Sooner or later, an asteroid large enough to wipe out a city or perhaps even an entire country will fly toward Earth. That’s why NASA conducted a dress rehearsal in 2022 to simulate the defense of our planet. The space agency deliberately crashed an unmanned spacecraft into a harmless asteroid to change its course. The effect turned out to be greater than expected.
The target was Dimorphos, a small moon orbiting the larger asteroid Didymos. Neither poses a threat to Earth, and altering Dimorphos’ orbit around Didymos was extremely unlikely to change that. The mission, the Double Asteroid Redirection Test (DART), was a success and showed that it is possible to change the course of an asteroid.
DART mission was more powerful than expected
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But accurate telescope observations have now revealed that the DART mission was so powerful that the rebounding Dimorphos also gave the larger asteroid Didymos a gravitational boost. As a result, both asteroids ended up in slightly different orbits around the sun.
“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 Applied Physics Laboratory at Johns Hopkins University (US) and one of the authors of the new study, published today in the journal Science Advances has been published. It is the first time that it has been possible to change the orbit of an asteroid around the sun.
Could the collision pose a danger to Earth?
Binary asteroids, such as Didymos and Dimorphos, share a gravitational center. That center is dominated by the more massive object, in this case Didymos, which is two hundred times as massive as its moon. But if you throw the smaller object out of balance, it will also affect the larger one.
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Prior to the DART mission in 2020, scientists calculated all possible scenarios. “What if this experiment puts the Didymos system on course to collide with Earth?” says Rahil Makadia, one of the study’s co-authors and a planetary defense researcher at the University of Illinois at Urbana-Champaign (USA).
They found that there would be no observable effect on Didymos. Didymos would feel the impact on Dimorphos, but would not be thrown off course by it.
An explosion of rubble acted like a rocket engine
At the time, NASA stated that the mission would only be successful if DART changed Dimorphos’ orbit around Didymos by 73 seconds. Instead, the small, van-sized spacecraft managed to shorten the asteroid’s orbit by 33 minutes – thanks to the force of the DART impact and the eruption of rocky debris ejected from Dimorphos’ impact.
Before the mission, astronomers suspected that Dimorphos was not a large rock, but a collection of boulders held together by the asteroid’s weak gravity. If you collided with a spaceship at a speed of 22,500 kilometers per hour, part of the asteroid would inevitably be thrown into space.
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But DART’s impact released much more debris than expected. That cloud of debris acted like a rocket jet, pushing the asteroid back forcefully, much more than predicted.
The unexpected consequences of the DART mission
“We immediately thought: This must have unforeseen consequences,” says Federica Spoto, a researcher at Harvard’s Center for Astrophysics and the Smithsonian, who was not involved in the new study. If Dimorphos was so profoundly affected by the spaceship, what would have happened to Didymos?
Since the end of the DART mission in 2022, Makadia and his team have been tracking Didymos and Dimorphos using telescopes. That was quite a challenge. They had to look through the right telescope at the right time to see the celestial body pass in front of a distant star.
Based on how the asteroid temporarily blocks starlight, the scientists were able to determine how fast and in which direction it is moving through space. The Makadia team found that the two asteroids had slowed down, but only by 0.22 millionths of a mile per hour.
Small changes can have big consequences
“That’s a minuscule change,” Makadia says. But over time, small changes can add up and drastically alter the orbit of asteroids. The scientists calculated where Didymos and Dimorphos could end up in the long term. “We don’t have to worry about Didymos impacting Earth,” Makadia says.
The calculations show that astronomers can detect unimaginably accurate changes in the orbit of asteroids. “This is impressive,” says Cristina Thomas, an astronomer at Northern Arizona University (USA), who was not involved in the study.
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In addition, the research team was able to accurately determine the density of Dimorphos and Didymos. Dimorphos has a density not much higher than that of water, which explains why its shape changed when DART flew into it. Didymos is considerably denser and more mountainous.
Important insights for protecting the earth
Knowing the density of different asteroids is important if you want to save the world. If you try to drastically change the orbit of an asteroid like Dimorphos by crashing a spacecraft into it, it could end up in multiple pieces of debris on Earth.
This won’t happen on an asteroid with the density of Didymos, but it also means that multiple interceptor rockets are needed to achieve the desired deflection.
Later this year, the European Space Agency’s (ESA) Hera spacecraft will arrive at Dimorphos to forensically examine the DART wreckage. This will undoubtedly provide new insights into humanity’s very first experiment in planetary defense.
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