'Spectacular observation': Hubble Telescope spots 37 space boulders ejected by DART asteroid impact

In a worst-case scenario, this technology might one day shield humanity from the same cosmic forces that led to the extinction of the majestic dinosaurs.

Boulders drifting at the speed of a tortoise

The swarm of boulders has been drifting away from the asteroid at a comparatively slow speed of 0.5 miles per hour (0.8 kilometers per hour), or "roughly the walking speed of a giant tortoise".

The boulders were measured using Hubble's High-Speed Photometer (HSP), which ranged in length from three to 22 feet (0.9 meters to 6.7 meters). These floating boulders would have originally made up only 0.1 percent of Dimorphos' mass.

"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," David Jewitt, a planetary scientist at the University of California at Los Angeles, said in a statement.  

"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,” added Jewitt, who has been tracking changes in the asteroid during and after the impact using Hubble. 

The team mentions that follow-up Hubble observations could deduce the precise trajectories of the boulders. Additionally, it could determine the origin or launch direction of the objects involved in the collision.

How did the boulders dislodge from the asteroid? 

The researchers are uncertain about how the boulders became dislodged from the surface of the asteroid. 

Fascinatingly, right before the impact, DART's DRACO imager captured surface photographs of Dimorphos, revealing a landscape resembling a collection of rubble. The images displayed an array of boulders in various shapes and sizes scattered across its surface. Consequently, the researchers formulated a hypothesis suggesting that these boulders were not the result of debris produced by the spacecraft's collision.

Nevertheless, they posit two potential scenarios: either the boulders were discharged as an ejecta plume during the DART crash, or seismic activity induced by the impact could have jolted the surface, propelling them outward.

An earlier study found that the collision, which occurred seven million miles (11.3 million kilometers) from Earth, resulted in the ejection of over a million kg of debris into space.

Astronomers will have a better understanding of the DART aftermath once the European Space Agency's Hera spacecraft reaches this asteroid. However, they will have to wait until the next few years, as this dedicated mission is expected to reach the asteroid in late 2026.

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