When NASA smashed its Double Asteroid Redirection Test (DART) craft into Dimorphos last September, it hit the asteroid so hard that it sent boulders flying off into space.
This is the report of astronomers using the Hubble Space Telescope, who detected 37 of the large rocks that ranging from 3–22 feet across moving away from the asteroid,
The boulders aren’t moving that fast — they are travelling away from Dimorphos at about half-a-mile per hour, or roughly the walking speed of a tortoise.
While this might seem a little undramatic, it is great news for science, as we will be able to study the ejecta when an upcoming space mission arrives at Dimorpos in a few years.
Based on Hubble photometry, the researchers have estimated that the boulders would have made up around 0.1 percent of Dimorphos’ total mass before the DART impact.
Planetary scientist astronomer David Jewitt of the University of California at Los Angeles said: “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 boulders are consistent with them having been knocked off the surface of Dimorphos by the 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.”
Launched from Earth in November 2021, DART slammed into the surface of Dimorphos at 23:14 UTC on 26 September last year at some 14,000 miles per hour.
The mission was developed to find out whether a simple, kinetic impact based on the transfer of momentum could provide a viable means of deflecting asteroids found to be on a collision course with our planet.
The 581-foot-diameter asteroid Dimorphos is the natural satellite of another near-Earth asteroid, Didymos — together they make up a little binary system.
The pair orbits the Sun at a distance of between 93–213.9 million miles, and were in no danger of smashing into the Earth.
Instead, the space agency chose Dimorphos specifically because it orbits another asteroid — meaning that any shift in its orbit would be easier to detect.
And the impact seems to have had the desired effect, altering the time it takes Dimorphos to orbit Didymos by around 33 minutes.
The collision also ejected an estimated 2,200,000 lbs of dust into space, producing a dust plume that temporarily brightened the binary system and producing a 6,200-mile-long dust tail that lasted for several months before dispersing.
According to Prof. Jewitt, the new discovery of the boulders opens up a new way to study the aftermath of the DART collision using the European Space Agency’s Hera.
This spacecraft — which will perform a detailed survey of Dimorphos — is scheduled to arrive at the binary asteroid system late in 2026.
Prof. Jewitt said: “The boulder cloud will still be dispersing when Hera arrives. It’s like a very slowly expanding swarm of bees!”
Eventually, he explained, the cloud “will spread along the binary pair’s orbit around the Sun.”
The experts do not believe that the boulders represent shattered parts of the tiny asteroid produced during the NASA experiment, but were already lying loose on Dimorphos’ surface.
Astronomers believe that Dimorphos likely formed from material shed into space by Didymos — perhaps because it spun too fast, or even because of a collision with another object.
Either way, Dimorphos is essentially composed of a pile of gravitationally bound rubble, and likely has an interior that is less solid and more like a bunch of tightly packed grapes.
In fact, various boulders can be seen scattered across the asteroid’s surface in the very last image DART transmitted mere seconds before impact — when it was at an altitude of just seven miles from the surface.
Prof. Jewitt has estimated that the DART experiment shook some two percent of the boulders on the asteroid’s surface loose — either as part of the previously noted dust plume, or perhaps as the result of seismic waves rebounding around inside the asteroid.
He said: “If we follow the boulders in future Hubble observations, then we may have enough data to pin down the boulders’ precise trajectories. And then we’ll see in which directions they were launched from the surface.”
He added that if the boulders came directly from from impact, they “could have been excavated from a circle of about 160 feet across on the surface of Dimorphos.”
