NASA’s pioneering OSIRIS-REx mission has successfully returned from its journey to the asteroid Bennu. The robotic spacecraft briefly set down on the celestial body in a first-of-its-kind attempt (by an American space agency) to collect pristine rock samples, before alighting and heading back to Earth on a three-year roundtrip journey. The samples impacted safely on Sunday in the desert at the DoD’s Utah Test and Training Range and Dugway Proving Grounds.
Even more impressive, the spacecraft performed its Touch-and-Go Sample Acquisition Mechanism (TAGSAM) maneuver autonomously through the craft’s onboard Natural Feature Tracking (NFT) visual navigation system — another first! Engadget recently sat down with Guidance Navigation and Control Manager at Lockheed Martin Dr. Ryan Olds, who helped develop the NFT system, to discuss how the groundbreaking AI was built and where in the galaxy it might be heading next.
The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security – Regolith Explorer) is America’s first attempt at retrieving physical samples from a passing asteroid (Japan has already done it twice). Bennu, being roughly 70 million miles from Earth when OSIRIS first intercepted it, presented far more of a challenge in landing than previous, larger targets like the also-not-particularly-easy-to-reach targets of the moon or Mars.
“There’s so many different factors,” in matching the myriad velocities and trajectories involved in these landing maneuvers, Olds told Engadget. “So many little details. A lot of what we’re doing is based on models and, if you have little error sources in your model that aren’t being taken into account, then those can lead to big mistakes. So it’s really, really important to make sure you’re modeling everything accurately.”
In fact, after OSIRIS-REx rendezvoused with Bennu in 2020, the spacecraft spent more than 500 days circling the asteroid and capturing detailed images of its surface from which the ground control team generated digital terrain models. “It takes a lot of research to make sure you’ve got all the effects understood,” Olds said. “We did a lot of that with our work on Natural Feature Tracking to make sure we understood the gravity field around the asteroid. Even little things like the spacecraft’s heaters turning on and off — even that produces a very, very tiny propulsive effect because you’re radiating heat, and on really small bodies like Bennu, those little things matter.”
Since the asteroid revolved around its axis, the surface transitioning from sunlit side to dark and back again, every four hours, the OSIRIS team had to, “design all of our TAG trajectories so that we were flying over the lit portion of the asteroid,“ Olds said. “We didn’t want the spacecraft to ever miss the maneuver and accidentally drift back into the eclipse behind the asteroid.” The NFT system, much like a Tesla, relies primarily on an array of visual spectrum cameras to know where it is in space, with a LiDAR system operating as backup.
LiDAR was initially going to be the primary method of navigating, given the team’s belief during the planning phase that the surface of Bennu resembled a sandy, beach-like environment. “We weren’t expecting to have any hazards like big boulders,” Olds said. ”So the navigation system was really only designed to make sure we would land within about a 25-meter area, and LiDAR was the system of choice for that. But quickly once we got to Bennu, we were really surprised by what it looked like, just boulders everywhere, hazards everywhere.”
The team had difficulty spotting any potential landing site with a radius larger than eight meters, which meant that the LiDAR system would not be precise enough for the task. They racked their brains and decided to switch over to using the NFT system, which offered the ability to estimate orbital state in three dimensions. This is helpful in knowing if there’s a boulder in the lander’s descent path. The spacecraft ultimately touched down within just 72cm of its target.
“We did have some ground-based models from radar imagery,” Olds said. “But that really only gave us a very kind of bulk shape — it didn’t give us the detail.” OSIRIS’s 17 months of flyovers provided that missing granularity in the form of thousands of high-resolution images. Those images were subsequently transmitted back to Earth where members of the OSIRIS-REx Altimetry Working Group (AltWG) processed, analyzed and reassembled them into a catalog of more than 300 terrain reference maps and trained a 3D shape model of the terrain. The NFT system relied on these assets during its TAG maneuver to adjust its heading and trajectory.
That full maneuver was a four-part process starting at the “safe-home terminator orbit” of Bennu. The spacecraft moved onto the daylight side of the asteroid, to a position about 125m above the surface dubbed Checkpoint. The third maneuver shifted OSIRIS-REx to Matchpoint, 55m above the surface, so that by the time it finished descending and came into contact with the asteroid, it would be traveling at just 10 cm/s. At that point the ship switched from visual cameras (which were less useful due to kicked-up asteroid dust) to using its onboard accelerometer and the delta-v update (DVU) algorithm to accurately estimate its relative position. In its fourth and final maneuver, was the craft — and its approximately eight-oz (250g) cargo — gently backed away from the 4.5 billion-year-old space rock.
Sunday’s touchdown was not the end of the NFT’s spacefaring career. An updated and upgraded version of the navigation system will potentially be aboard the next OSIRIS mission, OSIRIS-APEX. “Next year, we’re going to start hitting the whiteboard about what we want this updated system to do. We learned a lot of lessons from the primary mission.”
Olds notes that the asteroid’s small stature made navigation a challenge, “because of all those little tiny forces I was telling you about. That caused a lot of irritation on the ground … so we’re definitely wanting to improve the system to be even more autonomous so that future ground crews don’t have to be so involved.“ The OSIRIS spacecraft is already en route to its APEX target, the 1,000-foot wide Apophis asteroid, which is scheduled to pass within just 20,000 miles of Earth in 2029. NASA plans to put OSIRIS into orbit around the asteroid to see if doing so affects the body’s orbit, spin rate, and surface features.
Source: www.engadget.com