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.