In 2022, when NASA's $325 million spacecraft crashed into an asteroid called Dimorphos at 14,000 miles per hour, the Earth erupted in cheers and applause. NASA's Double Asteroid Redirect Test (DART) mission deliberately targeted Dimorphos, changing its orbit around the larger asteroid Didymos as a dress rehearsal for thwarting deadly space rocks that might one day hurtle toward Earth.
The world's first planetary defense experiment was considered a victory: the asteroid's orbit shrunk by 33 minutes, far exceeding the minimum threshold of 73 seconds.
But what the DART team didn't realize at the time was just how bizarre Dimorphos' reaction to the hit was. A new study published Monday in Nature Astronomy concludes that DART hit Dimorphos so hard that the asteroid changed shape.
Simulations of the impact showed that the spacecraft's sacrifice did not excavate a normal bowl-shaped crater. Instead, it left something like a dent. While man-made impacts blasted millions of tons of rock into space, massive amounts of rock were splashed onto its sides like a giant tidal wave. It widens the Dimorphos, turning it from a crouching sphere into a flat-topped oval—like an M&M candy.
The reason why this asteroid is fluid-like is because of its special composition. Sabina Raducan, a planetary scientist at the University of Bern in Switzerland and first author of the study, said that the asteroid is not a solid continuous rock, but more like "a pile of sand." The low-density asteroid, barely held together by its own gravity, would never react directly when a van-sized spacecraft flew in front of it.
Cristina Thomas, leader of the observing team at Northern Arizona University who was not involved in the study, said Dimorphos' response was "completely outside the realm of physics as we understand it in our daily lives." And "this has profound implications for planetary defense."
DART shows that a tiny spacecraft can deflect an asteroid. But the study suggests that hitting a similar disconnected space rock too hard could break it up, which in a real asteroid emergency could create multiple asteroids that hit Earth.
Planetary defense, as a concept, is clearly feasible. "We know we can do it," said Federica Spoto, an asteroid dynamics researcher at Harvard University and the Smithsonian Center for Astrophysics. "But we have to get it right.
There are many reasons for choosing Dimorphos as a target for DART. One of the most important reasons is its size: At 530 feet in diameter, it's about the size of a common variant of a rocky asteroid that could easily destroy a city.
Because Dimorphos is so small, it is difficult to observe from Earth, and little was known about it until the DART spacecraft glimpsed it up close as it neared its endpoint. But many scientists suspect it is a rubble pile, a collection of closely spaced boulders.
The few space missions that have visited similarly sized asteroids, even those with different geological compositions, have found them to lack cohesion. This makes them behave strangely. For example, when NASA's OSIRIS-REx spacecraft briefly touched down on the surface of the gravel asteroid Bennu to collect samples, it sank almost completely, like a pit of plastic balls.
That DART's collision knocked back Dimorphos so much suggests that such asteroids can be successfully deflected even if they have essentially no prior knowledge of their properties.
But early observations from ground-based telescopes, space observatories and LICIACube, a small satellite aboard the DART spacecraft, suggest that Dimorphos responds to this interstellar disruption in unexpected ways. Huge amounts of material were thrown out, and Dimorphos was soon surrounded by swarms of boulders and dragged along by a comet-like tail 20,000 miles long that lasted for months.
What other surprises will Dimorphos bring? The European Space Agency's Hera mission will launch in October this year and arrive at the end of 2026 to survey the asteroid's remains.