Aug 10, 2023 - Science

Study: NASA's asteroid deflection test spawned a swarm of space boulders

An image of the aftermath of DART's collision with Dimorphos, the target asteroid, captured by LICIACube. Photo: NASA/ITA

When NASA slammed a refrigerator-sized spacecraft into an asteroid last year, the impact spawned a swarm of at least 37 boulders that are now coursing through space, according to a new study published in the Astrophysical Journal Letters.

Why it matter: The crash was key in determining if it was possible to smash dangerous asteroids off of a collision course with Earth, but the study reveals a possible consequence of that planetary defense technique: the generation of smaller space rocks that could still reach the planet.

Reality check: Dimorphos, the targeted asteroid that's around 6.8 million miles from the planet, and the boulders ejected during the Double Asteroid Redirection Test (DART) have never threatened Earth, and NASA chose the asteroid for that reason.

Yes, but: But if an asteroid on course with Earth was targeted, the ejected debris from the impact could go on to make contact with the Earth, its atmosphere or satellites.

  • "The question is, 'Do you want to get hit by the big guy, or do you want to get hit by a swarm of little guys?'" lead researcher and UCLA astronomer David Jewitt told Axios. "And the answer usually is that it's better to be hit by the swarm."

How it works: Jewitt and his fellow researchers discovered the previously undetected boulders by viewing Dimorphos with the Hubble Space Telescope multiple times over several months.

  • They found that the boulders were broadly distributed around Dimorphos and the larger asteroid it orbits, Didymos.
  • The largest among the found rocks was around 22 feet in diameter, while the smallest was around three feet.
  • Jewitt said it's likely that the DART test also generated hundreds or thousands of smaller rocks or particles that could not be seen with Hubble.

What they're saying: If they hypothetically did reach Earth, Jewitt said the observed rocks would likely burn or break up in atmosphere.

  • If they were a bit larger, they could have more of an effect, as space rocks between 50 and 65 feet in diameter typically explode in the atmosphere, Jewitt said.
  • He pointed to the 62-foot meteor that blew up over Chelyabinsk, Russia, in 2013 as an example, but also stressed that the repercussions of rocks that size pale in comparison to the effects of a Dimorphos-sized asteroid hitting Earth.
  • "A 200-meter body like Dimorphos would come all the way through the atmosphere, probably, and explode with great energy near the ground," he said.

Context: NASA's DART mission successfully altered Dimorphos' orbit around Didymos by around 32 minutes.

  • Before impact, NASA said a change in Dimorphos' orbit by at least 73 seconds would be a success, meaning DART surpassed the minimum benchmark by over 25 times.
  • The spacecraft blasted over 992 tons of dust and rock from Dimorphos, which acted like "a little like a jet of air streaming out of a balloon" that propelled the moonlet closer to Didymos, the space agency said afterwards.

The big picture: The researchers hypothesized that the boulders were split from Dimorphos in two ways.

  • They may have been directly ejected from the spacecraft's impact site, which it actually captured in its last image transmitted just seconds before contact.
  • The rocks may have also been shaken off Dimorphos' surface by seismic waves from the impact.
  • Jewitt stressed that rocks the size of those discovered around Dimorphos are constantly being generated in space from natural asteroid collisions.

What's next: The European Space Agency's Hera spacecraft may see the boulders when it rendezvouses with Dimorphos and Didymos in 2026.

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