All right. Okay. We'll go ahead and get started. So this is our
two o'clock press conference: Welcome to Bennu.
So our participants today are Jeff Grossman from NASA Headquarters;
Dante Lauretta from the University of Arizona; Michael Nolan also from the
University of Arizona; and Amy Simon from NASA Goddard Space Flight Center
[No Audio]
-[Jeff] OSIRIS-REx is the third mission in NASA's New Frontiers Program.
These missions are competed missions
and led by principal investigators from the community, and they cost in the range
of a billion dollars each mission. OSIRIS-REx is the third mission that
joins New Horizons which is going to encounter a distant
Kuiper Belt object, Ultimate Thule, on new year's day, and
Juno which is currently orbiting Jupiter and studying its
atmosphere, and another mission to be announced next summer, it's
scheduled to be announced. So that's the New Frontiers Program.
The New Frontiers Program is one component of NASA's planetary mission fleet.
These missions range from all stages of development.
Some of them are just being developed others are in
primary operations, like OSIRIS-REx, and others
have completed, like the Dawn Mission and a few others, and they span the
whole solar system. these missions are NASA run,
and many of them have international partners, and a few of them shown here
are international missions that NASA participates in.
You may notice that there's many of these missions,
if you recognize them, are going to the small bodies
in our solar system. Small bodies, rather than planets or their large moons,
small bodies are bodies that range in size from
only a few meters across to hundreds of kilometers, like the
largest asteroids. The one that OSIRIS-REx is going to,
Asteroid Bennu, is about 500 meters across which is
about three city blocks in diameter.
These bodies are mostly fairly primitive bodies. They never
manage to form into large planets, and they tell us
many things about the early solar system and other things
about our solar system. The small bodies missions are spanning
our solar system, exploring it from one end to the other:
from out at Ultimate Thule where New Horizons is
in the Kuiper Belt which is about 40 times as far
from the sun as the Earth is, to Lucy which will be looking at
small bodies that share Jupiter's orbit when it goes out
there in the next decade, to a few missions in the main belt of
asteroids, including the completed Dawn Mission,
and the upcoming Psyche Mission, and several missions that
are studying objects in the inner solar system like OSIRIS-REx.
And the great thing about studying an inner solar system,
near-earth asteroid like Bennu, is that it's accessible, and that
enables OSIRIS-REx to do something great which is to bring a sample back to Earth.
The topics that these .... There are many problems that these
missions are attempting to solve.
These range from how our solar system formed,
to how the solar system got to be the way it is,
to what the properties and processes in the solar system are now,
to the hazards and resources that asteroids
pose to Earth or provide to explorers, and critically for OSIRIS-REx,
one of the things we hope to learn is about the inventory of volatile elements
and organic matter that may be present in the solar system.
And OSIRIS-REx, as you can see on this chart,
has a lot of its potential to answer questions
like that. And I think what we now know
and we'll hear in this press conference, is that we've gone to the right place to
answer these questions, and that Bennu is proving to be just the
asteroid we hoped it would be. And now we'll find out why.
-[Dante] Thank you, Jeff.
So I'm Dante Lauretta, professor of planetary science at the University of
Arizona and the principal investigator for the OSIRIS-REx Mission,
and also very excited to be here today, so thank you
for this opportunity. So we are here. We have arrived at Bennu.
Arrival was officially December 3rd of 2018,
and that was the morning I woke up and saw an event on my calendar that I put
there a decade ago, so it was truly a great day for us.
And a lot of the uncertainty has been removed about
this mission, right? One of the things that's characteristic of OSIRIS-REx is
that we're going to a target that we've only seen through our
telescopes. And we're using that telescopic knowledge to design the
mission, and understand the risk posture, but
really we were going to a new world, and we were going to reveal that world,
and then assess, in this early campaign of the mission,
the challenges that lie ahead. So here you can see, so far, the best
resolution image of the asteroid that we've
obtained so far. This was right before the arrival on December 2nd
when we were using our PolyCam high resolution imager
that was built at the University of Arizona, to get 33 centimeter
per pixel images, and then this is a multi-image mosaic that's
stitched together, to show the overall asteroid texture.
