Just how do you bring home pieces of asteroid? Carefully, that’s how, with grudging respect for the curveballs the asteroid can throw you.
Sixteen years after NASA’s OSIRIS-REx mission was first proposed and two years after the robotic spacecraft went into orbit around asteroid 101955 Bennu, mission team members are now counting down to the moment when it will descend to the surface, grab a sample—and then get out of there before anything can go wrong.
The sampling is set for next Tuesday, Oct. 20. If it works, it will be a first for the United States. (A Japanese probe is currently returning to Earth with samples from asteroid 162173 Ryugu.)
Though the mission plan has so far been executed almost flawlessly, an outsider might be forgiven for thinking there’s something a bit…well, counterintuitive about it. The spacecraft has no landing legs, because it will never actually land. Instead, the OSIRIS-REx spacecraft vaguely resembles an insect with a long snout—a honeybee, perhaps, hovering over a flower to pollinate it. The “snout” is actually an articulated arm with a 30.5 cm round collection chamber at the end. It’s called TAGSAM – short for Touch-And-Go Sample Acquisition Mechanism. You’ve doubtless heard the old expression, “I wouldn’t touch that with a 10-foot pole.” The TAGSAM arm is an 11-foot pole.
The OSIRIS-REx sampling arm hovers a few meters above Bennu's surface in a practice run for its planned touch-and-go maneuver.Gif: Goddard Space Flight Center/University of Arizona/NASA
The TAGSAM collector will gently bump the surface—and then try to kick up some rock and dust with a blast of nitrogen gas from a small pressurized canister. If all goes well, at least 60 grams of dirt will be caught in the collection chamber in the 5 to 10 seconds before the spacecraft pulls away.
The mission was conceived in 2004 at the Lunar and Planetary Laboratory at the University of Arizona. It was rejected twice by NASA before it won approval in 2011. The spacecraft was launched in 2016, reached Bennu in 2018, and has surveyed it for two years. If Tuesday's sample pickup is successful (if not, there are two backup nitrogen canisters), the ship will bring its cargo back in a sealed re-entry capsule for a parachute landing in the Utah desert on Sept. 24, 2023.
Think about that: nearly 20 years of work, seven years in space, US$800 million spent, and the moment of truth—actually touching the asteroid—will not even last a minute.
And all for just 60 grams? “That’s like a few sugar packets that you use for your coffee,” says Michael C. Moreau, the deputy project manager at NASA’s Goddard Space Flight Center in Maryland. But for scientists involved in the mission, that’s enough to conduct their experiments and put some aside for future research. Bennu, and other near-Earth asteroids like it, are interesting because they are rich in carbon, probably dating back 4.5 billion years to the formation of the solar system. Might they tell us about the origins of life on Earth? “We are now optimistic that we will collect and return a sample with organic material—a central goal of the OSIRIS-REx mission,” said Dante Lauretta, the OSIRIS-REx principal investigator at the University of Arizona, who has been on the mission team from the start.
Asteroid Bennu, imaged by OSIRIS-REx from a distance of 7 km.Image: Goddard Space Flight Center/University of Arizona/NASA
Bennu is not a friendly place for a spacecraft, especially a robotic probe operating on its own 334 million km from Earth, far enough away that commands from mission managers take 18 minutes to reach it. The asteroid is a rough, rocky spheroid, about 500 meters in diameter, and it spins fairly rapidly: a “day” on Bennu is about 4.3 hours long. It’s probably not solid. Scientists call it a rubble pile, a cluster of dirt and rock held together by gravity and natural cohesion. And when Moreau says Bennu threw them “a bunch of curveballs,” some of them were literal—pieces of debris being flung out into space from the surface, though not enough to endanger OSIRIS-REx.
An asteroid the size of Bennu has almost no gravity to speak of: an object on the surface would weigh 8 one-millionths of what it would on Earth. That means staying in orbit around it is very delicate business. OSIRIS-REx’s orbital velocity has been on the order of 0.2 km/hr, which means a tortoise could outrun it. More important, it can easily be thrown off course by solar wind, or heating on the sunlit side of the spacecraft, or other miniscule forces. “Wow, a[n extra] millimeter per second three days later changes the position by hundreds of meters,” says Moreau.
A false-color animated gif of asteroid Bennu to show subtle variations in its gravity. Red and orange mean higher elevations; hence, less gravity.Gif: University of Arizona/CSA/York/MDA/NASA
Which is an error they can’t afford. Scientists thought Bennu would have a fine-grained surface, but were surprised when the spacecraft’s images showed a rugged, craggy place with house-sized boulders. They had hoped to pick a touchdown spot 50 m across. But the best they could find was a depression they nicknamed Nightingale, all of 8.2 m wide, with a jagged rock nearby that they call Mount Doom. OSIRIS-REx has a hazard map on board; if it appears off-target it will automatically abort at an altitude of 5 meters.
So there will be some nail-biting on Tuesday, but after 16 years on the project, Dante Lauretta said they’re as ready as they can be. “It’s transcendental,” he said, “when you reach a moment that you’ve devoted most of your career to.”