Since December 2018, the OSIRIS-REx spacecraft has been orbiting the asteroid Bennu and trying to find out more about its chemistry and geology. And for good reason: “Bennu is a time capsule,” says Thomas Zurbuchen, the head of NASA’s Science Mission Directorate. “It has been out there for 4.5 billion years, and carries the history of that environment with it.”
If we study 1,640-meter-long Bennu and other asteroids like it in deep detail, we can potentially unravel how the ingredients that lead to the formation of planets like Earth—and, eventually, life—come together. Now comes the hard bit: getting a sample.
OSIRIS-REx has already paid off to some extent. In less than two years we’ve confirmed that Bennu is rich in organics and hydrated minerals. We’ve learned it probably once had massive rivers of water flowing through its insides. We’ve seen it’s an “active” asteroid—it loses mass through the ejection of debris. We’ve also learned, contrary to our initial beliefs, that the surface of the asteroid isn’t caked in fine grains like a sandy beach, but rather covered in boulders. It’s rough, rocky, and rugged.
But all these revelations pale in comparison to what we might learn if we’re able to get some samples of Bennu here for scientists to study in the lab. That’s precisely the goal come October 20, when OSIRIS-REx will plunge down toward the surface of the asteroid and attempt to scoop up some rubble and dust from the surface. It will be one of the hardest things NASA has ever attempted, taking place over a fraught 4.5 hours more than 200 million miles away from Earth. Then it has to bring the sample safely back over the course of the next three years.
Asteroid sample returns might already sound familiar to you—Japan’s Hayabusa 2 mission collected some material from asteroid Ryugu, and will bring them back this December. But unlike Hayabusa 2, which used high-speed space bullets to retrieve these samples, OSIRIS-REx will use what could be described as a “reverse vacuum cleaner.”
It’s called TAGSAM, short for “touch-and-go sample acquisition mechanism.” It’s an 11-foot-long arm with a collection head fitted on the end. As the head gets closer to the asteroid, it fires nitrogen gas onto the surface in an effort to stir up material toward the collection head—ideally any particles that are two centimeters or smaller. According to Olivia Billett, the mission science lead at Lockheed Martin, it’s an entirely new concept for extraterrestrial sample collection. “The TAGSAM head was designed by a Lockheed Martin engineer in his garage, just working with a can of compressed air and a plastic cup,” she says.
The sampler was initially designed for a sandy space beach with an area of 165 feet. As we now know, nothing like that exists on Bennu. So instead, the team has now settled on Nightingale, a 52-foot-diameter site sitting inside a crater that’s thought to be well preserved. There are boulders the size of buildings surrounding the site, and many other large rocks that could disrupt the sample collection or totally wreck the TAGSAM arm. But it still affords the best opportunity for a safe collection of meaningful material.
A couple of new capabilities were also developed to try to keep the probe safe during the procedure. One was to nix an imprecise lidar-based navigation and guidance system in favor of natural feature tracking (NFT), in which an optical camera continuously takes images of the surface throughout the 4.5-hour TAGSAM procedure and processes them to update the estimate of the spacecraft’s trajectory. According to Billett, this is the first time NFT is being used as part of a space mission. Since it takes more than 18 minutes for communications to go one way between Earth and OSIRIS-REx, autonomous control will be critical for adjusting maneuvers on the fly.
Here’s how the sample collection will work:
In preparation
The OSIRIS-REx team has spent weeks inputting all the proper command prompts to painstakingly maneuver the spacecraft so that by October 20, it will be in exactly the right spot to begin the sample collection process.
2,500 feet above the surface—4.5 hours from touchdown
The spacecraft begins a maneuver to depart orbit and transit toward Nightingale. Shortly after it leaves orbit, the TAGSAM arm is deployed, the spacecraft rotates into the proper orientation, and the navigation camera that enables the NFT system is redirected toward Bennu’s surface. From here, the NFT will be working to constantly determine OSIRIS-REx’s position and ensure its safety relative to a map of hazards on the surface.
410 feet—20 minutes from touchdown
The spacecraft’s solar arrays are folded into a “Y-wing” position. OSIRIS-REx fires its thrusters to perform a “checkpoint burn,” which ensures that the spacecraft is heading toward Nightingale.
177 feet—10 minutes from touchdown
Thrusters fire again for the “matchpoint burn” to match the spacecraft’s speed with the asteroid’s rotation. This sets up the precise contact and velocity for the touchdown, and the spacecraft is essentially in a free-fall descent to the surface.
16 feet
This is the final benchmark before the actual sample collection occurs. If the NFT thinks the TAGSAM arm is coming down on something dangerous flagged by the hazard map, it will automatically execute an abort burn that moves the spacecraft up and away from the surface. Billett says there’s about a 5.8% chance this might happen. Otherwise, it keeps heading down.
Touchdown
TAGSAM will make brief and very gentle contact with the surface for five to 10 seconds. During that time, the nitrogen gas bottle fires, and sample collection gets under way. Once it’s over, the thrusters will fire again and the spacecraft will head out to a safe distance from the asteroid.
The aftermath
The goal is for the TAGSAM head to pick up at least 60 grams of material (although it could potentially acquire as much as two kilograms). The team will run a series of experiments over the next week or so to verify if this has been achieved. It’ll start with visual evidence of the TAGSAM head from one of the onboard cameras. Then the team will measure the mass of the sample inside the TAGSAM head. You can’t really weigh things in the microgravity field of Bennu (a millionth the gravity at Earth’s surface), so this will focus on observing the spacecraft’s spin (which should change with added mass). If the team thinks a sufficient sample has been collected, it will be stowed in the sample return capsule.
But there’s actually a 30% chance OSIRIS-REx fails to pick up enough material. TAGSAM has two more nitrogen gas bottles for two more collection attempts. Because Nightingale will already have been disturbed by the first touchdown, a second attempt would most likely occur in January at a site called Osprey. That means we could potentially get samples from two different sites on Bennu—and it would be up to researchers on Earth to disentangle everything and figure out what originated from where.
That might seem like a hassle, but it would be a small price to pay to get some rubble from a rock 200 million miles away. Either way, OSIRIS-REx would be scheduled to leave Bennu later in 2021, and deliver the collected samples back home on September 24, 2023.
Correction 10/14: the initial version of the story stated that NFT was developed by Lockheed Martin for the US Army. NFT was developed as part of internal research and development by the company, not for the US military.