And there's a couple really interesting features here.
We'll hear more about the shape when Mike Nolan gets up and talks to us,
but we did relatively accurately predict the shape
of the object. We are looking at a bouldery surface.
Remembering that this is a sample return mission, we are going to have to send our
spacecraft down to make a brief, five-second,
touch-and-go contact with the asteroid to collect
material on the order of centimeter sized particles.
We are seeing some basins or depressions, and we're discussing the nature of those
and the origin of those. Some of those look like traditional impact craters,
others may be expressions of the interior structure or the evolution of
the asteroid. So there's a lot of excitement here, and
I just want to emphasize these are really early days in the mission.
This is kind of our first look at the asteroid, and we have a long
time to explore this and make the critical operational decisions about
where to go on the asteroid surface to get that sample.
So the good news is, when when you're working with NASA on a mission like this,
you like to have very clear and well-defined mission success criteria,
and we made sure that there would be no doubt as to whether or
not OSIRIS-REx was a success. And we're proud to announce today that
the first mission success criterion has been achieved.
We have rendezvoused with Asteroid Bennu, and we have begun our exploration of
this fascinating small world. The next one is probably the grand
challenge of the mission which is to contact the surface of the asteroid
and collect that sample. And then, of course, the mission
culminates in 2023 with the safe return of that material to Earth,
ultimate delivery to the NASA Johnson Space Center, and to the astromaterials
curation facility where it will then become available worldwide
to researchers across the planet. We have a phenomenal payload suite.
Our scientific instruments are truly
world class. In addition to the camera system that was provided by the
University of Arizona, I showed you the PolyCam imagery from right at the end of
our approach phase, we also have MapCam data and SamCam.
MapCam is our global mapper and it has the color filters, and we'll be showing
some color imaging data. And then SamCam is there to
document the sample collection activity.
The Goddard Space Flight Center and my colleague, Dr. Amy Simon, is the
deputy instrument scientist for the visible and infrared spectrometer.
And we'll be hearing some more
information about what that instrument has detected.
Arizona State University provided the OSIRIS-REx thermal emission spectrometer,
the OTES instrument, which is right here, and our Canadian Space Agency partners
provided the OSIRIS-REx laser altimeter which did just get its first data a few
days ago when we entered preliminary survey and started our
north polar pass. And we're still actually assessing
the nature of that information and processing that.
And then we're also really proud to fly a student collaboration experiment,
that's the regolith X-ray imaging spectrometer, the REXUS
instrument, from students at MIT and Harvard University, to look at
X-ray fluorescence from the surface and tell us about the elements that are
there. And they're also still in checkout and calibration phase, and we don't have
any information about the nature of the asteroid from them yet.
So with that, I'm going to hand it over to Dr. Simon to tell us about the
spectral analysis results from the mission.
[No audio]
-[Amy] Okay, so, as Dante said, we're all extremely excited to be here and to
finally be at the asteroid. And I'm representing the two
spectrometer instruments today. And so that's OVIRS, the visible and
infrared spectrometer, and OTES, the thermal emission spectrometer.
And the thing to remember about both of these instruments, is that they're not
imagers. They're point spectrometers. So whatever spot
we're looking at on the surface, we're seeing that ...
we're not making an image. We're seeing the spectrum of everything in
that spot at the same time. And so we started our observations on
approach to the asteroid on Sovember 2nd. So about 200 kilometers away is when we
got our first look at the asteroid. So for OVIRS, this is what we saw.
We have this beautiful spectrum, and you can see that the asteroid does
not fill our field of view. It only filled about 45 percent of the field of
view, actually. And so we were seeing the entire surface
on that side of the asteroid, plus dark space around it all at the same time.
And even so, we were able to make our first detection of a real spectral
signature that relates to the asteroid surface,
and that's the feature you see here at 2.7 microns.
And if you're wondering what that feature is - it's one of the things we
were hoping to find. So we're super excited and happy to say
that we saw the evidence of hydrated minerals on the surface of Bennu.
We saw this in every single one of the spectra we have taken to date.
And so that means we're seeing this widespread, hydrated minerals throughout
the surface which is really great news.
