NASA’s OSIRIS-REx mission triggered an unexpected explosion when it landed on asteroid Bennu in October 2020 to collect a valuable sample to bring back to Earth.
Mission scientists have described the dramatic recovery of the sample, which led to surprising discoveries about the nature of the asteroid, in two new studies. And the results aren’t just intriguing: The researchers say the findings could have implications for a possible future diversion mission, if the 1,640-foot-wide (500-meter) Bennu (one of the riskiest known NEOs asteroids) ever threaten to impact the planet.
“We expected the surface to be quite rigid, a bit like landing on a pile of gravel: a little dust flying and a few particles jumping,” Dante Lauretta, a planetary scientist at the University of Arizona and director of investigation of OSIRIS-REx, the mission told Space.com.
“But as we were bringing the footage back after the event, we were stunned,” he continued. “We saw a giant wall of debris moving away from the side of the sample. For spacecraft operators, that was really scary.”
Related: Asteroid Bennu’s Mysterious Missing Craters Suggest ‘Impact Shielding’ Protecting Surface
The aftermath of the impact was so unexpected that Lauretta, lead author of one of the two studies, campaigned for the spacecraft to revisit the area to figure out what happened. Six months after collecting the samples, in April 2021, researchers got another glimpse of the OSIRIS-REx landing site. When the spacecraft first arrived at Bennu, this site, called Nightingale, was in a 65-foot-wide (20 m) impact crater. After landing, mission scientists found a brand new 26-foot-wide (8 m) gaping hole in the surface, with displaced rubble and boulders strewn across the site.
It’s a surprisingly large scar; the scientists expected to dig a little about as wide as the sample collector itself, 12 inches (30 centimeters). “But we dug in,” Lauretta said. “There was clearly no resistance. The surface was smooth and flowed like a fluid.”
The probe sank up to 30 inches (70 cm), revealing pristine material that, unlike the asteroid’s surface, was unaltered by the constant pounding of cosmic rays and the solar windhigh-energy particle fluxes from the sun.
From measurements acquired during this repeat visit, Lauretta’s team calculated that the density of the surface material was only about 31 to 44 pounds per cubic foot (500 to 700 kilograms per cubic meter), a Lauretta said. For comparison, “one type Earth rock” has a density about six times higher, closer to 190 pounds per cubic foot (3,000 kilograms) per cubic meter.
A second study, based on measurements of the forces exerted on the probe during impact, confirmed these figures.
“The [surface] the rocks are very porous and there’s a lot of empty space between them,” Kevin Walsh, a geologist at Colorado’s Southwest Research Institute and lead author of the second study, told Space.com. “We expected that the small fine grains and dust would stick to the large rocks and fill the empty space and act like a glue to provide some strength, allowing the surface to push back more against the spacecraft. But it’s not there.”
Bennu’s soft and fluffy nature could complicate a possible future deflection attempt, if astronomers determine that the rock threatens to hit Earth. At 1,640 feet across, a strike from Bennu would cause a continent-wide disruption to our planet. And even if NASA estimates the risk of collision at 1 in 2,700 between the years 2175 and 2199Bennu is still one of the most dangerous asteroids currently known.
Additionally, scientists speculate that many asteroids sport a similar “rubble pile” structure: essentially conglomerates of rock, gravel, and earth held together by weak gravitational forces. The sampling experience at Bennu shows that it is almost impossible to predict how such a pile of rubble might react to an impact.
“Touchdown provided the first experience of really pressing something into the surface,” Walsh said. “And if we ever try to deflect something like that, we would need to know what the surface looks like so it doesn’t just absorb the impact.”
Lauretta added that the subterranean material appeared redder compared to Bennu’s bluish surface, suggesting that cosmic rays and other forms of space weather erode exposed space rocks. The reddish hues hint that organic molecules, like hydrocarbons, could be present inside the asteroid, which is of great interest to researchers trying to understand the origins of life on Earth.
Scientists will have to wait for the scheduled delivery of OSIRIS-REx in September 2023 to get their hands on the valuable material. During the dramatic sampling attempt, the probe collected nearly 9 ounces (250 grams) of asteroid dust, which is slightly less than the teams had hoped for but still four times more than they expected. need to perform the analysis, Lauretta said.
The OSIRIS-REx mission was recently extended and after the spacecraft drops its cargo on Earth next year, it will head for Apophisanother high-risk asteroid, which it will visit in 2029.
The results are described in articles published Thursday, July 7 in the journals Science and Scientists progress.
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