In 2022, when NASA’s $325 million spacecraft crashed into an asteroid called Dimorphos at 14,000 miles per hour, cheers and applause erupted back on Earth.
NASA’s Double Asteroid Redirection (DART) mission deliberately targeted Dimorphos to change its orbit around a larger asteroid, Didymos, as a suit exercise to thwart a deadly space rock that might one day be headed for Earth.
The world’s first planetary defense experiment was deemed a success: the asteroid’s orbit shrank for 33 minutes, well above the minimum threshold of 73 seconds.
However, what the DART team didn’t realize at the time was how strangely Dimorphos responded to the hit. The new study, published Monday in Nature Astronomy, concludes that DART hit Dimorphos so hard that the asteroid changed shape.
Impact simulations show that the spacecraft’s demise did not excavate the usual bowl-shaped crater. Instead, it leaves something that resembles an indentation. And although the artificial impact blasted millions of tons of rock into space, much of it splashed back over the side like a giant tidal wave. This expands the Dimorphos, turning it from a short-shaped ball into a flat-topped oval — like an M&M candy.
The fact that asteroids act like liquids depends on their strange composition. It’s not a continuous, solid rock, but rather a “pile of sand,” said Sabina Raducan, a planetary scientist at the University of Bern in Switzerland and lead author of the study. And a low-density asteroid held together only by its own gravity wouldn’t respond in such a simple way when this van-sized spacecraft flew into its face.
Dimorphos’ response is “completely outside the realm of physics as we understand it” in our daily lives, said Cristina Thomas, a leader of the mission’s observation working group at Northern Arizona University who was not involved in the study. And “this has far-reaching implications for planetary defense.”
DART shows that small spacecraft can deflect asteroids. However, studies show that hitting mutually exclusive space rocks too hard risks breaking them apart, which, in a real asteroid emergency, could create several asteroids headed towards Earth.
Planetary defense, as a concept, clearly works. “We know we can do it,” said Federica Spoto, an asteroid dynamics researcher at the Center for Astrophysics, Harvard and Smithsonian, who was not involved in the new study. “But we have to do it right.”
Dimorphos was chosen as a DART target for various reasons. One of the most important is its dimensions: at 530 feet wide, it is the right size for a common type of rocky asteroid that could easily wipe out a city.
Because it is so small and difficult to observe from Earth, little was known about Dimorphos before DART got a closer look at it during the spacecraft’s terminal approach. But many scientists suspect that this is a rubble pile, a collection of closely located rocks.
A number of space missions that have visited similar-sized asteroids — even ones with different geological compositions — have also found that they lack coherence. This makes them behave strangely. For example, when NASA’s OSIRIS-REx spacecraft briefly touched the surface of the rocky asteroid Bennu to steal samples, it almost sank completely into it, as if sinking into a plastic ball pit.
The fact that DART’s impact hit Dimorphos back so significantly suggests that deflecting this type of asteroid could be successful even when its properties were largely unknown beforehand.
However, early observations taken by public-based telescopes, space-based observatories, and LICIACube (a small satellite participating with the DART spacecraft) suggest that Dimorphos responded with unexpected drama to this act of interplanetary vandalism.
“A lot of material was thrown around,” said Dr. Thomas. Dimorphos was quickly surrounded by a group of rocks and followed by a 20,000-mile-long comet-like tail that lasted as long as the moon.
What other surprises might Dimorphos have in store? Hera, a European Space Agency mission launched this October, will arrive at Dimorphos in late 2026 to survey the asteroid’s wreckage.
However, Dr. Raducan was impatient, deciding to predict what Hera might find. His team ran impact simulations, hoping to see which virtual results best matched the brief post-impact observations taken from Dimorphos. The impassability of classical craters, and transmogrified asteroids, was not something most astronomers anticipated.
Like its previously studied asteroid sibling, Dimorphos responded in an unexpected way when angry at being poked by a robot. That means that if the world needs saving from the impending pile of rubble, no assumptions should be made.
“We need more space missions to asteroids,” said Dr. Raducan. “Just because we hit one asteroid doesn’t mean they’re all going to behave the same.”
2024-02-26 18:16:48
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