It's very similar to what we see in meteorites that have been analyzed here on Earth
in the laboratory. And once we got a little bit closer, we
were able to use OTES which has a bigger field of view, and also get a
strong signal. And again, they also see evidence of
hydrated minerals on the surface. So this is both spectrometers working in
concert to confirm what each other is seeing, and we're seeing great matches
with some of the meteorites. And so, why is this important? And that's
because, to get hydrated minerals in the first place, to get clays, you have to
have water interacting with regular minerals. Once you've made the clays, they
can actually bond and store some of that water in there, make it water bearing.
So these hydrated minerals have evidence of liquid water in Bennu's
past. So this is really big news, and this is a great surprise.
So with that I'm going to pass it over to Mike Nolan to talk about
the shape model.
-[Mike] All right, so, I'm Mike Nolan. I'm the science team chief, and I'm at the
University of Arizona, and I deducted the shape model. So in 1999, almost 20
years ago, I was a junior scientist at the Arecibo Observatory in Arecibo
Puerto Rico, working at this very large telescope,
and one of the ideas that we had going into this was
that we might be able to make observations that would someday be used
in planning space missions. And so here's the data I took on the
left of the actual radar image data from Arecibo. You can
see they're noisy and speckly, but then it was three million
kilometers away, so that's still pretty good.
And from that we made ... using a computer model we made the shape
model on the right which you can see up here on the table too.
We made this shape model. It's sort of a lumpy round thing.
And let's see if we get that to play again.
We did see one sort of obvious boulder feature and said
that we thought that was probably 10 or 20 meters across,
but then we couldn't really see any below that ...
the size of our pixels here is about 15 meters.
But so we knew there was one big one, and we got our overall shape, and people
always ask me, "how much do you believe this shape?" and I said, "well, I believe
that boulder's there; I think the sizes are about right;
some of those little wiggly details I'm not so sure about."
So then we got our ground-based model.
So on the right is the model that I just showed you that we made,
and on the left is the actual imagery that we got.
And so, as far as I can tell, every
single one of these little features on here has some meaningful
signature on the asteroid. The model works remarkably well.
There's ... it's missing some of the surface detail because it was a little
coarser than, of course, we would have liked,
we always like to do better, but everything in this model
is in, you can see in the picture... so there's there's our little bump,
and there's a boulder. Even this smaller boulder
is revealed there. This little valley you can see in the images.
So it worked remarkably well which is important because we made a lot
of our decisions about how to go forward with this mission based on this
model. And so the fact.... And people say, "are you sure?" I said, "I'm as
sure as I can be, and let's go do it." And we went and did it.
And yep, it really worked. So now we have, of course, now that
we're there, we're using images to make a new shape model.
And so on the right you can see the new shape model of
Bennu made from those images on the left. And so it's got a lot more detail.
It's a little more.... We have the models here. You can see that the real asteroid
is a little more squashed, perhaps, than the model we made.
And in fact, that was the one thing I said that
really might be true. When you look at what radar can do, it might be a little
different. But it worked remarkably well and what
that means is that pretty much all of those decisions we made about how to go
forward are really going to be.... We're accurate enough that we can really
use them and say, "Yes, we can. That's really what Bennu looks like, and
now let's go get our sample."
-[Dante] Thank you. So before I go on, let me just say,
the findings that Amy talked about are really,
really exciting for us. I mean, we targeted Bennu precisely because we
thought it had water-bearing minerals, and by analogy
with the carbonaceous chondrite meteorites that we've been studying,
organic material. That still remains to be seen. We have not detected the
organics, but it definitely looks like we've gone
to the right place. Both the OVIRS instrument, with the detection of the 2.7
micron feature, and the OTES instrument, with the
spectral matches to the CI and CM carbonaceous chondrites, really
is exciting. I mean, I know when Amy and Dr. Vicky Hamilton, who leads the
spectral analysis working group, first called me with the 2.7 micron results,
we were so excited, and it just was everything we hoped had come to pass.
Very early in the mission we had found out
that, in fact, Bennu is going to be able to provide the type of material that we
want to return and get back into our laboratories
to answer these fundamental questions about whether these carbonaceous
asteroids might have delivered these compounds to the surface of the early
Earth and led to habitability and maybe even
the origin of life. So let me go over a couple other results.
