Universe Today
NASA’s Double Asteroid Redirection Test (DART) spacecraft impacted the asteroid moonlet Dimorphos, which orbits the larger asteroid Didymos, in September 2022. The purpose of this mission was to test the kinetic impactor method, a potential strategy to alter the orbit of asteroids so they don't pose a threat to Earth. The test was a success, as images taken by the Italian Space Agency’s LICIACube (which traveled alongside the DART mission) after the impact showed. Combined with Earth-based observations, these confirmed that the moonlet's orbit changed noticeably.
According to a recent update from NASA, DART"s impact also altered the orbit of both asteroids around the Sun. Since Didymos and Dimorphos are part of a binary system and orbit each other around a shared center of mass, changes to one asteroid affect the other. As an international team of researchers described in a study that appeared in the journal Science Advances, observations revealed that asteroids' 770-day orbital period around the Sun changed by a fraction of a second post-impact. This marks the first time a human-made object has altered the path of a celestial body around the Sun.
When DART struck Dimorphos, follow-up observations showed that the moonlet's orbit was shortened by 33 minutes. The impact also produced a cloud of rocky debris that carried its own momentum away from the asteroid. This imparted momentum to the asteroid, known as a momentum enhancement factor, in addition to the impact alone. According to the new study, the momentum enhancement factor from DART's impact was about two, meaning the debris doubled the force imparted by the spacecraft alone. This is what led to Didymos' orbit being altered by 0.15 seconds.
*This LICIACube image, taken moments after impact on Sept. 26th, 2022, shows rocky debris fanning out from Dimorphos. Credit: ASI/NASA*
Thomas Statler, the lead scientist for Solar System small bodies at NASA Headquarters, said in a NASA press release:
This is a tiny change to the orbit, but given enough time, even a tiny change can grow to a significant deflection. 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.
To demonstrate that DART had a detectable influence on both asteroids, the researchers needed to measure Didymos’ orbit with extreme precision. To do this, the team combined radar and other ground-based observations and tracked how the asteroid passed in front of background stars (aka stellar occultations). This was challenging because tracking stellar occultations requires precise timing and being in the right location. For this, the team relied on volunteer astronomers around the globe who recorded 22 stellar occultations between October 2022 and March 2025. Said study co-lead Steve Chesley, a senior research scientist at JPL:
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. 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. This technique allowed the team to obtain extremely precise measurements of the asteroid’s speed, shape, and position.
*The above infographic shows the current orbit of Dimorphos around Didymos, and its projected orbit following the impact of DART. Credit: NASA/Johns Hopkins APL*
"The change in the binary system’s orbital speed was about 11.7 microns per second, or 1.7 inches per hour. Over time, such a small change in an asteroid’s motion can make the difference between a hazardous object hitting or missing our planet," added Rahil Makadia of the University of Illinois Urbana-Champaign and the lead author on the paper. However, NASA and the ESA have both determined that the modification to Didymos' orbit does not make it a threat to Earth. Studying changes in Didymos’ motion also helped the researchers calculate the densities of both asteroids, which revealed that Dimorphos is slightly less dense.
This supports the theory that it formed from rocky debris shed by a rapidly spinning Didymos that eventually clumped together to form a “rubble pile” asteroid. Although Didymos is not a Potentially Hazardous Object (PHO), meaning it poses no collision threat to Earth, the success of the DART mission demonstrates the effectiveness of the kinetic impactor method. The first step, however, is to detect PHOs far enough in advance that a kinetic impactor can be sent to rendezvous with them. This is the purpose behind NASA's Near-Earth Object (NEO) Surveyor mission: a next-generation space survey telescope, and the first to be built for planetary defense.
Further Reading: NASA
