April 24, 2024

Mission DART Didn’t Just Move Dimmorphos, It Made Boulders Lose

In further evidence that asteroids are not as tough as they seem, the Hubble Space Telescope has spotted large rocks escaping from Dimorphos after being hit by the DART mission. Although we only have one example, this adds to the picture of asteroids being loosely bound together, making them vulnerable to human interference.

Last year, NASA smashed a space probe into the small asteroid Dimorphos. This particular asteroid was chosen not only because it is close to Earth, but because it orbits a larger body, Didymos, making it easy to measure the effects of the impact.

The mission, named the Double Asteroid Redirect Test, or DART, hit Dimorphos at 6.7 kilometers per second (14,000 miles per hour) and shifted its orbit by 33 minutes, much more than expected. It also produced a dust trail that was observed by Earth-based and Hubble telescopes. However, that was not the end of the story, even if most people’s attention came forward. Hubble continued to monitor Dimorphos on occasion, and recent observations show the impact raised more than dust.

Scientists have counted 37 rocks leaving Dimorphos in images taken since the impact, ranging in size from 1 to 7 meters in diameter (3-22 feet). Instead of settling back on the asteroid, they are drifting slowly – so slowly that their average speed of 0.3 meters per second (0.7 mph) is compared to that of a giant tortoise. However, that is hardly enough to achieve escape velocity from the gravity of the two asteroids.

“This is an amazing observation – much better than I expected. We see a cloud of boulders carrying mass and energy away from the impact target,” UCLA Professor David Jewitt said in a statement statement. “The number, sizes and shapes of the boulders are consistent with the impact of being removed from the surface of Dimorphos.” The observations were also a technical triumph, with Jewitt noting; “Boulders are some of the faintest objects ever imaged within our Solar System.” Distinguishing objects this tiny from cosmic rays hitting the Hubble telescope was a significant challenge.

Dimorphos, with a dusty bluish tail extending diagonally to the upper right. The boulder circles pick out to distinguish them from background stars and cosmic ray-induced pixels.

Image Credit: NASA, ESA, David Jewitt (UCLA) Image Processing: Alyssa Pagan (STScI)

The European Space Agency’s Hera mission is scheduled to visit Dimorphos and Didymos in 2026 to conduct a post-impact survey of Dimorphos. “The boulder cloud will still be dispersing when Hera arrives,” Jewitt said. “It’s like a swarm of bees that is expanding very slowly and will eventually spread along the binary pair’s orbit around the Sun.”

The team thinks that the DART did not penetrate Dimorphos enough to create the boulders and hit the ones already loose on the surface.

This is the last complete image of the asteroid Dimorphos, as seen by NASA's DART (Double Asteroid Redirect Test) collider spacecraft two seconds before impact.  The Didymos Exploration imager and Asteroid Camera for Optical Navigation (DRACO) on board captured a 100-foot-wide patch of the asteroid.

A 30-meter-wide (98-foot) patch of the asteroid from the last complete image of Dimorphos, as seen by DART two seconds before impact.

Image Credit: NASA, APL

The disruption made Dimorphos ever so slightly less of a threat, should it one day hit Earth, as the total mass of those that escaped is estimated to be 0.1 percent of the pre-impact asteroid.

However, we will need a larger sample before we can assume that asteroids can be degraded by repeated impacts, in which case this is unlikely to have been considered a better approach than deflection. Dimorphos is thought to have been formed from matter lost by Didymos in a collision or even a quick spin. If so, the smaller asteroid may be a looser pile of rubble than other space rocks.

The observers would love to know the process that raised the boulders from the surface, and they hope to solve it by establishing the path of each rock. Alternatively, we could decide to do it again, this time with a second spacecraft nearby to record the whole thing.

The results are published in The Astrophysical Journal Letters.

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