I talked about the MapCam. That's the color imager. It has four
different band pass filters in the red, green, blue,
and infrared. And when we're looking at Bennu, one of
the other things that struck us early on, when some of the images were coming in
from PolyCam, like the one that we see over here on the right,
these are at really low phase angles, which basically means that the sun was
kind of right behind us, so there's very little shadows,
and all of those color variations or contrast variations that you see on the
surface are real. So we're seeing a lot of different kinds
of reflectivity in the surface material, probably related
to the composition. So we are seeing a fair bit of compositional diversity
which wasn't something, necessarily, that we would have predicted or expected from
the ground-based data. In fact, as we had watched Bennu rotate
in various telescopes and with spectrometers attached to them,
we didn't see a lot of spectral variation or even reflectance variation.
So to see this wide range of reflectance properties is really surprising.
And we see it in the color. So over there on the left is a MapCam red/green/blue
color composite image. You can see the dark spots showing up really nicely.
Bennu is bluish. We had thought that from the beginning. It's a B-type
asteroid, and that means it reflects more blue light than red light.
And we are seeing that color variation that we expected showing
up in the ratios, and there'll be more
information about the implications of the color variations
during our presentations tomorrow morning.
We also care a lot about the thermal properties of the asteroids, so we tied....
Mike mentioned, you know, thinking about planning a
spacecraft mission based on his data processing. Well, a lot of
the safety of the spacecraft and how we design the spacecraft was
based on that information. And we took his shape model
along with observations made by the Spitzer Space Telescope, and came up with
what we thought the thermal profile would be of the asteroid.
And then you design the spacecraft hardware to be able to survive those
thermal regimes, particularly during the sample acquisition event.
And what the graphs over there on the right show is the OVIRS data
at four microns, so out in the infrared part of that
instrument's capabilities, compared to what we predicted
the thermal flux, or the infrared radiation, coming off the surface of the
asteroid would be based on the ground-based shape model.
And then you see the similar kind of plot down below this is the
OTES 14 micron channel, so farther out into the mid-infrared
where you're really getting a lot of the thermal emission, or heat flux, off the
surface of the asteroid, those are the blue circles compared to
the prediction using the asteroid shaped model in the red curve.
So the good news is, again, the shape model,
Mike and his team at Arecibo, and also using Goldstone planetary radar system
in California, really nailed it. The planning that
we did, the design of the spacecraft, the observation profile, they're all spot-on.
We are ready to go. There is nothing about this asteroid
that is going to prevent us moving forward into the
intensive science mapping campaign that we have planned for 2019.
And then I just want to highlight a couple of the really interesting
geologic features. So I would say, Mike, the one thing was that 10 meter
size boulder turned out to be substantially larger, on the order of 50
meters. And there's a fair number of 10
meter-sized boulders, and the boulder geology is fascinating. I definitely
can't go into all of it here. This is one area
that looks like it's a disaggregated breccia,
and that's something I'll talk about during my presentation tomorrow.
But we're looking at a wide variety of the reflectance variations even as we
get down into the tens of centimeter scales. We're seeing clasts or rocks
that look very bright, connected to some that are very dark, and then when we look
at a feature like this, they look like they're maybe falling apart due to some
kind of thermal stresses, or fracturing, or weathering pattern.
We do see rocks that appear to be outcrops, that they're
some just tip-of-the-iceberg reflection of something that appears to
extend much deeper down into the interior of the asteroid, like this
feature here. And it's got a lot of linear features
associated with it which also may [be] due to thermal
stressing and fracturing on the asteroid surface.
And then we also see these perch boulders. There's some that, like
especially the big one, that we even saw on the ground-based radar data and
and several others, that are 10 meters or larger, 10 to 50 meter size range,
and they appear to just be resting right on top of the asteroid surface like they
fell back on, or something weathered around them and left them exposed.
So the bottom line is this is an interesting world from a geologic
perspective. There's a lot going on here in terms of processes on the
surface of the asteroid, modification of the material,
and that's something we want to get a good handle on because as we're going
through our sample site selection process, we want to understand
where do we have the best opportunity to get this most primitive,
oldest material from the dawn of the solar system, and where might we get
material that would tell us about the geology of the asteroid and the
processing that happens on the surface of the asteroid.
