The OSIRIS-REx mission is an asteroid sample return mission
and the main goal is to collect 60 grams of pristine sample from
the surface of the near-Earth asteroid Bennu. Bennu’s rocky
surface presented a challenge for the original LIDAR-based
approach to the Touch-and-Go sample acquisition event. As an
alternative, the project developed an onboard, optical
based approach called Natural Feature Tracking. NFT requires
detailed, 3-dimensional maps of the surface of Bennu. In order
to build those 3-dimensional maps, we needed many many images
of particular patches of the surface on Bennu itself. So, all
these surveys that we’ve done throughout the mission have been
at distances between several kilometers from the asteroid,
down to just a few hundred meters from the surface. And
based on those different perspectives and what the
shading of the terrain is, it is possible to build a 3D model.
Very similar to how we have two eyes that allow us to have depth
perception of what we see, NFT requires multiple features
across the field of view to have a depth perception. The original
TAG accuracy requirement was to touch the surface within a
50-meter diameter circle. Based on Bennu’s rough surface, our
actual performance has to be much better than that, at
10-meters or less. And even within that 10-meter area, there
could be smaller, localized hazards that may damage the
spacecraft. Right now, we are working on a flight software
patch to help us avoid hazards as we come in for our sample
event. When we do Natural Feature Tracking, that is all
autonomous onboard the spacecraft – where we load the
features, or a catalog to the spacecraft – so we are telling
it where we expect these features to be. So, as the
spacecraft makes its progress down to taking the sample, we’re
recording images and then locating those landmarks to make
sure we’re on the right track. And if there are any issues with
the trajectory or we see landmarks where we shouldn’t, we
know something’s up and we can back away and wave off the
attempt and try again.