Recalcitrant asteroids present a real risk of collision with Earth. Scientists estimate that an asteroid measuring several miles across crashed into Earth 65 million years ago and wiped out the dinosaurs, among other forms of life, in a mass extinction. Unlike the dinosaurs, humanity can avoid this fate if we start practicing how to knock down an asteroid that approaches Earth.
It is much more insidious than how it is portrayed in science fiction movies like Deep Impact. Planetary scientists first need to figure out how asteroids are assembled. Are they flying piles of broken rocks, or something much bigger? This information can help provide strategies on how to successfully deflect a threatening asteroid.
As a first step, NASA conducted an experiment to smash an asteroid to see how it was disturbed. The DART (Double Asteroid Redirection Test) impact of the spacecraft on the asteroid Dimorphos occurred on September 26, 2022. Astronomers using the Hubble Space Telescope continue to follow the results of the cosmic collision.
A surprise was the discovery of several dozen rocks lifted from the asteroid after the collision. In the Hubble images they look like a swarm of bees moving very slowly away from the asteroid. This could mean that an asteroid impact that approaches Earth could result in a cluster of menacing rocks heading in our direction.
The famous 1954 rock song “Shake, Rattle and Roll,” will be the theme music for the Hubble Space Telescope’s latest discovery about what happened to asteroid Dimorphos after NASA’s DART (Double Asteroid Redirection Test) experiment. DART intentionally impacted Dimorphos on September 26, 2022, slightly altering its orbit trajectory around the larger asteroid Didymos.
Astronomers using Hubble’s extraordinary sensitivity discovered a host of rocks that may have been shaken off the asteroid when NASA purposely slammed the half-ton DART impactor spacecraft into Dimorphos at approximately 14,000 miles per hour.
The 37 free-flung boulders range in size from three feet to 22 feet across, based on Hubble photometry. They drifted away from the asteroid at more than half a mile an hour—about the walking speed of a giant tortoise. The total mass of these detected rocks is about 0.1% of the mass of Dimorphos.
“This is an amazing observation—better than I expected. We saw a cloud of rocks that carried the mass and energy from the impact target. The numbers, sizes, and shapes of the boulders are consistent with those that fell from the surface of Dimorphos due to the impact,” said David Jewitt of the University of California in Los Angeles, a planetary scientist who tracked the changes of DART during the Hubble impact.
“This tells us for the first time what happens when you hit an asteroid and see the material come out to its maximum size.
Jewitt says this opens up a new dimension for studying the aftermath of the DART experiment using the European Space Agency’s upcoming Hera spacecraft, which will arrive at the binary asteroid in late 2026. Hera will perform a detailed post-impact survey of the targeted asteroid.
“The stone cloud will still disperse when Hera arrives,” said Jewitt. “It’s like a very slowly expanding swarm of bees that eventually spreads out in the binary pair’s orbit around the sun.”
The rocks were probably not broken pieces of a small asteroid caused by the impact. They are already scattered over the surface of the asteroid, as seen in the last close-up image taken by the DART spacecraft just two seconds before the collision, when it was only seven miles above the surface.
Jewitt estimated that the impact shook two percent of the asteroid’s surface rocks. He said Hubble’s rock observations also provide an estimate of the size of the DART impact crater. “The stones could have been excavated from a circle about 160 feet wide (the width of a football field) above Dimorphos,” he said. Eventually Hera will determine the actual size of the crater.
In the past, Dimorphos may have formed from material ejected into space by the larger asteroid Didymos. The parent’s body may be spinning quickly or losing material from a visible collision with another object, and other scenarios. The ejected material formed a ring that gravitationally coalesced to form Dimorphos. This would make it a flying pile of rocky debris held together by a relatively weak gravitational pull. Therefore, the content may not be strong, but has a structure more like a bunch of grapes.
It is unclear how the rocks were removed from the asteroid’s surface. They may be part of an ejecta plume photographed by Hubble and other observatories. Or a seismic wave from the impact could have rattled the asteroid—like hitting a bell with a hammer—the shaking would dislodge the debris.
“If we follow the rocks in future Hubble observations, then we will have enough data to pinpoint the exact paths of the rocks. And then we will see in which directions they are launched from the surface,” said Jewitt.
The findings are published in The Astrophysical Journal Letters.
David Jewitt et al, The Dimorphos Boulder Swarm, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/ace1ec
Provided by HubbleSite NewsCenter
Citation: Hubble spots boulders escaping from asteroid Dimorphos (2023, July 20) retrieved July 20, 2023 from https://phys.org/news/2023-07-hubble-boulders-asteroid-dimorphos.html
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