In the best world we would get both kinds of material, or multiple kinds of
material, depending on what we see on the surface of this asteroid.
And then we were starting to look at where we might be able to go get a
sample. I know it's very early days here, but as the PI of the mission,
I have to think about that second mission success criteria:
safely contact the asteroid sample... asteroid surface and collect a sample.
And it's a little early, but we are seeing these
basins and possible impact craters. This one here,
it's over here on the left of the image, is about 20 meters in diameter
which is of order of our navigation capability in terms of accurate
targeting a location on the asteroid surface.
And we are seeing the finest grain material seeming to accumulate in these
low basins here. We haven't mapped the entire asteroid surface yet.
We've only been at this for a week, and our team is going through this in a
very systematic manner, and this is just the preliminary survey
data. When we get to the detailed survey next year, when we're in our site
selection campaign, we're going to get about a factor of
six to seven times better resolution. So right now we're at 33 centimeters per pixel.
When we do the global mapping campaign,
what we call our "baseball diamond" which refers to the different look angles so
we can get stereo imaging and different shadowing features,
we'll be getting down to five centimeter per pixel resolution.
And then once we've got regions of interest selected on the asteroid
surface, and we get into this reconnaissance phase,
we'll be doing low altitude flyovers of potential sample sites,
and we'll be getting sub centimeter per pixel resolution of these
regions of interest and potential sample sites. So we are
going to take our time. Right now we're still in the navigation campaign of the
mission, which means we're learning to fly. One of
the big challenges of OSIRIS-REx is accurate navigation of the spacecraft in
this microgravity environment. It's more akin to docking than it is to
landing, when we go in to get that sample. So a lot of the expertise from NASA
and from our partners at Lockheed Martin that relate to space situational
awareness or rendezvous with a target in Earth orbit, like the
International Space Station, come into play with OSIRIS-REx. So we
want to take our time and make sure that we know how to fly, and that's what the
navigation campaign is about. We're getting quick,
early-look data in preliminary survey. We're planning an orbital insertion in
orbit A, and we actually do have an estimate on
the mass, and I'll be talking about that in the presentation tomorrow as well as
our radio science team and Dan Shearer's presentation,
but in orbit (and that'll be a record for the smallest object ever orbited by
a spacecraft) we'll really transition over to
what we call landmark tracking optical navigation, where we look at
features on the surface of the asteroid, and use that to determine the spacecraft
position. So it really won't be till the end of
February where the science campaign, what we call the site selection campaign
of the mission, starts. And the science team is kind of in charge of where the
spacecraft is pointing and the data that we're collecting,
processing all of that information. And once even we have a site,
which we anticipate by the end of 2019, we're going to spend about six months
designing the descent down to the asteroid surface and rehearsing various
approaches to make sure we do it in a safe and
systematic manner, so that we go and get that sample
approximately July of 2020. We're confident in mission success.
One of the other things we did along those lines in terms of our readiness
for TAG, or the Touch And Go sample
acquisition event, is to extend the robotic arm that's called TAGSAM,
the Touch And Go Sample Acquisition Mechanism, that was provided by our team
members at Lockheed Martin. And we got it out in front of the
SamCam. And the image that you see over here on the right is an
in-flight image, one of my favorite images, because you can actually see
sunlight passing through the filter screen. This is basically an air filter,
and I call TAGSAM a space vacuum cleaner. We're actually going to blow
high purity nitrogen down into the regolith of the asteroid,
and push it up into this filter. And the fact that we can see the shadow of the
filter screen on the inner portion of the TAGSAM,
makes me look forward to that day in 2020 when we take the same image after
the sample acquisition event and we can see outlines of grains there.
So, fingers crossed. After sample acquisition, these images
from SamCam will help us verify successful sampling, and then we
can start thinking about stowing that in the sample return capsule,
and bringing it back to Earth, which will occur in September of 2023.
