of the entire Didymos-Dimorphos pair. “In our case, we found that the beta parameter was around two because of the dart effect,” Makadia said.
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