Spacecraft’s impact changed asteroid’s orbit around the sun in a save-the-Earth test, study finds

CAPE CANAVERAL, Fla. – An asteroid that NASA used for target practice a few years ago was nudged into a slightly different route around the sun, findings that could help divert a future incoming killer space rock, scientists reported Friday.

It’s the first time that a celestial body’s orbit around the sun was deliberately changed. The asteroid that NASA’s Dart spacecraft slammed into was never a threat to Earth.

“This study marks a notable step forward in our ability to prevent future asteroid impacts on Earth,” the international research team wrote in Science Advances.

The changes were slight — reductions of just one-tenth of a second and one-half of a mile (720 meters) to a solar lap spanning two years and hundreds of millions of miles (kilometers), according to the scientists.

“Even though this seems small, a tiny deflection … can add up over decades and make the difference between a potentially hazardous asteroid hitting or missing the Earth in the future,” lead author Rahil Makadia, of the University of Illinois Urbana-Champaign, said in an email.

For any save-the-planet tests, “the key isn’t delivering a huge shove at the last minute. The key is delivering a tiny shove many years in advance,” he added.

Launched in 2021 on the world’s first planetary defense exercise, the Dart spacecraft deliberately plowed into Dimorphos, which orbits a bigger asteroid, Didymos, as they circle the sun together. The space agency quickly determined that the 2022 strike trimmed the smaller asteroid’s orbit around its bigger companion.

But it took until now for scientists to confirm, based on observations from around the world, that the impact cut the duo’s travel time around the sun by 0.15 seconds. With each solar orbit lasting 769 days, that’s a real-time slowdown of just over 10 micrometers per second, shrinking the asteroids’ 300-million-mile (480-million-kilometer) orbit by 2,360 feet (720 meters).

The researchers said all the boulders and other debris flung off Dimorphos in the crash provided as much push to Dimorphos as the spacecraft itself — a doubling of momentum. Last summer, a U.S.-Italian team estimated that 35 million pounds (16 million kilograms) of rock and dust were ejected.

The good news is that even with the change in the asteroids’ course, Earth remains safely out of their way for the foreseeable future. That’s why this rubble-packed system was picked for the mission, said Steven Chesley of NASA’s Jet Propulsion Laboratory, who took part in the study.

“While it is just a single experiment, it is nonetheless an important data point that will be relevant to any future asteroid deflection missions,” Chesley said in an email.

Scientists expect to learn even more about the impact’s aftermath when the European Space Agency’s Hera spacecraft reaches the asteroids in November. Dimorphos is 525 feet (160 meters) in diameter. Fast-spinning Didymos is 2,560 feet (780 meters) across with, according to the latest study, 200 times more mass than its sidekick.

Unlike Dart, Hera will not strike but will tag along for months of surveying. A pair of small experimental probes will peel away and attempt to land.

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