How they did it
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 JPL. “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.
More about DART
The DART spacecraft was designed, built, and operated by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, for NASA’s Planetary Defense Coordination Office, which oversees the agency’s ongoing efforts in planetary defense. It was humanity’s first mission to intentionally move a celestial object.
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