In 2022, NASA’s Double Asteroid Redirection Test (DART) mission deliberately struck the asteroid Dimorphos, which orbits a larger asteroid called Didymos. Now scientists have confirmed that DART not only changed Dimorphos’ orbit around its binary companion, but it also changed the entire binary system’s orbit around the Sun.
“The change in the binary system’s orbital velocity was about 11.7 microns per second, or 1.7 inches per hour,” Rahil Makadia of the University of Illinois Urbana-Champaign said in a statement. “Over time, such a small change in an asteroid’s motion can make the difference between a dangerous object hitting or missing our planet.”
The goal of DARTS was to see if a kinetic impactor—in this case, the DART spacecraft—could deflect one asteroid and prove that if an asteroid of the same size was on a collision course with Earthwe could knock it out of the way.
The article continues below
The Didymos – Dimorphos double asteroid system was a safe place to practice this. Didymos is the larger asteroid, about 2,788 feet (850 meters) across, and the smaller 560-foot (170-meter) asteroid Dimorphos orbits Didymos. Because astronomers had previously accurately measured Dimorphos’ orbital period and radius, any deflection caused by the impact would also be clearly measurable. Additionally, because Dimorphos is gravitationally bound to Didymos, the DART impact could not accidentally knock it toward Earth.
The DART impact took place on September 26, 2022, with the spacecraft traveling at a speed of 6.6 kilometers per second. It hit Dimorphos enough that the orbital period around Didymos was shortened from 11 hours and 55 minutes to 11 hours and 23 minutes. Before impact, the goal had been to push Dimorphos so that its orbital period was reduced by a minimum of 73 seconds, meaning the mission was a huge success.
New analysis of the data, led by Makadia and Steve Chesley of NASA’s Jet Propulsion Laboratory (JPL), has shown that DART’s efforts to push Dimorphos got a helping hand from the cloud of debris, called ejecta, that it sprayed into space as it excavated another new crater.
As this ejection cloud raced away from Dimorphos, it also carried momentum away. And since momentum is always conserved, this gave Dimorphos an extra push. Scientists call this extra push the “momentum enhancement factor,” and in the case of Dimorphos and the DART effect, the momentum enhancement factor had a value of two. This means that the loss of thrust doubled the thrust imparted on Dimorphos by the initial DART impact.
Because Dimorphos and Didymos are connected by the invisible bonds of gravitythis extra push pulled both Dimorphos and Didymos off course in their orbits sunand changes their orbital period by 0.15 seconds.
This may not sound like much, but as Makadia refers to, it would be sufficient to push a dangerous asteroid away from Earth (as long as the asteroid is detected in time).
To this end, NASA intends to launch a new space telescope called the Near-Earth Object (NEO) Surveyor sometime after September 2027. NEO Surveyorits mission will be to find as many of the remaining undiscovered asteroids near Earth’s orbit as possible.
Determining the change in Didymos and Dimorphos’ orbit around the sun was a labor of love, especially for the 49 amateur astronomers who traveled to the ends of the earth in search of stellar occultations.
A stellar occultation is when an object passes in front of a star from our point of view. When an asteroid occults a star, we can learn about the asteroid’s shape, size, position and orbit.
Stellar occultations are unfortunately difficult to observe – they can only be seen from random narrow tracks above the Earth’s surface. However, thanks to the amateur astronomers who traveled around the globe between October 2022 and March 2025 to witness 22 stellar occultations of the Didymos–Dimorphos system, Makadia and Chesley’s team were able to calculate the change in the binary system’s orbit around the Sun.
“When combined with years of existing ground-based observations, these stellar occultation observations became key to helping us calculate how DART had changed Didymos’ orbit,” Chesley said. “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 commitment of dozens of volunteer occultation observers around the world.”
Based on the orbital changes, it was also possible to calculate the densities of the two asteroids. Didymos has a density of 2,600 kilograms per cubic meter, while Dimorphos is less dense than assumed, with 1,540 kg per cubic meter/, making it something of a loosely composed rock pile. This supports the hypothesis that Dimorphos was formed from material once spun by Didymos.
These latest findings from the DART mission were published on March 6 in the journal The progress of science.
