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OSIRIS-REx will gather dust from Bennu in a 3- to 5-second “touch and go” encounter.


NASA to sample asteroid for clues to life on Earth

Twelve years ago, Dante Lauretta, a budding meteorite scientist at the University of Arizona in Tucson, was close to a breakthrough. His team was examining carbon-rich meteorites and had detected whiffs of triphosphate—the “TP” in adenosine triphosphate, or ATP, the molecule that powers life. But they couldn’t nail it because of terrestrial contamination. One sample, for instance, came from a meteorite that had crashed into an Australian manure ditch. Lauretta needed something pristine.

Soon after, his mentor, Michael Drake, called. Lockheed Martin wanted to build a spacecraft for NASA that would go to an asteroid and return a fresh sample to Earth, he said. Could Lauretta craft a science case for it? Could he ever. “I’d been doing that for the past year,” he recalls.

On 8 September, Lauretta will be a space flight away from getting his wish, with the launch of the $1 billion Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer (OSIRIS-REx). Although it won’t be the first spacecraft to bring asteroid dust back to Earth—Japan’s Hayabusa 1 returned several thousand grains of dust in 2010—the scoop of grit it delivers in 2023 could reveal new insights into the unaltered building blocks of the solar system and the types of amino acids and other organic molecules that asteroid impacts delivered to an early Earth. “We are bringing back scientific treasure,” says Lauretta, who took over as principal investigator following Drake’s death in 2011.

The samples will come from Bennu, a half-kilometer-wide asteroid that’s as black and dense as coal. It’s a “B-type” asteroid, one of the parent bodies suspected to be responsible for the carbon-rich meteorites that Lauretta studied. Its orbit at times brings it nearly as close to Earth as the moon, so astronomers have studied it well. Nevertheless, mystery remains. There’s a small chance, for instance, that a hint of blue in its reflection could point to a past episode of heating that might have destroyed the anticipated organic molecules, says Mike Zolensky, a space dust scientist at NASA’s Johnson Space Center in Houston, Texas.

Upon arrival in August 2018, the spacecraft will survey Bennu from 240 meters above the surface. It will study the Yarkovsky effect, in which photons emitted from the sun-heated surface of a small, rotating asteroid generate a minuscule force that can alter its orbit. Because the effect varies greatly depending on subtle differences in shape and reflectivity, scientists want an opportunity to study it up close. The effect can also be used to trace asteroid orbits back in time, in order to identify the events that created them. Team scientists hope to confirm suspicions that Bennu was born hundreds of millions of years ago in collisions within the asteroid belt.

As it traces out the lumpy contours below, the spacecraft will also identify up to 12 sites for potential sampling. Lauretta and his team will then narrow these candidates based on accessibility and safety: The spacecraft can’t reach Bennu’s poles, for example, and it will avoid slopes, boulders, and pebbles. After that, the science comes in, along with debates. Some, interested in the early solar system, will favor pristine samples recently exhumed from the asteroid’s interior; others will want surface dust that has been blasted by radiation and endured “space weathering.” Ideally, investigators will find a shallow crater where both types of material coexist. It will ultimately be NASA and Lauretta’s call. “It’s the billion-dollar decision,” Lauretta says.

Credits: (DIAGRAM) G. Grullón/Science; (Images) NASA’s Goddard Space Flight Center Conceptual Image Lab

OSIRIS-REx won’t land on Bennu. It will pogo off it. Extending an arm with a vacuum chamber at its end—a scheme that engineers first demonstrated with a Solo plastic cup and an air compressor—the spacecraft will make contact with the surface for 3 to 5 seconds, blasting it with nitrogen to kick dust into the chamber, before rebounding off. Ideally, it will be a “safe, smooth, slow high-five of that surface,” says Christina Richey, a deputy program scientist at NASA headquarters in Washington, D.C.

The dust harvest—anywhere from 60 to 300 grams—“won’t be much, but NASA scientists have become masters at working with practically nothing,” says Hap McSween, a planetary scientist at the University of Tennessee, Knoxville. McSween leads the curation of the Bennu samples, which will be kept at the Johnson Space Center. The majority will be reserved for future scientists, following the model set by Apollo’s moon rocks.

Will these pricey and precious samples be worth it? Alan Rubin, a meteorite scientist at the University of California, Los Angeles, says that meteorites offer something better—clean samples of an asteroid’s interior—and they’re cheap. “Go some millimeters below the fusion crust” of a meteorite, he says, “and that stuff is still pristine.”

But team members say that although carbon-rich meteorites probably come from asteroids like Bennu, they don’t know for sure. And they believe the dust sample will tell stories about Bennu’s past—biographical information that could not be gleaned from a meteorite. Mark Sykes, director of the Planetary Science Institute in Tucson, is curious to see whether the dust samples show that Bennu has a wide mix of materials—say, rocks formed at both high and low temperatures. That would indicate that, early on, the asteroid’s orbit migrated toward or away from the sun, which could support the notion that the solar system “was a pretty rough and tumble environment,” Sykes says.

The greatest challenge for the mission will be keeping its samples safe and unsullied—the very thing Lauretta was after in the first place. When the mission’s capsule falls onto Utah’s West Desert in 2023, it will be protected by a heat shield and slowed by a parachute. But Hayabusa 1’s similar return capsule, carrying samples from a silicate asteroid, suffered an O-ring leak, contaminating its contents with earthly organics. NASA’s Stardust mission, which raced through a comet’s tail, catching dust as it went, was also plagued by contamination, and its samples were deeply modified by their high-speed collection. OSIRIS-REx, designed for a gentle encounter, aims to do better.

Even if OSIRIS-REx succeeds, its cargo will not be the first carbon-rich asteroid dust in human hands. Hayabusa 2, launched in 2014 and set to return at least 100 milligrams of dust in 2020, ought to hold that distinction. But the U.S. and Japanese teams see their mirrored purposes as a strength, offering a chance to check each other’s results. Nor can any one asteroid tell a universal story. “You don’t go to one of them and say, ‘I understand how all these things work,’” Sykes says. 

*Correction, 6 September, 5:08 p.m.: A previous version of the story incorrectly stated that Hayabusa 2 would return 100 micrograms of asteroid dust.