A potentially city- or even country-leveling asteroid will inevitably head toward Earth. In 2022, NASA conducted a crucial test run to simulate planetary defense, intentionally colliding an uncrewed spacecraft with a non-threatening asteroid to alter its course. New analysis reveals the impact was even more effective than initially anticipated.
The target was Dimorphos, a small moon orbiting the larger asteroid Didymos. Neither asteroid posed a threat to Earth, and altering Dimorphos’ orbit around Didymos was considered unlikely to change that. The mission, known as the Double Asteroid Redirection Test (DART), successfully demonstrated the feasibility of altering an asteroid’s trajectory – a key step in developing planetary defense capabilities.
DART Mission More Powerful Than Expected
Precise telescope observations have now revealed that the DART mission’s force wasn’t limited to Dimorphos. The recoil from the impact also imparted a gravitational nudge to the larger asteroid Didymos, subtly shifting both asteroids’ orbits around the sun. This unexpected outcome provides valuable data for refining future planetary defense strategies.
“By hitting the moon so hard, we also moved the giant object next to it a little bit,” said Andy Rivkin, an astronomer at the Johns Hopkins University Applied Physics Laboratory, and a co-author of the new study published today in the journal Science Advances. This marks the first time humanity has successfully altered the orbit of an asteroid around the sun.
Could the Collision Have Posed a Threat to Earth?
Binary asteroid systems, like Didymos and Dimorphos, share a common center of gravity, dominated by the larger object – in this case, Didymos, which is 200 times more massive than its moon. Yet, disrupting the balance of the smaller object can also affect the larger one. The findings underscore the complex dynamics at play when considering asteroid deflection strategies.
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Prior to the DART mission in 2020, scientists modeled all possible scenarios. “What if this experiment puts the Didymos system on a collision course with Earth?” said Rahil Makadia, a co-author of the study and a planetary defense researcher at the University of Illinois at Urbana-Champaign.
They determined there would be no observable effect on Didymos, and that the impact on Dimorphos wouldn’t alter its course.
Debris Explosion Acted Like a Rocket Engine
NASA initially stated the mission would be considered successful if DART changed Dimorphos’ orbit around Didymos by 73 seconds. Instead, the spacecraft – roughly the size of a van – shortened the orbit by 33 minutes, thanks to the force of the DART impact and the resulting burst of rocky debris ejected from Dimorphos.
Before the mission, astronomers hypothesized that Dimorphos wasn’t a solid rock, but a collection of boulders held together by the asteroid’s weak gravity. A spacecraft impact at 22,500 kilometers per hour would inevitably send some of the asteroid flying.
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However, the DART impact released far more debris than expected. This cloud of ejecta acted like a rocket exhaust, powerfully pushing the asteroid backward – more than predicted.
The Unexpected Consequences of the DART Mission
“We immediately thought this must have unforeseen consequences,” said Federica Spoto, a researcher at the Harvard and Smithsonian Center for Astrophysics, who was not involved in the new research. If Dimorphos was so significantly affected by the spacecraft, what would happen to Didymos?
Since the end of the DART mission in 2022, Makadia and his team have tracked Didymos and Dimorphos using telescopes. This proved challenging, requiring precise timing to observe the asteroids as they passed in front of distant stars.
By measuring how the asteroids temporarily blocked starlight, the scientists determined their speed and direction. Makadia’s team found that the two asteroids were slowed, but by only 0.22 millionths of a mile per hour.
Small Changes Can Have Large Consequences
“That’s a minuscule change,” said Makadia. But over time, small changes can accumulate and drastically alter asteroid orbits. The scientists calculated where Didymos and Dimorphos could end up in the long term. “We don’t need to worry about Didymos impacting Earth,” Makadia stated.
The calculations demonstrate astronomers can detect incredibly precise changes in asteroid orbits. “This is impressive,” said Cristina Thomas, an astronomer at Northern Arizona University, who was not involved in the study.
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The research team also accurately determined the density of Dimorphos and Didymos. Dimorphos has a density only slightly higher than water, explaining why its shape changed when DART impacted it. Didymos is significantly denser and more rocky.
Important Insights for Earth Protection
Knowing the density of different asteroids is crucial for planetary defense. Attempting to drastically alter the orbit of an asteroid like Dimorphos by impacting it with a spacecraft could cause it to break apart into multiple fragments headed toward Earth.
This won’t happen with an asteroid with the density of Didymos, but it also means multiple interceptor rockets would be needed to achieve the desired deflection.
Later this year, the Hera spacecraft from the European Space Agency (ESA) will arrive at Dimorphos to forensically examine the wreckage of DART. This will undoubtedly yield new insights into humanity’s first experiment in planetary defense.
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