So, this is kind of our report card. For those of you who don't know, the name
OSIRIS-REx, as I like to say, is an awesome acronym that
describes the scientific objectives of the mission. So the first one,
Origins, looks like we've got to the target that is like our
most volatile and organic rich carbonaceous meteorites.
From the spectral interpretation we thought there would be clay minerals on
the surface of this asteroid, and as Amy said, that we have found, both in
OVIRS and OTES data, strong convincing evidence that the
surface is dominated by these clay minerals.
From the resource identification and, kind of the feed forward to asteroid
mining, Bennu appears to be a very water-rich target.
And water is the most interesting, and possibly lucrative,
commodity that you would mine from an asteroid. So you could process it into
rocket fuel in-situ, in space, and from security which relates to the
fact that Bennu is a potentially hazardous asteroid
with a non-negligible probability of impacting the Earth
well into the 22nd century. We have already, from our initial preliminary
survey flybys, characterized the mass. Combining that with the shape model we
have a constraint on the asteroid density.
And then the orbit of the asteroid, and folding all of that information into
predicting the likelihood of an Earth impact, that's still work to go as we
move forward. I would say the one area that we were
surprised by the asteroid, and where the challenge ahead lies,
is that we were working towards one boulder
on the order of 10 meters, we've got hundreds of boulders on the
order of 10 meters. The average grain size was predicted to
be on the centimeter size scale. That's still to be determined as we get into
the higher resolution imaging campaigns, but
certainly, it's a little more rugged of an environment than we had expected
based on our mission planning and the ground-based observational data analysis.
So I will leave you there with this portrait of Asteroid Bennu,
and I think we're ready to move on.
-[Moderator] Okay so, questions. Please remember to
state your name and affiliation.
Hi, Sid Perkins, I'm a freelancer, and
related to your last point in looking at the population
distribution of boulders and sizes on the surface,
is that what you would expect? Are you seeing something of a Brazil nut effect
where the big stuff rises and the small things kind of
settle, or what what can you say about that
versus your expectations if you had any?
-[Dante] Right, well, certainly our expectation was
that there would not be as many boulders of
this size scale on the surface of the asteroid. So that is the the big
surprise so far that we've encountered with the asteroid.
In terms of what's causing the boulder distribution, it looks like there's
multiple processes at work here, and like I said it's really early days.
We've literally had seven days to think about it.
Plus getting ready for all these events that are going on here at the AGU.
So, I almost say come to the talks tomorrow because we've got
three different presentations on boulders and geology, plus posters
on Wednesday afternoon. There's a lot going on to explain the boulder
distribution and their morphologies on the surface of this asteroid.
Seth Borenstein, AP. Two quick questions:
first, the mass of Bennu, if you can give us that,
and second, have you seen anything that's different than, for example,
Ceres and Vesta, you know, which would be the ones you'd be compared to
in terms of past missions. Is there something that you've
seen on Bennu that we haven't seen on any
other asteroid yet?
-[Dante] Sure so, in terms of the mass of
the asteroid we work in units called GM which is the gravitational
coefficient times the mass of the object. From the ground we had predicted about a
value of 5.2, and we're seeing a little lower than
that, around 4.9. Probably a better value is
the density, and the density's coming in at just
under 1200 kilograms per cubic meter, remembering that your average rock is
around 3 000. So it's really got a lower density than even these
carbonaceous chondrite meteorites, which tells us that it probably is made
of these hydrated minerals which are generally low density, but it's also got
some porosity or cave like structures in its interior
to the tune of maybe 40 percent of that asteroid
may be void space in the interior to get
down to such a low density. In terms of comparison to Vesta and
Ceres, it's really not fair because this would be a boulder on the
surface of those objects. Those are really planetary scale bodies.
And especially something like Vesta which has gone through planetary
differentiation, core formation, volcanism on its surface,
Bennu is a totally different creature compared to those objects.
It really looks like a pile of rubble in the sense that it
probably originated from a larger asteroid.
We have some working hypotheses that it
came from a much larger body, on the order of 100 kilometers in diameter.
And that was probably shattered in a catastrophic collision,
on the order of 800 million to a billion years ago.
And Bennu is just an accumulation of rubble from that event.
