Hubble Sees Boulders Escaping From Asteroid Dimorphos

The popular 1954 rock song “Shake, Rattle and Roll,” could be the theme music for the Hubble Space Telescope’s latest discovery about what is happening to the asteroid Dimorphos in the aftermath of NASA’s DART (Double Asteroid Redirection Test) experiment. DART intentionally impacted Dimorphos on September 26, 2022, slightly changing the trajectory of its orbit around the larger asteroid Didymos.

Astronomers using Hubble’s extraordinary sensitivity have discovered a swarm of boulders that were possibly shaken off the asteroid when NASA deliberately slammed the half-ton DART impactor spacecraft into Dimorphos at approximately 14,000 miles per hour.

The 37 free-flung boulders range in size from three feet to 22 feet across, based on Hubble photometry. They are drifting away from the asteroid at little more than a half-mile per hour – roughly the walking speed of a giant tortoise. The total mass in these detected boulders is about 0.1% the mass of Dimorphos.

“This is a spectacular observation – much better than I expected. 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,” said David Jewitt of the University of California at Los Angeles, a planetary scientist who has been using Hubble to track changes in the asteroid during and after the DART impact. “This tells us for the first time what happens when you hit an asteroid and see material coming out up to the largest sizes. The boulders are some of the faintest things ever imaged inside our solar system.”

Jewitt says that this opens up a new dimension for studying the aftermath of the DART experiment using the European Space Agency’s upcoming Hera spacecraft, which will arrive at the binary asteroid in late 2026. Hera will perform a detailed post-impact survey of the targeted asteroid. “The boulder cloud will still be dispersing when Hera arrives,” said Jewitt. “It’s like a very slowly expanding swarm of bees that eventually will spread along the binary pair’s orbit around the Sun.”

The boulders are most likely not shattered pieces of the diminutive asteroid caused by the impact. They were already scattered across the asteroid’s surface, as evident in the last close-up picture taken by the DART spacecraft just two seconds before collision, when it was only seven miles above the surface.

This is the last complete image of the asteroid Dimorphos, as seen by NASA’s DART (Double Asteroid Redirection Test) impactor spacecraft two seconds before impact. The Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) imager aboard captured a 100-foot-wide patch of the asteroid. The DART spacecraft streamed these images from its DRACO camera back to Earth in real time as it approached the asteroid. DART successfully impacted its target on September 26, 2022.
Credits: NASA, APL