If all goes as planned, on Sunday morning a bell-shaped space capsule the size of a mini-fridge will come screaming down through the atmosphere toward a Utah desert.
Inside will be some precious cargo: about a cup's worth of rock and dust that a NASA spacecraft collected from an asteroid called Bennu that was, at the time, more than 200 million miles away.
This will be the biggest amount of extraterrestrial material to be brought back to Earth by any nation since the Apollo astronauts hauled home moon rocks, and it's the culmination of NASA's first mission to bring home samples of an asteroid.
The 4.5-billion-year-old pebbles inside the return capsule are thought to be pristine leftovers from the early days of the solar system, when the planets were first forming.
Scientists want to study these rocks to learn more about the chemistry that ultimately led to the emergence of life on Earth — assuming the capsule parachutes down unscathed and its contents don't get destroyed in a crash landing.
"That would be just heartbreaking, right?" says Dante Lauretta of the University of Arizona, the principal investigator for NASA's OSIRIS-REx mission, who compared that scenario to fumbling a football in the end zone while trying to make what would be a winning touchdown in the final seconds of a game.
Lauretta, who has devoted about two decades of his life to this asteroid-sampling effort, says that he and his colleagues have already endured a number of heart-pounding, nail-biting episodes along the way. Now, they await the final one.
"To me, the moment the parachute opens, I'll know we've made it. We're home," says Lauretta.
The anxiety of the mission's critical moments often gives him strange, vivid dreams. In one dream, for example, he found himself in a gift shop on the rocky asteroid.
"I was like, 'How did I get this job? I'm not supposed to be selling Bennu memorabilia,'" Lauretta recalls, saying that his dream-self then thought, "If we can build a gift shop, why am I stressing so much about getting all this sample? I could just pick some up right now."
In a more recent dream, with the day of the sample's return drawing ever closer, his dream-self opened up the capsule and saw, sitting on top of the black asteroid dirt, a shining green gem. "And I grabbed it and popped it in my mouth," he says, laughing.
In real life, no one will be tasting the asteroid rocks, although Lauretta says they might get a whiff at some point. He expects a smell like rotten eggs or ammonia.
"I would never want to touch it or eat it," says Lauretta, "because we worked so hard to get it."
Meeting the "troublemaker"
The van-sized OSIRIS-REx spacecraft launched in 2016, and it took about two years to reach Bennu, which is about as big as the Empire State Building.
Researchers consider Bennu to be a potentially dangerous asteroid, as it appears to have about a 1 in 1,750 chance of impacting the Earth between now and the year 2300.
When the probe arrived at Bennu in 2018, researchers were surprised that instead of the smooth surface that they'd expected, the asteroid was studded with huge boulders.
The team had to figure out how to maneuver the spacecraft around obstacles with nicknames such as "Mount Doom" so that it could reach a spot the size of a few parking spaces.
In October of 2020, the spacecraft briefly touched down, contacting the surface with an arm equipped with a special collection device at the end.
Shockingly, the arm basically plunged in, as the asteroid behaved more like a liquid than a solid, suggesting it's composed of rocky bits just barely held together by gravity.
At first the researchers were ecstatic, as they managed to recover far more asteroid stuff than their original goal of about two ounces, or 60 grams.
Once the spacecraft had backed away from the asteroid and they stopped to assess their treasure, however, their elation turned to alarm.
They'd grabbed so much that a Mylar flap that was supposed to seal up the collection device had gotten jammed open by a rock that appeared to be about an inch across. This created a gap that allowed pebbles and dust to escape. Images from an on-board camera showed them floating away.
That moment "was gut-wrenching," says Jason Dworkin of NASA's Goddard Space Flight Center, a project scientist for the mission. "Seeing those particles drifting away....every particle is a discovery not made. It was difficult."
Mission managers scrambled to get this collection device into its return canister so that the canister could be closed up, trapping everything inside.
"We had planned to have a very leisurely operation to stow it," says Dworkin. "That was out the window. We had to move quickly."
