In 2022, a NASA spacecraft intentionally collided with the small asteroid Dimorphos during a planetary defense test. The objective was assess whether humanity would be able to protect Earth from cosmic threats, such as space rocks. New observations now reveal that the planetary defense test was a successaltering the orbit of Dimorphos, as well as that of a larger asteroid called Didymos.
Dimorphos and Didymos form a binary pair, meaning the asteroids orbit each other while also orbiting the Sun — and a measurable change in one will affect the other.
The new data shows that the time needed for Didymos and Dimorphos to complete a solar orbit, which takes about 770 days, permanently decreased by less than a second after the Double Asteroid Redirection Test (DART), according to a study published Friday in the journal Science Advances.
“The change in the binary system’s orbital velocity was about 11.7 micrometers per second, or 4.3 centimeters per hour,” study lead author Dr. Rahil Makadia, a planetary defense scientist who worked on the DART team and recently completed his doctorate in aerospace engineering at the University of Illinois Urbana-Champaign, said in a statement. “Over time, such a small change in an asteroid’s motion could make the difference between one hitting our planet or not.”
According to the study authors, the DART mission marks the first time that a man-made object has altered the trajectory of a celestial body in its orbit around the Sun — and, if an asteroid is found on a collision course with Earth in the future, this may not be the last time.
The consequences of a collision
Although Didymos and Dimorphos never represented one, the binary system provided NASA with the perfect scenario to evaluate how effectively a spacecraft could be used as a diversion tool.
But to assess the test’s success, researchers needed to measure how Dimorphos and Didymos were altered by the impact.
Didymos is shaped like a top and is believed to be a debris asteroid — essentially a collection of dust and rocks held together loosely by gravity. Dimorphos, also a debris asteroid, likely formed from fragments that clumped together after being expelled by Didymos.
When the DART probe collided with Dimorphos, a huge cloud of debris was released into space, estimated at 16 million kilograms (35.3 million pounds). Although the 170-meter-diameter (560-foot) space rock lost just 0.5% of its mass, the debris released was 30,000 times greater than the mass of the spacecraft, according to previous research.
Scientists determined that the force of the fragments expelled by the asteroid had an even greater impact than that of the spacecraft when it collided with the space rock. This increase in thrust helped reduce the time needed for the two asteroids to orbit the Sun.
Previous research has shown that Dimorphos’s 12-hour orbit around Didymos has shortened by 33 minutes.
The new study highlights that the enormous amount of material expelled by the asteroid system also increased the speed at which both space rocks orbit the Sun, reducing the total orbital time by 0.15 seconds.
To measure this orbital change, astronomers relied on ground-based observations of Didymos, as well as data from when the asteroid passed directly in front of stars. Known as stellar occultations, these movements allow scientists to measure an asteroid’s exact position, speed and shape.
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But detecting, from our perspective on Earth, the moment a star blinks for a fraction of a second as an asteroid passes in front of it is incredibly challenging. The study’s conclusions depended on 22 stellar occultations recorded between October 2022 and March 2025 by volunteer astronomers around the world.
“When combined with years of existing ground-based observations, these stellar occultation observations became essential in helping us alter Didymos’ orbit,” said Steve Chesley, senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and co-lead author of the study, in a statement. “This work is highly dependent on weather conditions and often requires travel to remote regions with no guarantee of success. This outcome would not have been possible without the dedication of dozens of volunteer occultation observers around the world.”
Patrick Michel, principal investigator for the European Space Agency’s Hera mission, which will launch in 2024 and fly over the area after the collision with the DART asteroid, was amazed that such a small difference in the two asteroids’ orbits was possible to measure.
“We knew that such a small change could occur without posing any risk to the Earth, but actually measuring it was another challenge that the team faced with extreme competence,” Michel wrote in an email. “To do this, well-organized international coordination is necessary, as it is necessary to precisely time the oscillations caused by the passage of an asteroid in front of a star, as observed by different observers around the planet. If this is done correctly, as in this study, it is possible to carry out measurements with incredible precision.”
Monitoring risky asteroids
More observations and measurements of DART’s effect on space rocks will be shared once the Hera probe enters orbit around the asteroid system later this year. Hera will capture and share the first images of Dimorphos this fall, Michel said.
Meanwhile, NASA’s Near-Earth Object Surveyor mission, currently in development, could detect dark, dangerous asteroids that have remained largely invisible to ground-based observatories.
Identifying potentially dangerous asteroids and understanding how a small change in orbit can lead to a significant deviation are aspects that are closely linked to the way space agencies envision protecting the Earth.
“The team’s incredibly precise measurement once again validates kinetic impact as a technique for defending Earth against the dangers of asteroids and shows how a binary asteroid can be deflected by impacting just one member of the pair,” said Thomas Statler, chief scientist for small solar system bodies at NASA, in a statement. He did not participate in the study.
If an asteroid that poses risks to our world is found early enough to be deflected, a kinetic impactor like DART could be sent to propel the space rock, or its companion, into a more benign orbit that avoids Earth.
