3/9/2026 Ian J. O'Neill, NASA
The spacecraft changed the binary system’s orbit around the sun, confirming that a kinetic impactor can be an effective planetary defense technique for deflecting a near-Earth object.
Written by Ian J. O'Neill, NASA
In September 2022, NASA’s Double Asteroid Redirection Test spacecraft intentionally crashed into the asteroid moonlet Dimorphos. A new study, conducted by aerospace engineers Rahil Makadia, B.S. '20, Ph. D. ’25, and his adviser, Siegfried Eggl, in The Grainger College of Engineering, University of Illinois Urbana-Champaign, and 12 international collaborators, shows the impact didn’t just change the motion of Dimorphos around its larger companion, Didymos; it also shifted the orbit of both asteroids around the Sun.
"Being able to show that DART was the first mission to actually change an asteroid's orbit about the Sun before Hera arrives is simply amazing. This study really shows how powerful the combination of ultra-high precision astrometry and modern astrodynamics has become," Eggl said.
Linked together by gravity, Didymos and Dimorphos orbit each other around a shared center of mass in a configuration known as a binary system, so changes to one asteroid affect the other.
As detailed in the study, observations of the pair’s motion revealed that the 770-day orbital period around the Sun changed by a fraction of a second after the DART spacecraft’s impact on Dimorphos. That change marks the first time a human-made object has measurably altered the path of a celestial body around the Sun.
“This is a tiny change to the orbit, but given enough time, even a tiny change can grow to a significant deflection,” said Thomas Statler, lead scientist for solar system small bodies at NASA Headquarters in Washington. “The team’s amazingly precise measurement again validates kinetic impact as a technique for defending Earth against asteroid hazards and shows how a binary asteroid might be deflected by impacting just one member of the pair.”
When DART struck Dimorphos, the impact blasted a huge cloud of rocky debris into space, altering the shape of the asteroid, which measures 560 feet, or 170 meters wide. Because the debris carried its own momentum away from the asteroid, it gave Dimorphos an explosive thrust — what scientists call the momentum enhancement factor. More debris being kicked out means more oomph. According to the new research, the momentum enhancement factor for DART’s impact was about two, meaning that the debris loss doubled the punch created by the spacecraft alone.
Earlier research showed that the smaller asteroid’s 12-hour orbital period around the nearly half-mile wide, or 805 meters, shortened by 33 minutes. The new study shows the impact ejected so much material from the binary system that it also shortened the binary system’s orbital period around the Sun by 0.15 seconds.
“The change in the binary system’s orbital speed was about 11.7 microns per second, or 1.7 inches per hour,” said Makadia, who led this study. “Over time, such a small change in an asteroid’s motion can make the difference between a hazardous object hitting or missing our planet.”
Although Didymos was not on an impact trajectory with Earth and it was impossible for the DART mission to put it on one, that change in orbital speed underscores the role spacecraft — aka kinetic impactors in this context — could play if a potentially hazardous asteroid is found to be on a collision course in the future. The key is detecting near-Earth objects far enough in advance to send a kinetic impactor.
To prove DART had a detectable influence on both asteroids — not just on the smaller Dimorphos — the researchers needed to measure Didymos’ orbit around the Sun to exquisite precision. So, in addition to making radar and other ground-based observations of the asteroid, they tracked stellar occultations, which occur when the asteroid passes exactly in front of a star, causing the pinpoint of light to blink out for a fraction of a second. This technique provides extremely precise measurements of the asteroid’s speed, shape, and position.
Measuring stellar occultations is challenging. Astronomers have to be in the right place at the right time with several observing stations, sometimes miles apart, to track the predicted path of the asteroid in front of a specific star. The team relied on volunteer astronomers around the globe who recorded 22 stellar occultations between October 2022 and March 2025.
“When combined with years of existing ground-based observations, these stellar occultation observations became key in helping us calculate how DART had changed Didymos’ orbit,” said study co-lead Steve Chesley, a senior research scientist at NASA’s Jet Propulsion Laboratory. “This work is highly weather dependent and often requires travel to remote regions with no guarantee of success. This result would not have been possible without the dedication of dozens of volunteer occultation observers around the world.”
Studying changes in Didymos’ motion also helped the researchers calculate the densities of both asteroids. Dimorphos is slightly less dense than previously thought, supporting the theory that it formed from rocky debris shed by a rapidly spinning Didymos. This loose material eventually clumped together to form Dimorphos, a “rubble pile” asteroid.
“This study is exciting because now we have actual predictions for the mass and density of Dimophos that can be validated once the European Space Agency’s Hera mission arrives in about a year’s time,” Eggl said.
The study, “Direct detection of an asteroid’s heliocentric deflection: The Didymos system after DART,” was written by Rahil Makadia, Steven R. Chesley, David Herald, Davide Farnocchia, Nancy L. Chabot, Shantanu P. Naidu, Andrew S. Rivkin, Alexandros Siakas, Damya Souami, Paolo Tanga, Sotirios Tsavdaridis, Kleomenis Tsiganis, Sébastien Bouquillon and Siegfried Eggl. It is published in the journal Science Advances. DOI:10.1126/sciadv.aea4259