So, in my job as the spacecraft science phase lead, I get to work hand-in-hand with the
scientists and the navigation team to plan out this mission and how OSIRIS-REx is going
to execute this. For me personally this is an ideal combination. I've always loved deep space
exploration and the science behind it, so I get to work one-on-one with the science teams to identify
really what their science goal is and translate that for the spacecraft team to something that
we can actually execute, piece by piece, as the spacecraft goes through its commanding.
So, one of the challenges that OSIRIS-REx has been facing is the microgravity environment that
we are in when we orbit Bennu. We don't have the strong pull of gravity below us, so instead, the
non-gravitational forces in the spacecraft affect our trajectory a lot more than what we're used to.
So over the course of a few weeks, we could end up on a number of different trajectories,
and that means our uncertainty in planning and our responsibility to keep the spacecraft safe
is very challenging. So we have to constantly update our pointing on board the spacecraft,
both for maneuvers and for science pointing purposes.
And we have to constantly reanalyze the sun angles on the spacecraft to make sure
that the power and thermal and telecom attitudes are safe for the spacecraft to navigate.
So, when we're orbiting around Bennu, the force of gravity on the spacecraft is about
one millionth what the force of gravity at Mars is for our spacecraft,
so that's a significant change in the forces that spacecraft feels,
and it means that the non-gravitational forces, such as solar radiation pressure, are within an
order of magnitude of gravity, which is a very new environment for us to be operating in.
So, OSIRIS-REx comes with an instrument suite that was uniquely designed to observe the asteroid,
but the spacecraft itself was also designed to observe this asteroid. Our science mission
requires us to observe and map all over the surface of the asteroid under a number of
different lighting conditions. To support that, all of our subsystems have been tuned for this
purpose. So our solar arrays are gimballed so we can continue to keep a power positive margin
as we look at different sun angles and different sites around the surface of the asteroid. Our
thruster suites, we have very low thrust engines that allow us to do very tiny, precision pointing
maneuvers around the asteroid. On board, our software, for instance, has a targeting software.
It allows us to load up a list of targets from the science teams and tell the spacecraft to
execute them autonomously as it maps out the surface of Bennu for us, and additionally, we
have an engineering camera which we use to support the optical navigation that this mission needs.
So, before launch we expected that the surface of Bennu would be covered in a fine-grained material
much like the lunar regolith on the surface of the Moon. This was supported by the observations that
the science teams had done of Bennu before launch. They used ground-based astronomy,
radio astronomy, and infrared imaging to deduce the thermal inertia of the surface material on
Bennu. When we got to Bennu, we found, to our surprise, that instead it's covered in a lot of
rocky material and it's a much rougher surface than we were expecting. We've had to adapt our
science plan and our sample acquisition plan to this new rocky surface of Bennu instead.
Before launch, we had developed two methods to guide ourselves in for TAG. We had a primary
method which was to use our lidar to do a ranging acquisition. As we approach the TAG site and
guide ourselves in, that was our primary method that we did the majority of our development on.
We developed a backup method using natural feature tracking which is an optical navigation
used on board autonomously to image the site, compare it to a catalog, and guide ourselves in.
When we got to Bennu, the rocky surface of Bennu really changed our entire approach to TAG.
Originally, we had expected to be able to find a 25 meter radius site that could be safe to TAG in,
that would be full of this fine-grained material, and wouldn't have uneven surfaces that could
be dangerous for the spacecraft. Unfortunately, Bennu did not know about our expectations for it,
and when we got there, we found that there weren't sites large enough to meet that need. Without that
large site, the lidar approach could no longer support the increased performance that we needed,
and the natural feature tracking with its higher precision targeting
could get us to a site that was smaller and could fit within the smaller smooth surfaces of Bennu.
So, we've just completed our Match Point rehearsal, and we got to see
a second demonstration of how well NFT has been used to predict our TAG profile.
And in our last rehearsal, we saw NFT come in within two meters
of the TAG site which is really impressive when you consider that our original design
was to come within 25 meters of a TAG site. So NFT's onboard performance and the tuning that
our ground team has done to perfect NFT over the last few years has allowed us to get an
order of magnitude better performance out of the spacecraft than it was designed for, and that's
what we need to be able to TAG successfully on this small TAG site that Bennu has for us.
For me the most exciting part about OSIRIS-REx has been the same thing that causes the challenges:
it's the dynamic nature of this mission and how much we are dependent on what we learn from the
asteroid. Every few weeks our trajectory has to change, and we're having to redesign the mission
as we fly it which gives us a whole new challenge for mission operations we haven't had before.
Traditionally, mission operations, from a spacecraft engineer point of view,
we operate the spacecraft, and we don't always see the results of data we're taking for a long
time. On Bennu the science observations and the results we're getting out of that,
feed right back into the spacecraft trajectory and our overall proximity operations phase,
so we get to see first-hand what we're learning about this asteroid with the
science teams, which has been a really rewarding experience for our team and for me personally.
[Room Tone]