Bennu has a lot of challenges for sample collection. The one that we focused on the
most before launch was the actual sample head and our kind of vacuum cleaner to get the dust
into a head when there's no vacuum or no air to help you. So, we've got that one we think.
And then when we arrived and we saw how rocky that surface was, that all of a sudden gave us
a whole slew of other challenges which basically fall into the categories of precision
navigation and maneuver performance. So, we've got a landing site we've got a hit that's now
five times smaller. We've got huge boulders the size of three-story buildings parked on
the edge of that landing site, so getting in and out safely has also become quite a challenge.
The navigation on O-REx for descent to the surface is done on board.
It's an optical navigation system. It involves kind of three different teams of people here,
both here, in Arizona, Goddard people are involved. It's a huge bunch of people. So,
the science guys with all the mapping have built what amounts to a 3D map of that asteroid down to
centimeter precisions, and then we figure out the flight path to get to
the surface. And then along that flight path we pick out features to recognize,
to use as guide points to tell us where we are on the way down, and that we're on track, and
we have two opportunities to correct the course; that's the checkpoint and the match point burns.
One corrects us to the tag site, and one stops the relative motion and takes us straight down.
So the optical system figures out where we are well enough to do those burns. Those burns are
computed on the fly in real time. So we put pieces of that digital asteroid map on board.
The software on board is smart enough to account for lighting configurations and everything else.
It looks at where it's supposed to be, says "well, my camera's pointing this way,
that feature ought to be up here, I don't know precisely where I am." There's an uncertainty,
so it draws a box around where the feature ought to be and starts looking for it.
One feature will give you a direction. Multiple features give you direction,
position, the whole nine yards. So we're looking for up to five features in each image to tell us
precisely where we are in three space, and then we can also use changes in position to do velocity
and figure out how we're moving and propagate the position. And that's how we get down there.
The whole process of taking the images, figuring out where they are,
correlating them to onboard stuff, that's the natural feature tracking system that was developed
as, actually, a backup to the lidar system that was put in place before launch.
So, the surface of Bennu is extremely rough, much rougher than we had planned on. It does
pose hazards to the vehicle in the process of getting that sample.
They fall into two basic categories: there's a tip over hazard and there's a back away hazard.
So, when we come down we're going to touch the surface. The head's likely to move because we
won't be perfectly straight down. There will be some lateral velocity. The head's going to stick,
the vehicle is going to tip, the controller will try to hold us still, but it won't be completely
successful, depending on how fast we actually touch, so there is a hazard to the arm that
if there's a boulder nearby, we could lean over into it with the arm. The arm has high pressure
gas bottles on it. It's got plumbing for the gas to feed the vacuum cleaner; it's got pyros
on it which are basically explosives. You don't want to hit that arm on something.
So that's one hazard. There is also a hazard that, having touched, gotten the sample, tipped over,
now we decide to get out, we're going to back away at the attitude we're at at the time, which may be
a little off from straight up and down, and if we tip over too far and then back away, because of
the size of the obstructions around the TAG site, we could fly into something. So there's been a lot
of analysis of the site using the really high precision digital elevation maps and all the
slopes of the facets and which way will we tip and what might we hit depending on which way we tip.
All that's gone into convincing ourselves that we can get down and get out safely.
So, the head comes into contact with the surface. There's a spring in the arm so
that the vehicle will keep pushing towards the surface. The spring will soak that up.
It holds the head in contact with the ground, we fire the gas bottles, the bottles will stir
up the dirt under the head. We only give the gas one way out and that's through the vents
in that sample head, so the stirred up dirt is carried with the gas. The gas goes out the vents,
the dirt is kept by screens in the head, and that's how we collect the sample.
We just got finished with Match Point rehearsal, and we've used that imagery to estimate,
based on where we look like we're going to come down and what's under us, how much
sample we might be able to pick up based on the surface properties and all the properties of the
that we see, and the estimate that came in after match point rehearsal from the science team was
around 100 grams, maybe more depending on things that we can't tell from imagery like
how much will the surface compress, what's under the particles we can see, but the current guess is
around 100 grams if we were to come down right where we were headed at match point rehearsal.
The sample collection measurement is done using a change in momentum of the spacecraft.
So, if you think about ... the analogy I usually use is an ice skater,
arms out - spins slowly, arms in - spins faster. So, that's a momentum issue. We
have done ... we just do two positions of the spacecraft: we do the arm straight out,
we do the arm straight up. We spin the spacecraft, and we see what it takes to spin the spacecraft,
and we can make an estimate of the inertia of that spacecraft with an empty head
and we know it's completely empty right now because we haven't touched a thing.
So we do that measurement before we go down. After we come back up, we do the same two positions,
and based on the change in that estimated inertia, we can tell ... we can estimate
how much mass must be sitting in the head because it's the only thing that's changed.
We can take three sample attempts from actual contact. We have three bottles we can fire, so we
can basically run the vacuum cleaner three times. We can do any number of aborts part way down
and get out and still have our three attempts. Those don't use up a bottle. We have lots of fuel.
We can add sample into the head. It was built for that, so there's no problem
there if we get a little bit but we want more we can go back. There is uncertainty as to
if we do fire the bottle, the bottle has a huge amount of energy in it, given the rocky surface
and how hard it was to find sampleable spots, we don't know what firing that bottle is actually
going to do to the sampleability of the site. So there is a chance that if we fire the bottle
and we want to go back, the site may or may not still be suitable for sampling.
So, even if we have to abort on the first attempt because we think we're coming down on a hazard,
and we have the ability to know that, we have a backup site, Osprey, that is in a lot of ways it's
a little easier to get to. It has fewer hazards within the site. It has some on the edge but none
really within the site, but it doesn't look as sampleable. That's why it's the backup, but we do
have that option, so we aren't going to take ... we aren't going to go for broke on the first shot
and risk the vehicle when we know we have the equipment to take more than one sample attempt.
The most exciting part for me has been kind of rising to the challenge that Bennu presented, and
you know, working with all the various parts of this team
to figure out how we're going to change this thing, from going to a 25 meter beach to
a five meter pile of rocks with buildings parked around it, and you know, watching how these guys
have figured out ways around pretty much every problem that's come along. It's been a lot of fun.
[Room Tone]