It's very different than Vesta, maybe more like Ceres in the fact that
it's got these water-rich minerals and so on, but again Ceres is kind of a
planetary scale object, and you're going to have a lot more active geology and
you know, surface expressions and cryovolcanism, and things like that that
you would never expect to see on Bennu.
Harvey Lyford - Freelance
I'm wondering first of all, whether this is the smallest
object in space that we've orbited with a spacecraft,
and secondly, how much gravity can it have
for you to have a stable orbit around it. I wonder how that works.
-[Dante] Sure, so it will be the smallest object ever orbited by a spacecraft.
That will happen on New Year's Eve of this year, so pay attention for that
exciting event. And the acceleration due to gravity
at the surface of the asteroid is estimated at about 10
micro g's. So 1g is the average acceleration at the surface of the Earth.
So we are one divided by 100,000 times that. So it is a really low
gravity environment, and I mentioned the navigation challenge,
and that's the reason why. You've got a gravitational force on the spacecraft
that is comparable to solar radiation pressure,
as well as outgassing from the spacecraft, or even thermal radiation
from the asteroid, so that's why the navigation team gets
two months to learn how to fly
as they get into orbit at the end of the year.
Alex Witze with Nature.
Just sticking with the Dante theme, I wanted to ask you to talk more about
the grain sizes. You can sample only up to two
centimeters. You've got a very, very rugged look here, as you pointed out.
You're noting these basins as places with the small grain material to maybe
try and sample. How much are you worried about that
two centimeter sampling limit?
-[Dante] Well, you know I don't worry about things
that are out of my control, right? So we did everything we could to plan this
mission. I am confident we're going to find some fine-grained
regions. The challenge will be how large they are and what that
means for our navigation accuracy as we go into to
get the spacecraft precisely targeted to the location that we want on the surface
of the asteroid. There's an interesting puzzle going on
here, because one of the key pieces of evidence for the average grain size was
a parameter we call thermal inertia, which basically tells
you how fast the asteroid surface heats up and how fast it cools off.
And usually lower thermal inertias correspond to smaller grain sizes.
So that's one of the reasons we estimated the average grain size on the
order of one or two centimeters. We're still seeing that low thermal
inertia. I didn't go into it on the thermal plot, but when we looked at that
profile, the thermal curve as Bennu rotates,
it's very consistent with the thermal inertia
that we estimated from the ground.
So there's some physics that we need to go back and take a look at, and see...
and maybe there still is that fine grain material. There's still plenty of
unresolved regions on the asteroid surface,
and it may be dominated by centimeter scale particles. Remembering our pixel
is 33 centimeters at this point, so we don't know what's happening inside those
single pixel regions. And as a point of reference, the pixel
size is just about the same size as the diameter of the TAGSAM head.
So we have to get TAGSAM to one pixel on this image,
and there's plenty of regions that we don't know what's happening at that
scale. So that's why we have the year ahead of us to do the detailed
mapping and ultimately the high resolution reconnaissance.
Christoph Siedler: Spiel Online
Dante, you mentioned that it would be unfair to compare
Bennu to Ceres, for example, or Vesta. Can you compare it to Ryugu, the asteroid
that Hayabusa2 is currently at just to
have a vague idea how these are similar or different?
-[Dante] Does anybody want to take that? I've been answering a lot of questions.
Anybody else?
-[Amy] Well, I guess at least from the spectral point of view, we can
say that, based on Ryugu, we weren't expecting to
see these hydrated minerals so easily from so far away. So that was actually a
big surprise that it was such a strong signal
immediately, without even a full field of view filled in.
So, I think we probably do have more hydrated minerals than they do.
We'll know a lot more once we enter that science mapping phase and can look at
variations on the surface and get better ideas of where things are
concentrated.
-[Mike] Okay, one other thing is, so we knew Bennu looked like this. We've
known Bennu looked like this for a long time, but we didn't know Ryugu looked
like it does. And so when we saw those images of Ryugu,
we were looking at those, and say well, that is likely...
since overall they look kind of the same, we expect maybe they'll look
the same in detail. And then when they're actually so
different, that was even more exciting,
that you have these things that are superficially very similar,
but then in detail they're really quite different. And so that's another
exciting aspect of going forward here.
