NASA’s asteroid-smashing DART sent out a swarm of boulders

24 Jul 2023

Image of the asteroid Dimorphos, showing boulders around the asteroid with circles to highlight them. Image: NASA, ESA, David Jewitt (UCLA); Alyssa Pagan (STScI)

The Hubble Space Telescope has detected 37 boulders slowly drifting away from the asteroid Dimorphos, which was hit with a spacecraft last year.

NASA’s Double Asteroid Redirection Test (DART) appears to have shaken a swarm of boulders off the asteroid it impacted last year.

The DART spacecraft completed its 10-month journey last September, when it successfully collided with the asteroid Dimorphos in humanity’s first planetary defence test.

The mission was an experiment to see if an asteroid’s path can be changed by crashing into it, using a technique known as kinetic impact. Since then, multiple observations have taken place to learn more about the impact and the asteroid itself.

In recent images taken from the Hubble Space Telescope, researchers have detected dozens of boulders drifting away from Dimorphos. NASA said these boulders may have been shaken off the asteroid’s surface when the DART spacecraft impacted at roughly 14,000 miles per hour.

The 37 boulders range in size from three feet to 22 feet across, based on the Hubble images. These boulders appear to be drifting away from Dimorphos incredibly slowly – little more than a half-mile per hour (roughly the walking speed of a giant tortoise).

Despite the number of boulders in the swarm, the total mass of these ejected rocks is believed to be only 0.1pc of the mass of Dimorphos. Close up images of the asteroid – just before the DART spacecraft’s impact – suggest these boulders were already scattered across the asteroid’s surface before the collision.

A grey rocky surface with rocks of different sizes visible. A close up image of the Dimorphos asteroid.

The last complete image of the asteroid Dimorphos, seen by NASA’s DART spacecraft two seconds before impact. Image: NASA, APL

David Jewitt, a planetary scientist from the University of California, said the results of the observation were “much better than I expected”.

“This tells us for the first time what happens when you hit an asteroid and see material coming out up to the largest sizes,” Jewitt said. “The boulders are some of the faintest things ever imaged inside our solar system.”

“We see a cloud of boulders carrying mass and energy away from the impact target. The numbers, sizes and shapes of the boulders are consistent with them having been knocked off the surface of Dimorphos by the impact.”

Jewitt said the boulder swarm presents new possibilities for studying the aftermath of the DART experiment using the European Space Agency’s upcoming Hera spacecraft.

This spacecraft is scheduled to arrive at the asteroid in late 2026, where it will perform a post-impact survey of the Dimorphos.

“The boulder cloud will still be dispersing when Hera arrives,” Jewitt said. “It’s like a very slowly expanding swarm of bees that eventually will spread along the binary pair’s orbit around the sun.”

At the time of the DART impact, the James Webb Space Telescope and the Hubble Space Telescope took simultaneous observations of the collision to observe the results with various instruments.

10 things you need to know direct to your inbox every weekday. Sign up for the Daily Brief, Silicon Republic’s digest of essential sci-tech news.

Leigh Mc Gowran is a journalist with Silicon Republic