NASA is getting its first taste of an asteroid ever. On October 20, about 334 million kilometers from Earth, the agency̵
The journey to Bennus’ surface will not be easy. The spacecraft will have to navigate past a towering boulder nicknamed Mount Doom and then to a sampling area no larger than a few parking spaces. “We may not be successful on our first attempt,” said Dante Lauretta, the mission’s lead scientist and planetary scientist at the University of Arizona in Tucson. But if it works, he says, “I hope the world sees this as good news – something we can be proud of with all the insanity going on this year.”
There and back again
Launched in 2016, OSIRIS-REx is $ 800 million, NASA’s first asteroid launch mission. It follows two missions by the Japanese aviation agency (JAXA) that have pushed dust off the surface of the asteroids – including some retrieved last year that are currently on their way back to Earth for analysis. Before JAXA’s missions, scientists learned about the contents of asteroids mainly by studying meteorites that had fallen to the ground – and they can become polluted as they travel through the atmosphere and hit the planet.
Scooping a sample directly from an asteroid provides a pristine glimpse 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 enticing because it can contain material rich in organic compounds found throughout the solar system, including in life on earth.
But first, OSIRIS-REx will have to pull the sampling out. When Lauretta and his colleagues chose Bennu as their target, they thought the 500-meter-wide asteroid would be relatively slippery and easy to land on. But after OSIRIS-REx arrived and began circling Bennu in 2018, the craft got a closer look and found large, dangerous boulders1.
So mission engineers developed an automated system to guide the spacecraft down to the surface. It collects images when the spacecraft crashes and compares them to previously taken images of the same target region. OSIRIS-REx can then track whether it is safe on its preselected path. If not, it can autonomously interrupt and fly away from the asteroid, waiting for another chance to land.
Its target is a 16-meter-wide crater called Nightingale, which offers a relatively smooth surface for landing. If you could stand in the middle of the Nightingale, you would feel pebbles and fine-grained sand under your feet, says Erica Jawin, a planetary scientist at the Smithsonian National Museum of Natural History in Washington DC who has studied the geology of Bennus.2. Mount Doom would weave over you, roughly the height of a two-story building, which is “pretty scary,” she says.
OSIRIS-REx will come down towards Nightingale with its 3.3 meter long robot arm outstretched. When it touches the asteroid, a feat scheduled for 18:12 U.S. Eastern time, it releases a puff of nitrogen gas that will blow up on the surface and kick small grains into a cloud of asteroid debris. A sampling unit will list some of these particles and store them.
The process, which lasts only 10-15 seconds, is more of a ‘fist bump’ than a landing. As soon as the spacecraft has finished hovering, it will return to a safe distance and scientists will assess how much material it collected. NASA wants at least 60 grams of rock and dust – but close will be good enough. “If it’s 58 grams, we stow and come home,” Lauretta says.
If the spacecraft collects 40 grams or less, scientists will likely return it to another location on Bennu, called the Osprey, to grab more. (It may not be appropriate to try Nightingale for the second time because the original nitrogen purge will have pushed small rocks on the surface to unsafe locations, making a ‘double dip’ dangerous, Lauretta says.) Sampling at Osprey would likely take place in January; regardless, the spacecraft will leave Bennu in March and eventually land on Earth with its precious cargo in 2023.
Bennu has been through a lot in his lifetime. It formed about 100 million years ago and a billion years ago when it broke away from a larger ‘parent’ body during a cosmic collision in the solar system’s asteroid belt. But Bennu has preserved traces of his parent. While orbiting the asteroid, OSIRIS-REx discovered that some of the boulders on Bennu were shot through with veins of ancient carbon-rich material known as carbonate. The carbonate was probably formed as the ice melted and trickled through the parent body, causing wet reactions inside its rocks.
“I was surprised to see 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 measure centimeters wide and can extend more than a meter long – much larger than carbonate tears seen in some meteorites. According to Lauretta, the large veins suggest that Bennus’ parent body once had a massive system of hot water flowing through it – meaning it had plenty of active geology. Some fragments of these carbonates may lie on the ground at Nightingale and be collected by OSIRIS-REx.
Scientists plan to compare the Bennu 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 getting two delicious cakes cut on my birthday,” says Queenie Hoi Shan Chan, a planetary scientist at Royal Holloway, University of London, in Egham, England, who works at Hayabusa2. Ryugu appears to have less watery material on the surface than Bennu has; by comparing the samples, scientists will be able to better understand how common aqueous processes and organic materials are on asteroids, Chan says.
Scientists will also search Bennus’ rocks for clues on how to protect the earth from asteroids. Bennu orbits dangerously near Earth and has a small chance of smashing into the planet sometime in the twenty-first century. Studies have shown that the asteroid is more of a loose lump of rubble than a solid rock. By carefully examining the consistency of Bennus’ rocks, scientists were able to suggest ways to deflect or break apart threatening asteroids near Earth.
“Any sample from Bennu will be incredibly useful – a critical addition to the collection of planetary samples we have on Earth,” says Jawin. “It probably does not like too much that we were there and stole some of its rocks.”