In the end, they estimate that they managed to safeguard about 8 ounces of asteroid rock, but they won't know for sure until they open the canister up.
Once they do, they expect to see everything from dust to pebbles to larger rocks like the one that Lauretta calls "the troublemaker" — the one that kept their collection device from sealing up.
"We're really excited to see it," says Lauretta. "What is it? It must have been hard and different than probably the bulk of the asteroid."
Heading home
Early Sunday morning, mission operators will tell the OSIRIS-REx spacecraft to jettison the sample return capsule, sending it on a collision course with Earth.
This will occur when the spacecraft is about 67,000 miles away, and the capsule will be aimed at a target zone in the desert that's about 250 square miles.
"It's the equivalent of throwing a dart across the length of a basketball court and hitting the bullseye," says Rich Burns, the project manager based at NASA's Goddard Space Flight Center.
NASA says the capsule will enter the atmosphere at about 36 times the speed of sound, encountering temperatures "twice as hot as lava." As it falls towards the high mountain desert of the Utah Test and Training Range, it will deploy parachutes to slow itself down.
At least, that's the plan.
In 2004, a parachute failure doomed a NASA mission called Genesis that was returning with samples of solar wind particles. Lauretta says his team has watched video of that mishap "way too many times."
No matter what happens on Sunday, a recovery crew will be waiting to swoop in once the OSIRIS-REx capsule reaches the ground.
Some researchers will take samples of soil and other materials from the landing site — just in case some kind of contamination occurs, despite their best efforts, and they later have to figure out what came from the asteroid and what was a terrestrial interloper.
No one expects any kind of alien life, says Lauretta. Bennu has been pummeled with radiation for eons, making it highly inhospitable. His team worked with NASA's planetary protection office, which found that returning bits of this asteroid to Earth required no special precautions.
"We're more worried about Earth organisms contaminating the sample than we are the sample doing something to the biosphere," he says.
If the capsule's homecoming proceeds without a hitch, the asteroid sample will remain utterly untainted — making it different from space rocks that regularly fall to Earth and get collected as meteorites.
The capsule will be whisked away to a nearby clean room, where its heat shield will be removed to expose the sample-containing canister.
The biggest reveal
The next day, on Monday, workers plan to fly this canister to NASA's Johnson Space Center in Houston, Texas, which houses the repository for outer space samples like moon rocks.
Lauretta says that on Tuesday morning, if all goes according to the schedule, they'll open the canister inside a special lab.
"I expect we're going to see dust adhering to the surfaces as soon as we open the canister," he says, adding that this asteroid dust will immediately get put into microscopes and other instruments.
Over the following days, workers will detach the collection device, which was locked into its protective canister.
"It'll be a slow process, very deliberate," says Dworkin. "There are lots of fasteners that need to be removed. Every screw head could have Bennu material inside of it that we have to pluck out to preserve."
The biggest reveal should come on October 5 or 6, when they open the collecting device itself.
Researchers expect to hold a press conference on October 11 to go over everything that got collected and their initial findings. About a quarter of the asteroid sample will immediately get farmed out to researchers, who have set up their labs to analyze it.
Some of it will get saved for the future, so that researchers can study it with technology that hasn't even been invented yet — just like some Apollo moon rocks got left in sealed containers for about half-a-century before being opened up and studied.
The asteroid sample's arrival is the first event in what NASA officials are calling their "asteroid autumn."
In early October, the space agency will launch the Psyche mission to a metal-rich asteroid, a nickel-iron one that looks like the building block of planetary cores.
And, in November, NASA's Lucy mission will fly by an asteroid called Dinkinesh, its first asteroid rendezvous in a planned 12-year tour that will take it to multiple space rocks that orbit the Sun at the distance of Jupiter. These asteroids are thought to be remnants from the formation of the outer planets.
OSIRIS-REx has more asteroid adventures ahead of it as well. After delivering its cargo, the spacecraft will speed past Earth and keep going. Its next stop is a near-Earth asteroid called Apophis, which it will reach in 2029.