In a groundbreaking achievement for planetary science and defense, NASA has successfully altered the orbital path of an asteroid system around the sun—a feat never before accomplished by humanity. This milestone comes from the Double Asteroid Redirection Test (DART) mission, which in September 2022 deliberately crashed a spacecraft into Dimorphos, the smaller member of a binary asteroid system, to test whether such an impact could change the asteroid’s trajectory in a way that might one day protect Earth from hazardous space rocks.
Dimorphos is a moonlet orbiting a larger asteroid called Didymos. The DART mission targeted Dimorphos with a 570-kilogram spacecraft traveling at roughly 22,530 kilometers per hour, aiming to impart enough force to slow the moonlet’s orbit around Didymos. Initial results showed that the impact shortened Dimorphos’s orbital period by about 30 minutes, confirming that a kinetic impact can effectively nudge an asteroid’s motion. However, recent detailed analyses have revealed something even more profound: the collision did not just alter Dimorphos’s orbit around its companion but also changed the entire binary asteroid system’s orbit around the sun.
This discovery was detailed in a study published in the journal Science Advances. Researchers found that by slowing Dimorphos’s orbit around Didymos, the impact indirectly caused the binary system’s heliocentric (sun-centered) orbit to slow by approximately 12 microns per second, equivalent to a reduction of about 370 meters per year. While this change is minuscule on a human scale, it represents the first time humanity has successfully modified the orbit of a natural celestial body around our star.
Rahil Makadia, the study’s lead author and a planetary defense scientist formerly at the University of Illinois Urbana-Champaign, emphasized the importance of this result in advancing Earth’s planetary defense capabilities. “If [an asteroid] is ever on its way to hitting the Earth, we can more confidently now say that we have the ability to push them around and away from the Earth,” Makadia stated. Although Dimorphos and Didymos themselves do not pose a threat to Earth, their selection as DART’s target was strategic: the binary system serves as an ideal testbed to assess and refine methods for asteroid deflection.
The researchers employed a combination of radar measurements and observations of the asteroid system's transit across the sun to compare its orbit before and after the DART impact. By doing so, they could detect the subtle changes in the system’s orbital velocity around the sun. This method allowed them to measure the slight but definitive deceleration of the binary’s heliocentric orbit, confirming a key hypothesis that had been proposed even before the DART impact occurred.
“This was something we had thought about even before the DART impact,” Makadia explained. “But what we didn’t know was the extent to which this would happen and whether or not we would be able to measure it at all.” The successful measurement validates theoretical predictions and demonstrates the precision of current observational technologies.
The study also delved into the physics of the impact itself, introducing the concept of the “momentum enhancement factor.” This factor quantifies how much additional momentum was transferred to Dimorphos beyond the direct push of the spacecraft, due to the ejection of rocks, dust, and other debris caused by the collision. The researchers found that this ejecta effectively doubled the force exerted by the spacecraft alone, significantly amplifying the impact’s effect on the asteroid’s trajectory.
In addition to confirming the impact’s effectiveness, the team achieved a first in estimating the masses of Dimorphos and Didymos separately. This information is critical for understanding not just the immediate effects of kinetic impacts but also the physical composition and structure of these bodies. Masatoshi Hirabayashi, an associate professor of aerospace engineering at the Georgia Institute of Technology and a DART collaborator who was not involved in the new study, pointed out that these findings could have broader scientific implications. “Knowing the asteroids’ respective mass and densities could help scientists better understand their structure,” he said, describing this knowledge as a “key piece of information of how this binary asteroid formed.”
The DART mission’s success and the new study’s findings mark a significant step forward in planetary science and defense. They demonstrate that human technology can interact with and alter the dynamics of celestial bodies in a controlled, measurable way. This capability is crucial as the threat of potentially Earth-impacting asteroids remains a real