NASA is about to experience its first taste of an asteroid. On October 20, about 334 million kilometers from Earth, the agency̵
Traveling to the surface of Bennu will not be easy. The spacecraft will have to move past a towering rock nicknamed Mount Doom, then on a sampling area no larger than a few parking spaces. “We may fail on our first attempt,” said Dante Loretta, the mission’s chief researcher and planetary scientist at the University of Arizona in Tucson. But if he succeeds, he says, “I hope the world sees this as good news – something we can be proud of with all the madness that’s happening this year.”
To there and back
Launched in 2016, the $ 800 million OSIRIS-REx is NASA’s first asteroid sampling mission. Following are two missions from the Japan Aerospace Exploration Agency (JAXA), which removed dust from the asteroid’s surface – including some extracted last year, which are currently being returned to Earth for analysis. Prior to the JAXA missions, scientists learned about the contents of asteroids mainly by studying meteorites that fell to Earth – and they can be contaminated as they travel through the atmosphere and hit the planet.
Sampling directly from an asteroid offers a pristine view of rocks left over from the formation of the solar system more than 4.5 billion years ago. Each asteroid has its own story to tell about how it formed and evolved over time; Bennu is particularly attractive because it may contain material rich in organic compounds found in the solar system, including life on Earth.
But first, OSIRIS-REx will have to take the sample. When Loretta and his colleagues chose Benu as their target, they thought the 500-meter-wide asteroid would be relatively smooth and easy to land. But after OSIRIS-REx arrived and began orbiting Bennu in 2018, the ship looked closely and found large, dangerous rocks1.
So the mission’s engineers developed an automated system to direct the spacecraft down to the surface. It collects images as the spacecraft descends and compares them with previously taken images from the same target region. OSIRIS-REx can then monitor whether it is safe on its pre-selected path. Otherwise, it may break autonomously and fly off the asteroid, waiting for a second chance to land.
Its target is a 16-meter-wide crater called Nightingale, which offers a relatively smooth landing surface. If you could stand in the middle of the Nightingale, you would feel pebbles and fine sand under your feet, says Erica Javin, a planetary scientist at the Smithsonian’s National Museum of Natural History in Washington, D.C., who studied Bennu’s geology.2. Mount Doom would rise above you, about the height of a two-story building, which is “quite embarrassing,” she says.
OSIRIS-REx will descend to Nightingale with its outstretched 3.3-meter robotic arm. When it touches the asteroid, a feat scheduled for 6:12 p.m. Eastern time, it will release nitrogen gas, which will blow to the surface, launching small grains into a cloud of asteroid debris. A sampling device will direct some of these particles and store them.
The process, which will take only 10-15 seconds, is a “punch” rather than a landing. As soon as the spacecraft is finished, it will retreat to a safe distance and scientists will assess how much material it has collected. NASA wants at least 60 grams of rocks and dust – but close will be good enough. “If it’s 58 grams, we go home and go home,” says Loretta.
If the spacecraft collects 40 grams or less, then scientists will probably return it to a second place in Bennu, called Osprey, to take some more. (He can’t go in for Nightingale sampling a second time because the original nitrogen layer will have pushed small stones to the surface in uncertain places, making a dangerous “double immersion,” Loretta says.) Osprey sampling is likely to happen in January ; however, the spacecraft is scheduled to take off from Bennu in March and will eventually land on Earth with its valuable cargo in 2023.
Anatomy of asteroids
Bennu has been through a lot in his life. It formed between about 100 million years ago and one billion years ago when it broke away from a larger “parent” body during a cosmic collision in the asteroid belt of the solar system. But Bennu has kept track of his parent. While in orbit around the asteroid, OSIRIS-REx discovered that some of Bennu’s rocks had been shot through with veins of an ancient carbon-rich material known as carbonate. The carbonate probably formed when the ice melted and penetrated the mother’s body, causing watery reactions in its rocks.
“I was surprised when I saw these veins,” said Hannah Kaplan, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and lead author of Science paper3 who announced the discovery. They are centimeters wide and can extend more than a meter – much larger than the carbonate veins observed in some meteorites. According to Loretta, the great veins suggest that Bennu’s parent’s body once had a massive system of hot water flowing through it – meaning there was a very active geology. Some fragments of these carbonates may lie on the ground at Nightingale and be taken from OSIRIS-REx.
Researchers plan to compare Bennu’s samples with those currently returning to Earth from Ryugu, the larger asteroid that JAXA’s Hayabusa2 spacecraft visited last year. “I feel like a spoiled child cutting two delicious birthday cakes,” said Quinny Hoi Shan Chan, a planetary scientist at Royal Holloway, University of London in Egham, UK, who is working on Hayabusa2. Ryugu seems to have less water-rich material on its surface than Bennu; By comparing the samples, researchers will be able to better understand how common aqueous processes and organic materials are in asteroids, Chan said.
Scientists will also search the rocks of Benu to suggest how to protect the Earth from asteroids. Bennu orbits dangerously in orbit near Earth and has little chance of entering the planet in the twenty-second century. Studies show that the asteroid is a loosely stacked pile of debris rather than a solid rock. By carefully studying the consistency of Bennu’s rocks, scientists could suggest ways to deflect or smash threatening asteroids near Earth.
“Each Bennu sample will be amazingly useful – a critical addition to the collection of planetary samples we have on Earth,” says Jawin. “He probably won’t care too much that we were there and stole some of his rocks.”