Asteroid Sample-Return Mission Arrives to Collect Primordial Rocks of the Solar System
As the OSIRIS-REx spacecraft arrives at its target asteroid Bennu, scientists on the ground prepare for a new bounty of planetary samples
Now flying in formation with the asteroid Bennu, the OSIRIS-REx spacecraft will spend the next eighteen months surveying this pristine piece of the primordial solar system: mapping its composition, studying its motions, and working out the whys and wherefores of similar such objects. This initial survey is in anticipation of Independence Day 2020, when the spacecraft—the size of a UPS truck with the mobility of a hummingbird—will press its sample collection mechanism against Bennu to bring home a sealed canister of premium, grade-A asteroid for analyses at laboratories around the world.
“We will have seen Bennu from a point of light, and once back on Earth, down to its constituent atoms. It’s pretty amazing. There’s no other body that that’s true for,” says Dante Lauretta, the mission’s principal investigator, from his office at the Lunar and Planetary Laboratory at the University of Arizona. He thinks for a moment, and adds, “Maybe Wild 2.”
The comet Wild 2 was sampled by NASA’s Stardust mission in 2004. It was the agency’s first sample-return mission since the Apollo program, though it didn’t approach the audacity of what Lauretta and his team are doing at Bennu. Stardust collected particles in the comet’s wake, the largest of which was about a millimeter, and found amino acids essential for life, changing the scientific understanding of cometary formation. OSIRIS-REx, on the other hand, will take home up to 4.4 pounds of the carbonaceous asteroid. It is impossible to predict what its quarry will reveal, as constituents of Bennu are believed to be older than the solar system itself, but studying such ancient material is likely to fill in gaps in our models of solar system formation and the path that ultimately led to life on Earth.
Sample-return missions are exactly what they sound like, grabbing some celestial specimen in its natural habitat and bringing it home for analysis. Though planetary scientists have worked wizardry with landers and rovers, their mechanical proxies are still frustratingly limited in the science they can do. Robots’ scientific payloads are limited by mass and power, while spectrometers on Earth can be the size of a building. A synchrotron might be a kilometer across. Those are Star Trek sizes. The idea behind sample return is that if we can’t bring the tools to the target, we’ll bring the target to the tools.
“I was in this building in 2008 when the Phoenix lander was on the Martian surface, and those first scoops of Mars wouldn’t shake free from the robotic arm for analysis,” Lauretta says. “They finally figured it out. They warmed it up, and it released and made its way to the mass spectrometer, and we were scratching our heads and trying to make sense of it. And I thought to myself: If I had one grain that I could swab from that scoop, I could tell you a hundred times more information than what you just got off that instrument.”
Not all areas of planetary study are advanced by sample analysis. A geophysicist hoping to understand a planetary object might not reach for a shovel of alien regolith at first. NASA has an established exploration cadence for understanding planetary bodies: flyby, orbiter, lander, rover, sample-return mission and then a human mission. The moon checked each box. Mars 2020, NASA’s next rover set to launch in its namesake year, will begin the sample caching process. It will bottle Mars dirt for a future lander to gather up and blast back home. After that, you send astronauts.
“For decades, samples were glaringly missing from the study of Mars,” says Lindy Elkins-Tanton, the director of the School of Earth and Space Exploration at Arizona State University. “As advanced as we are with remote instrumentation, it is amazing how much more we learn when we’ve got it in our hands. There is just no substitution.”
Though planetary scientists study Martian meteorites for insight into the history of that planet, the meteorites cannot answer the question of whether Mars was ever an abode of life. Moreover, scientists do not know precicely where or when the samples originated before crashing down to Earth. Though meteorites from Mars discovered on Earth can be dated accurately, they are considered to be a likely biased sample, young relative to the Martian surface.
