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Seven years after it left for the near-Earth asteroid Bennu, NASA’s OSIRIS-REx spacecraft is returning with a celestial souvenir. On the morning of Sunday, September 24, as it passes by Earth the probe will release a canister holding about 9 ounces of space rock. The container will plummet through the atmosphere, its parachute will unfurl, and it will touch down in the Utah desert at about 8:55 am Mountain time.
Assuming its contents survive the journey unscathed, the return will mark a tantalizing step forward for planetary science. Researchers have long salivated over the prospect of examining pristine asteroid fragments. While meteorites—which are often broken chunks of asteroids—fall from the sky all the time, they’re immediately contaminated by the ground they smash into. This will be a rare look at an untainted rock from space, and it will help scientists understand what Bennu is made of and where it came from. If the mission is successful, it will be only the third asteroid sample return in history—following Japanese space agency missions to Ryugu and Itokawa.
To planetary scientist Dante Lauretta, the mission’s principal investigator, it’s also “a little bittersweet,” because the program is now coming to an end. Still, he says, “I’m excited to get it into the laboratory, so we can do all this amazing science.” His University of Arizona team will study the composition of the dust and rock fragments in the container and trace any organic molecules they may harbor. The scientists will also be able to compare samples of Bennu to Ryugu.
But first, the capsule, which is circular and about the size of an ice chest, has to make it safely down to Earth. That will mean slowing from 28,000 miles per hour to just 11. Lockheed Martin built the spacecraft for NASA and is responsible for the capsule recovery. “We have done sample returns before, so we have that experience,” says Sandra Freund, a systems engineer at Lockheed and the OSIRIS-REx program manager, referring to previous NASA missions that collected materials from a comet and the solar wind. “We know we can do this, but there’s always a risk when you’re bringing something back to Earth. You’ve got atmospheric reentry, which is a very fiery experience. You’ve got parachutes that need to deploy. So there are a number of things that need to go just right.”
The capsule’s built-in heat shield is designed to save it from burning up at 5,000 degrees Fahrenheit, as a meteor or unprotected satellite that size hurtling through the atmosphere would. “Any time you want to bring a payload through the atmosphere, you need protection for it. It can be pretty gnarly,” says Todd White, a scientist at NASA Ames Research Center in Mountain View, California, who worked on the spacecraft’s thermal protection system. The heat shield is made of a lightweight chopped carbon fiber infused with resin, and it’s ablative, meaning that it slowly burns off. “It looks nice and brown on the back and white on the front—but when it lands it’ll look charred and crispy,” White says.
First, the capsule will deploy a small drogue chute to keep itself stable. Then seven minutes into its descent, it will open its main parachute and drift to the ground for six more minutes. Recovery helicopters will get the first view of its rapid descent. Relatively soft soil should cushion the impact when it lands within the Department of Defense’s remote Utah Test and Training Range and Dugway Proving Grounds. It’s an active range, though, so before NASA personnel make their approach to retrieve the container, a military representative will check the area to make sure there’s no unexploded ordnance.
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Safe landing won’t be this mission’s first challenge. The OSIRIS-REx team initially expected the probe to scoop up fine-grained sand, like the Japanese space agency encountered on the asteroid Itokawa in 2005. But Bennu’s surface wasn’t like that. It turned out to be much rockier, more like a collection of pebbles loosely bound together in low gravity. It offered little resistance to OSIRIS-REx’s robotic arm as it tried to collect the sample, and bits of regolith got jammed in what was perhaps an overly full collection head. Some of the material that OSIRIS-REx managed to grab accidentally spilled back into space. Because of how that sample collection went, Lauretta expects some asteroid dust to be coating the outside of the canister. He’s looking forward to studying it before they finally open the container and peer at the precious material inside.
In 2020, Japan’s Hayabusa2 mission brought back 0.2 ounces of dust from Ryugu, which is a C-type, or carbonaceous, asteroid that appears dark like soot. Bennu is a B-type, which means it’s rich in carbon and other minerals. It’s possible they have bigger differences, including their origins, which could be revealed by the rock samples. “I’m hoping we can extract the history of this object. By comparing it to the history we’re gleaning from the Ryugu samples, we’ll understand the diversity within the solar system,” says Deborah Domingue, a scientist at the Planetary Science Institute in Tucson, Arizona, who conducted research on Ryugu material.
For example, scientists could determine whether Bennu has been hit by many other objects over the years, or whether it was knocked off of a larger parent body by another asteroid. These comparisons matter because asteroids are building blocks; rocky bodies like them glommed on to each other 4.5 billion years ago to assemble the planets we know today—and no two planets in our solar system are alike, Domingue says.
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While this is only the third asteroid sample captured from space, NASA has previously sampled other celestial objects. The Genesis mission brought back solar wind particles in 2004. Two years later, the Stardust mission returned a little piece of a comet. Like OSIRIS-REx, both landed in the Utah desert, and Stardust employed a similar capsule. NASA is also planning its first Mars sample return mission for later this decade, when some of the Perseverance rover’s painstakingly drilled rock samples will be sent to Earth, again to Utah.
OSIRIS-REx’s sample return is the first phase of what NASA has dubbed “Asteroid Autumn.” After a long delay, the space agency’s Psyche mission is scheduled to launch on October 5 toward a large, metal-rich member of the asteroid belt. (If the federal government shuts down on October 1 over disagreements on spending bills, it could be delayed once again.) On November 1, the Lucy spacecraft will snap photos while flying by an inner belt asteroid designated 1999 VD57.
Because a handful of asteroids may harbor water ice, iron, nickel, or platinum-group metals, there’s commercial interest in mining them. California company AstroForge tested its refining methods on asteroid-like material in Earth orbit earlier this year, and it plans to launch a demonstration mission early next year to fly by an asteroid and study its composition. The TransAstra Corporation, also based in California, has been investing in mining technologies and space tugs, and has earned some NASA grants. It’s not clear yet whether the new industry will be viable. Earlier space mining outfits Planetary Resources and Deep Space Industries have already gone out of business.
Following its Earth flyby, OSIRIS-REx will continue on to explore the near-Earth asteroid Apophis, which it will reach in 2029. It will be rechristened OSIRIS-APEX, to denote its extended mission.
In the meantime, NASA and its partners will repackage the retrieved capsule. They’ll purge it with nitrogen gas to make sure it doesn’t get contaminated by terrestrial microbes, then ship it to NASA’s Johnson Space Center in Houston. Within a week or so, it will be partially disassembled in a lab, and the OSIRIS-REx team will see its contents for the first time. On October 11, NASA will broadcast a reveal of the capsule. For now, Lauretta and his fellow scientists will just have to wait.
