Hubble sees chunks of rock loose from the asteroid Dimorphos

July 21, 2023

(Nanowerk News) The popular 1954 rock song “Shake, Rattle and Roll,” could serve as the theme music for the Hubble Space Telescope’s latest discovery about what happened to the asteroid Dimorphos after NASA’s DART (Double Asteroid Redirection Test) experiment. DART accidentally crashed into Dimorphos on September 26, 2022, slightly altering its orbital trajectory around the larger asteroid Didymos.

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

The 37 free-throwing boulders range in size from three feet to 22 feet, based on Hubble photometry. They were moving away from the asteroid at less than half a mile per hour – about the walking speed of a giant tortoise. The total mass in these detected boulders is about 0.1% the mass of Dimorphos. Image of the asteroid Dimorphos, with compass arrow, scale bar, and color buttons for reference. The north and east compass arrows show the orientation of the figure in the sky. Note that the relationship between north and east in the sky (as seen from below) is reversed relative to the directional arrows on an earth map (as seen from above). (Images: NASA, ESA, David Jewitt (UCLA); Alyssa Pagan (STScI))

“This was a spectacular observation – much better than I expected. We saw a cloud of boulders carrying mass and energy away from the impact target. The number, size, and shape of the boulders is consistent with boulders being 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 used Hubble to track asteroid changes during and after DART impacts. “This tells us for the first time what happens when you hit an asteroid and see the biggest material come out. The boulders are some of the faintest objects ever imaged within our solar system.”

Jewitt says this opens up a new dimension for studying the results of DART experiments using the European Space Agency’s forthcoming Hera spacecraft, which will arrive at the binary asteroid in late 2026. Hera will conduct a detailed post-impact survey of the targeted asteroid. “Stone clouds will still be spreading when Hera arrives,” said Jewitt. “It’s like a very slowly evolving swarm of bees that will eventually spread out along the orbit of a binary pair around the Sun.”

The boulders are most likely not small asteroid fragments caused by the impact. They are already scattered across the asteroid’s surface, as seen in the last close-up image taken by the DART spacecraft just two seconds before impact, when it was only seven miles above the surface.

Jewitt estimates that the impact shook up two percent of the boulders on the asteroid’s surface. He said observations of the rock by Hubble also provided an estimate of the size of the DART impact crater. “The stones could have been excavated from a circle about 160 feet (the width of a football field) on the surface of Dimorphos,” he said. Hera would eventually determine the true size of the crater. This is the last complete image of the asteroid Dimorphos, as seen by NASA's DART (Double Asteroid Redirection Test) impact spacecraft two seconds before impact This is the last complete image of the asteroid Dimorphos, as seen by NASA’s Double Asteroid Redirection Test (DART) impactor spacecraft two seconds before impact. The Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) imagers on board captured the asteroid field as wide as 100 feet. The DART spacecraft streamed these images from its DRACO camera back to Earth in real time as it approached the asteroid. DART successfully reached its target on September 26, 2022. (Image: NASA, APL)

Once upon a time, Dimorphos may have formed from material spilled into space by the larger asteroid Didymos. The parent body may be rotating too fast or it may lose material from a fleeting collision with another object, among other scenarios. The ejected material forms rings which gravitationally combine to form Dimorphos. This would make it a pile of flying rock debris loosely held together by the relatively weak pull of gravity. Therefore, the interior may not be solid, but has a structure closer to that of a bunch of grapes.

It’s not clear how the boulders were lifted from the surface of the asteroid. They could be part of the ejecta plumes photographed by Hubble and other observatories. Or the seismic waves from the impact may have rattled the asteroid – like hitting a bell with a hammer – shaking up surface debris.

“If we follow the boulders in future Hubble observations, then we may have enough data to pin down the stones’ precise trajectories. And then we will see which way they launch from the surface,” Jewitt said.

The DART team and the LICIACube (Light Italian CubeSat for Imaging of Asteroids) have also studied boulders detected in images taken by the LUKE (LICIACube Unit Key Explorer) camera of LICIACube minutes after the kinetic impact of DART.

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