NASA's novel sampler spacecraft relied on complex visual maps and inertial meters to figure its relation to the asteroid Bennu.
“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.
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 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.
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.
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