How OSIRIS-REx will Steer Itself to Sample an Asteroid

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.

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