NASA's OSIRIS-REx spacecraft successfully launched on September 8 from Cape Canaveral, Florida. It will spend the next two years traveling to the asteroid Bennu. After meticulous study of the asteroid, OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security-Regolith Explorer) will eventually touch Bennu's surface and take a small sample before returning to Earth.
So how does a space-faring robot with no legs or landing gear snatch up asteroid material and bring that sample home to Earth? It uses a highly specialized tool called the Touch-And-Go Sample Acquisition Mechanism, or TAGSAM for short.
HOW IT WORKS
The TAGSAM looks like a pogo stick with a wide suction cup at the bottom. The "stick" is a 10-foot reticulated arm; the suction cup is a sample collection head that's about the diameter of a dinner plate and as thick as a dictionary. During launch, the whole mechanism was tucked safely inside the spacecraft, and it will stay there during the voyage to Bennu. Following the mapping and characterization of the asteroid, a process that will last two years, the OSIRIS-REx team will identify a scientifically interesting spot, and the sampling phase will begin. The spacecraft will release a protective cover—the team calls it the "garage door"—and the TAGSAM arm will fully extend. OSIRIS-REx's human support team on Earth will then rehearse how they will collect the sample. They will check thrusters, maneuverability, and the collection arm's dexterity. They want to be sure that everything is behaving as expected. When the team feels comfortable, the actual collection will begin.
The Touch-and-Go Sample Arm Mechanism (TAGSAM) is tested in a Lockheed Martin facility. Image credit: Lockheed Martin Corporation
The spacecraft will approach Bennu at 10 centimeters per second, the pogo stick perpendicular to the surface. On contact, the collection head will disturb the asteroid surface, and as it presses into the asteroid, it will release a burst of nitrogen gas. This will create a dust-up of sorts, sending regolith—the loose soil and other material covering the solid rock—into a collection chamber. After two years of travel and another year of study, OSIRIS-Rex’s direct contact with Bennu will last just about five seconds.
Scientists have a few expectations about what will happen after that contact. Remember how the Philae lander touched down on comet 67P/Churyumov–Gerasimenko and then bounced around? That resulted in a bad outcome for Philae but turned out to be great news for the OSIRIS-REx team, because it is counting on the bounce. After the sample collection, the arm’s contact with the asteroid will spring the spacecraft outward. To measure how much material it has collected, it will begin a spin maneuver. The mass of the collected sample will alter the angular momentum of the spinning spacecraft. Changes in spin from before and after collection will reveal how much material it has captured. If an insufficient amount is collected, the spacecraft will be able to “kiss” the asteroid two more times.
Team members are confident that they will get the sample they seek. "We've tested this arm extensively over the last decade," Rich Kuhns, program manager of OSIRIS-REx, said at a press conference held at the Kennedy Space Center on the day of the launch. "We've exposed it to vacuums. We've exposed it to temperature. We've tested it both pre- and-post vibration, and we've tested it over a very wide range of materials." Insufficient collection has never been a problem during testing. The team intends to collect a minimum of 60 grams of asteroid regolith.
Christina Richey, the deputy program scientist of OSIRIS-REx, tells mental_floss that testing suggests the TAGSAM will collect closer to its maximum capacity—just under 5 pounds of material.
The cameras carried by OSIRIS-REx will record TAGSAM's contact with Bennu's surface. So even if TAGSAM fails to capture a single atom of regolith, it will have performed an invaluable science experiment. Very little is known about random mechanics in a micro-gravity environment. Just by watching how the regolith behaves when stimulated, scientists will have new data for constructing models.
Once its prodding and spinning tasks are completed, the arm will bring the collection head to the sample return capsule, where the head will detach. Once the capsule seals and the sample is secured, the spacecraft will begin its journey back to Earth.
FROM MACH 35 TO 10 MPH
Returning home with a sample of Bennu is the (relatively) easy part. That's because the sample return capsule is proven technology. In 1999, NASA sent a spacecraft called Stardust to comet Wild 2. Like OSIRIS-REx is meant to do, Stardust collected a sample and brought it back to Earth. Its sample capsule detached and landed successfully in Nevada. OSIRIS-REx will use the same design. In 2023, when OSIRIS-REx arrives back at Earth, it will eject its capsule, and the sample will land using parachutes.
"When it re-enters the environment, it's traveling 27,000 mph," said Kuhns. "By the time it gently touches down, it's moving less than 10." It’s scheduled to land at the Utah Test and Training Range, a U.S. Air Force installation in the West Desert of Utah. From there, NASA will bring the capsule to the same facility where samples from the Apollo program and the Stardust mission are stored and studied—the Johnson Space Center in Houston. Between now and then, NASA will invest in cutting-edge laboratories and equipment for sample analysis.
What happens next—how the sample will be analyzed—is still being decided. Right now, the team is focused on the mission at hand. "OSIRIS-REx has always had the strategy to go slowly and carefully and methodically," Dante Lauretta, the leader of the mission, said at the press event. "That's still going to be our plan." That’s one of the reasons OSIRIS-REx launched on time and under budget. When the sample collection capsule lands on Earth, the team will still have two years of funding to perform a full sample analysis, with all the attendant science.
In the future, scientists who haven't been born yet will have pristine Bennu sample material to work with. Only 25 percent of the sample will be used by scientists today. Most will be studied at NASA, but 4 percent will go to the Canadian Space Agency, a mission partner that provided the spacecraft's laser altimeter, and another 0.5 percent will go to the Japanese Space Agency, reciprocating for the sample of the asteroid Itokawa (sampled by its Hayabusa spacecraft) that it provided to the United States in 2010. The rest—75 percent of the sample—will go into long-term storage for scientists of the future, who will be able to study it using tools and techniques not yet conceived.
The purpose of studying the regolith is to analyze its chemical composition. Scientists will be looking for volatiles and organic molecules such as amino acids. This will help explain the role of meteorites in the creation of life on Earth. If they helped us along, they might well have helped other planets develop life as well.
As for OSIRIS-Rex’s timeline, after its successful launch, the next step will be to go into orbit around the Sun before meeting Earth again in September 2017. It will then fly under Antarctica in order to bend its trajectory and slingshot to Bennu. (The trajectory adjustment is necessary because the asteroid is located 6 degrees off of the orbital plane of Earth.) It will make its approach of Bennu in 2018, where it will spend a year, and another year in the sampling process. The return window for its voyage to Earth opens in March 2021.
After OSIRIS-REx reaches home two years later and jettisons the sample capsule, it will remain in space. It will likely still have fuel and be fully functional, with a working payload of cameras, spectrometers, and a laser altimeter. At that time, NASA will have to decide whether to extend its mission, possibly sending it back to deep space where it can continue its charge of exploring the unknown.