NASA’s OSIRIS-REx mission unleashed an unexpected explosion when it landed on the asteroid Bennu in October 2020 to collect a valuable sample to carry home to Earth.
Mission scientists have detailed the dramatic sampling that led to startling discoveries about the nature of the asteroid in two new studies. And the results are not only intriguing: The researchers say the results could have implications for a possible future diversionary mission should the 1,640-foot-wide (500-meter) Bennu (one of the riskiest known near-Earth asteroids) ever threaten to affect the planet.
“We expected the surface to be quite rigid, like landing on a pile of gravel: a bit of dust flies away and a few particles bounce up,” said Dante Lauretta, a planetary scientist at the University of Arizona and director of investigators at OSIRIS -REx mission to Space.com.
“But when we brought the pictures back after the event, we were stunned,” he continued. “We saw a huge wall of debris flying away from the sample site. For spacecraft operators, that was really scary.”
Related: Asteroid Bennu’s mysterious missing craters suggest “impact armor” protecting the surface
The effects of the impact were so unexpected that Lauretta, lead author of one of the two studies, lobbied for the spacecraft to revisit the area to understand what happened. Six months after sampling, in April 2021, the researchers got another look at the OSIRIS-REx touchdown site. When the spacecraft first arrived at Bennu, this place called Nightingale was in an impact crater 20 m wide. After landing, the mission’s scientists found a brand new 8m wide gaping hole in the surface, with displaced debris and boulders strewn across the terrain.
That’s a surprisingly large scar; Scientists had expected them to be about as wide as the sampler itself, 12 inches (30 centimeters). “But we’ve settled in,” said Lauretta. “There was obviously no resistance whatsoever. The surface was soft and drained like a liquid.”
The probe sank as deep as 70 cm, revealing pristine material that, unlike the asteroid’s surface, was unaltered by the asteroid’s constant beatings cosmic rays and the solar windthe streams of high-energy particles from the sun.
From measurements obtained 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), Lauretta said. For comparison: “A typical Earth Rock” has a density about six times higher, more like 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 numbers.
“That [surface] Boulders are very porous and there’s a lot of void space between them,” Kevin Walsh, a geologist at Southwest Research Institute in Colorado and lead author of the second study, told Space.com. “We expected that small, fine grains and dust would stick to the large boulders and fill the cavity and act as an adhesive to give some strength, allowing the surface to push back against the spacecraft more. But it’s not there.”
Bennu’s soft, fluffy nature could complicate a possible future diversionary attempt should astronomers determine that the rock is threatening to hit Earth. At 1,640 feet wide, a strike from Bennu would cause a continent-wide disruption on our planet. And this despite the fact that NASA estimates the probability of a collision at 1 in 2,700 between the years 2175 and 2199Bennu is still one of the most dangerous asteroids currently known.
In addition, scientists believe that many asteroids share a similar “debris heap” structure: essentially collections of rock, gravel, and soil held together by weak gravitational forces. The sampling experiment at Bennu shows that it is almost impossible to predict how such a pile of debris will react to an impact.
“The touchdown provided the first experience of really pushing something into the surface,” Walsh said. “And if we’re ever trying to deflect something like that, we’d need to know what the surface is like so it doesn’t just absorb the impact.”
Lauretta added that the underground material looked redder compared to Bennu’s bluish surface, suggesting that cosmic rays and other forms of space weather Erode the exposed space rocks. The reddish hues suggest that organic molecules such as hydrocarbons might be present inside the asteroid, of great interest to researchers trying to understand the origins of life on Earth.
Scientists will have to wait until the scheduled delivery of OSIRIS-REx in September 2023 to get their hands on the precious 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 but still four times more than they need to conduct the analysis, Lauretta said.
The OSIRIS-REx mission was recently extended, and after the spacecraft drops its cargo on Earth next year, it will will go to Apophisanother high-risk asteroid he will visit in 2029.
The findings are described in papers to be published in the journals on Thursday (July 7). Science and scientific advances.