Swarm of 37 boulders spotted by Hubble around asteroid hit by DART

By Ashley Strickland, CNN

(CNN) — The Hubble Space Telescope has spotted a swarm of boulders around the asteroid Dimorphos, which NASA’s DART spacecraft intentionally hit last fall.

The Double Asteroid Redirection Test spacecraft, weighing about 1,200 pounds (544 kilograms), slammed head-on into Dimorphos at 13,000 miles per hour (20,921 kilometers per hour) on September 26, 2022, to change the space rock’s velocity.

It was the first time humanity set out to change the movement of a celestial object, and the results showed how this kinetic impact technology could be used to deflect asteroids that may be on a collision course with Earth. Neither Dimorphos nor Didymos, the larger asteroid it orbits, poses a threat to Earth.

The DART impact was successful, changing Dimorphos’ orbital period around Didymos by 33 minutes. This first test of planetary defense, which took place 7 million miles (11.3 million kilometers) from Earth, also released over 1,000 tons of material into space.

Some of that material includes 37 boulders, ranging in size from 3 feet to 22 feet (0.9 meters to 6.7 meters) in diameter, according to new data captured by Hubble. The rocks, likely shaken loose from Dimorphos’ surface after impact, are drifting away from the asteroid at about a 0.5 mile per hour (0.8 kilometer per hour), or the walking speed of a giant tortoise, according to a Hubble news release.

Scientists estimate the boulders represent about 0.1% 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,” said planetary scientist David Jewitt, a distinguished professor at the University of California, Los Angeles, 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.”

Upcoming missions

Jewitt and his colleagues have used Hubble to track changes in Dimorphos both during and after the DART impact, but another mission will take an even closer look.

The European Space Agency’s Hera mission is set to launch in 2024. The spacecraft, along with two CubeSats, is expected to arrive at the asteroid system in late 2026.

Hera will study both asteroids, measure physical properties of Dimorphos, and examine the DART impact crater and the moon’s orbit, with the aim of establishing an effective planetary defense strategy.

“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.”

Surface boulders and other possible theories

Researchers believe the boulders were already sitting on the surface of Dimorphos, based on the final close-up photos taken by the DART spacecraft before impact. It’s much less likely that the rocks are shattered pieces of the asteroid, according to the Hubble observation team monitoring Dimorphos.

Jewitt estimated that 2% of the boulders on the surface were released into space after the crash. The rocks were likely ejected at the same time as the debris trail, also captured by Hubble. It’s also possible that a seismic wave from the impact lifted the rocks.

“The boulders could have been excavated from a circle of about 160 feet across (the width of a football field) on the surface of Dimorphos,” he said.

Future observations from Hera could help scientists pin down the actual size of the impact crater left by DART.

Scientists think Dimorphos may have formed from material shed by Didymos as it collided with another object, according to the European Space Agency. The material from Didymos would have formed a ring that eventually came together due to gravity, so Dimorphos may be what’s known as a rubble pile asteroid — rocky debris loosely held together by gravity, rather than a solid space rock.

Studying the DART experiment’s aftermath can help space agencies determine whether this impactor technology is the right approach to deflecting asteroids that may pose a threat to Earth in the future — or if it may result in creating more rocky hazards heading toward the planet.

The-CNN-Wire
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