Elkins-Tanton is part of the Mars 2020 science team and serves as the principal investigator of NASA’s Psyche mission to study a metal asteroid, thought to be a planetary core, set for launch in 2022. She says that right away, scientists would study Martian samples for organic materials and their isotopic makeups. Such study of isotope ratios would give a strong indication of whether the material was created by life.
Researchers would also date the sample, “something we can’t do with any accuracy with robots,” Elkins-Tanton says. “It takes super, super fine work in isotope laboratories to get the exact age of a mineral grain or upper rock.” Scientists presently lack absolute dates for rocks on the surface of Mars, and “samples would help to solve some of these long-standing arguments about when Mars was wet. What were the different eons, the eras of different chemical activity on the surface on Mars?”
Spacecraft of every flavor are inherently limited by the scientific hardware they fly. By the time Galileo arrived at Jupiter in 1995, its instrumentation was ten years old. Though technology leapt forward during that decade, poor old Galileo could leverage none of it. Sample missions, on the other hand, are essentially future-proof, says Ryan Zeigler, NASA’s Apollo sample curator. As technology advances, samples can be pulled from storage and revisited for new analysis.
“I grew up in lunar science with a bone-dry moon,” he says. “On Earth, almost every rock has a mineral inside with water tied up inside of it. But when scientists looked at the Apollo samples, they didn’t see that.” This lack of water was factored into models of how the moon formed, how it evolved, and in turn, suggested what the Earth was once made of. “And then ten years ago, we had better instruments and looked again at the glasses and minerals in the lunar samples and found water in both.” The lunar models had to be reworked. “If there are volatiles in the moon, is the giant impact hypothesis viable? Yes, but scientists had to tweak the way the giant impact worked to keep volatiles around. That was significant.”
Such analyses will pay dividends when astronauts return there. “It costs a lot of money to send anything to the moon, so any resource utilization we can do on site is key. And we can use the composition of the moon from Apollo samples to understand what we can use.” Zeigler explains that metals in the lunar regolith might be used to make habitats. Water might also be extracted. “Scientists have come up with a half dozen different ways of making oxygen from lunar soil, using the Apollo samples, on a small scale, to practice on. If I can produce large amounts of water on the moon, or hydrogen and oxygen—that’s rocket fuel! Which in turn enables human exploration of other parts of the solar system.”
All samples of celestial objects are handled and stored by the Astromaterials Research and Exploration Science Division of NASA’s Johnson Space Center in Houston. Each time a new sample is collected, new facilities are built to suit its source and keep the sample isolated and unsullied. Though OSIRIS-REx won’t return its Bennu samples until 2023, Johnson will soon start construction on new set of labs to house Bennu and also part of the asteroid Ryugu, which will soon be sampled by the Japanese Aerospace Exploration Agency (JAXA) spacecraft Hayabusa-2.
The NASA center has already conducted studies for how to store Mars samples; it’s just a matter of getting that mission close enough to the finish line to mobilize cranes and bulldozers for the new storage facilities on Earth. Likewise, the astromaterials division is keeping an eye on the Japanese mission Martian Moons Exploration (MMX), which will launch in 2024 and sample the larger of Mars’ two moons, Phobos.
Closer to home, there is CAESAR, a finalist for NASA’s New Frontiers program, which would sample comet 67P/Churyumov-Gerasimenko in 2038 if it is approved for funding. “We are already looking at what it would take to curate samples from a comet,” Zeigler says. “Luckily we have a lot of time, because it’s challenging. It’s cold, there’s gas involved, there are volatiles involved. It’s not impossible, but it’s going to require us to relearn how we do this and come up with protocols for how we handle entirely new types of samples.”
Getting the samples back on Earth, though extraordinarily challenging, is only half the battle. The real science begins once they are safe and sound in storage.
“One reason the Apollo samples are still useful to science,” Zeigler says, “is because we have spent time and effort to take good care of them, so that they tell us about the moon, and not Houston.”
David W. Brown is author of One Inch From Earth, the story of the scientists behind NASA’s mission to Europa. It will be published next year by Custom House.