Before the asteroid Bennu could hit the earth, NASA decided to "start"

The OSIRIS-REx spacecraft is about to perform a difficult operation: dig a rock sample from the asteroid Bennu and send it back to Earth.

　　 NASA is about to "start" an asteroid for the first time. On October 20, at a distance of about 334 million kilometers from the Earth, NASA’s OSIRIS-REx spacecraft will approach a dark diamond-shaped asteroid named "Bennu" for the purpose of being on its surface Stay for a few seconds-this time is long enough to collect dust and pebbles. If successful, OSIRIS-REx will send these carbon-rich rubbles back to Earth, where scientists can find clues to the history of the solar system.

The journey to the surface of Bennu is not easy. OSIRIS-REx will have to pass through a tall boulder-nicknamed "Mount Doom" (Mount Doom), and then enter a sampling area no larger than a few parking spaces. "Our first attempt may not be successful," said Dante Lauretta, the mission's lead researcher and planetary scientist at the University of Arizona. But if it succeeds, he said, "I hope everyone will take it as good news-we can be proud of it. There have been too many crazy things this year."

　　 back and forth

　　 The US$800 million OSIRIS-REx was launched in 2016 as NASA’s first asteroid sampling mission. Earlier, two missions of the Japan Aerospace Exploration Agency (JAXA) have obtained dust samples from the surface of asteroids-some of the dust sampled last year is currently returning to the earth, ready to be analyzed. Before the JAXA mission, scientists mainly used meteorites that fell to the earth to understand the composition of asteroids, and these meteorites may be contaminated when they travel through the atmosphere and hit the earth.

　　 digging samples directly from asteroids can study the rocks left over from the formation of the solar system more than 4.5 billion years ago. Each asteroid has its own story, telling how it formed and evolved over time; Bennu star is particularly attractive because it may contain substances rich in organic compounds, and these organic compounds in the entire It can be found in the solar system, including life on earth.

　　 But first, OSIRIS-REx(Hermes Lindy Bag) must work hard to complete the sampling. Lauretta and his colleagues chose Bennu as the target because they believed the 500-meter-wide asteroid was relatively stable and easy to land. But after OSIRIS-REx arrived in 2018 and began to detour Bennu, he took a closer look and found a large and dangerous boulder.

　　 Therefore, the mission engineer developed an automated system to guide the spacecraft to the surface. The system collects images as the spacecraft descends and compares them with previous images of the same target area. After that, OSIRIS-Rex will track whether it is safely walking on its pre-selected path. If it is not safe, it will automatically suspend operations and fly away from the asteroid, waiting for the second chance to land.

Its target was a 16-meter-wide crater named "Nightingale"-providing a relatively stable surface for landing. Erica Jawin, a planetary scientist at the Smithsonian's National Museum of Natural History in Washington, DC, said that if you can stand among the "Nightingales," you will feel the pebbles and fine sand under your feet. She has studied Bennu's geology[2]. Doom Mountain will rise above you, about the height of a two-story building, which is "quite scary," she said.

When 　　 OSIRIS-Rex landed on the "Nightingale", it would extend a 3.3-meter-long robotic arm. When it comes into contact with an asteroid—planned to be at 6:12 pm Eastern Time, it will release a burst of nitrogen that blows onto the surface of the asteroid, blowing up small particles from a pile of asteroid debris. Then, a sampling device will take some of the particles and store them.

This process will only last 10-15 seconds. It is not so much a landing as a "fist". Once OSIRIS-Rex completes sample collection, it will retreat to a safe distance and leave it to scientists to evaluate how much material it has collected. NASA hopes to collect at least 60 grams-but it doesn't matter if it is less. "If it is 58 grams, we will be able to stop work and go home." Lauretta said.

　　 If the material collected by OSIRIS-Rex is only 40 grams or less, then scientists are likely to send it back to the second location on Bennu-"Osprey" (Osprey), and collect more. (Lauretta said that OSIRIS-Rex cannot enter the "Nightingale" for secondary sampling, because the first nitrogen will blow the small rocks on the surface to an indeterminate location, making the "second dip" dangerous.) In the "Osprey" Sampling may take place in January; however, OSIRIS-Rex will leave Bennu in March and will eventually land on Earth with its precious samples in 2023.

　　Analysis of asteroids

　　 Benu has experienced a lot in his life. It was formed about 100 million to 1 billion years ago, when it was separated from a larger "parent body" during a cosmic collision in the asteroid belt of the solar system. But Benu retains traces of his mother. When OSIRIS-REx orbited the asteroid, it detected that some boulders on its surface were filled with paleocarbonate veins. Carbonate may be formed when the ice melts and flows through the parent body, causing a water reaction inside the rock.

This discovery was announced in a paper in Science[3]. The first author of the article and planetary scientist Hannah Kaplan at NASA’s Goddard Space Flight Center said that seeing those carbonate veins “makes me feel very Surprised". They are about a few centimeters wide and can extend more than a meter in length—much larger than the carbonate veins seen in some meteorites. According to Lauretta, the larger carbonate veins indicate that Bennu's parent body had a huge hot water system flowing through it-which means it has many active geological formations. Some fragments of these carbonates may be located on the ground of the "Nightingale" and picked up by OSIRIS-Rex.

The researchers plan to compare Bennu's sample with a sample of the asteroid Ryugu, which is currently on its way back to Earth; Ryugu is a larger small one visited by JAXA's Hayabusa 2 spacecraft last year. planet. "I feel like a spoiled child who can cut two delicious cakes on his birthday." said Queenie Hoi Shan Chan, a planetary scientist at Royal Holloway College, University of London, who participated in the Hayabusa 2 project. The surface of the Dragon Palace seems to contain less water-rich material than Bennu; by comparing samples, researchers will be able to better understand the common water-bearing processes and organic matter on asteroids, Chan said.

　　 Scientists will also look for clues on how to protect the earth from asteroids on Bennu's rocks. Bennu runs dangerously near the earth and has a very small chance of hitting the earth sometime in the 22nd century. Studies have shown that this asteroid is more like a pile of loose gravel than a solid rock. By carefully studying the density of Bennu's rock, scientists can propose methods to transfer or split near-Earth asteroids that pose a threat to the Earth.

"Any sample from Bennu will be extremely useful-it is a vital addition to the planetary samples we collect on Earth," Jawin said. "Bennu probably wouldn't mind that we stole it too much. Some of the rocks."