-[Moderator] I believe we have a question on the chat
[inaudible]
Hello? Okay cool, we're on. I have a question from vickisnath
with hotpoprobot.com. The question is, "Currently, Bennu is the second most
hazardous object, in terms of possible collision with Earth, according to NASA's
Palermo Technical Hazard Scale. How will the mission help us in improving the
risk index associated with asteroid Bennu?
-[Dante] So, one of the big uncertainties in the trajectory of the asteroid is a
phenomenon called the Yarkovsky effect. And the Yarkovsky effect relates to the
fact that the asteroid gets heated up by the sunlight,
and then as it rotates into it's afternoon and it's evening, it releases
that energy back out into space as heat. And it's a retrograde rotator which
means it's afternoon is in the direction that it's moving on
its orbit. So that emission of heat acts like a thruster
and applies the brakes and slows the
asteroid's orbital velocity down, therefore shrinking the semi-major axis
of its orbit. We're going to study that phenomenon in
unprecedented detail, and we're going to do it in two
directions: from the top down and the bottom up.
Mike talked about the radar data starting in 1999.
We got even better data in 2005, and then in 2011 we were able to
range to the asteroid, we didn't get the imaging resolution, but we
were able to get its position relative to the Earth. And because of
that 12-year arc, we actually measured the Yarkovsky effect in a
change in the asteroid's orbit. And then we combine that with Spitzer
Space Telescope observations to get the thermal emission,
and that was how we got that asteroid mass estimated to begin with.
So now we're going to be able to extend the baseline from 1999, to 2011, all the
way to the end of our encounter in 2021, so we'll have 22 years of tracking on
the asteroid's orbit, and that'll improve our understanding of
how much the semi-major axis is changing as a result of this phenomenon.
But in addition, because we're going to get this phenomenal
5 centimeter resolution global shape model, and we're going to get the thermal
emission from the OTES instrument, we're going to be able to evaluate the
Yarkovsky effect from first principles. Do we understand
the physics of thermal radiation and how it changes the
asteroid's orbit. And that'll shrink the uncertainty,
and therefore the impact probability will change.
Don't know if it'll go up or down, but it'll change as a result of that
improved knowledge.
-[Jeff] I'd also like to add, that knowledge can
be then applied to other asteroids, even ones that we
haven't yet discovered, which may prove to be hazardous,
and help understand the hazards posed by many asteroids.
-[Moderator] Great. Do we have any other questions from reporters in the room?
-[Questioner] I've got a really stupid question.
What's to say that isn't just a giant
chunk of impact breccia from a giant collision
that just sort of came together...
-[Dante] Okay, how do we know it's not an impact breccia?
It may be an impact breccia.
I mean it is... we expect a pile of rubble that
coalesced after a catastrophic disruptive impact
in the main asteroid belt hundreds of millions of years ago.
And, in fact, when I showed you that disaggregated assemblage
that looked like it was kind of breaking down in-situ. There's very
different reflectivities there. So, it's definitely a pile of rubble,
and whether or not it originated in an impact remains to be seen,
but that's okay, right? So there's nothing wrong with that.
Seth Borenstein again from AP. Just at the bottom right,
you know, the bulge that you saw, Michael, you know decades ago,
is that the 50 meter boulder you're talking about?
-[Mike] Yes -[Seth] And and how big is the opposite one?
-[Mike] The opposite ones, I recall...
It's about the same size around but it's not as tall. So it's
more like 20 tall, roughly. -[Seth] And is there a good... how do you
describe this shape? I mean is there something similar
in people's everyday life that this looks like to you?
-[Mike] Well, I mean, to first order it's round, but I think of it as sort of
two cones sitting on top of each other. That's sort of the way I think
about it is right here's a cone and there's a cone,
and with the bridge in the middle. That's the overall shape, and we think
it's a round thing where the spin has basically formed this
big bulge around the equator.
-[Seth] Okay... because it's not quite that round. I mean it's that bulgy...
-[Mike] You start with something round, and then
then as it spins up, the bits at the equator kind of move out.
and that's sort of how we think of it. We think of it as looking that way,
and also, most likely, actually having formed that way.
-[Dante] Yeah, I'd say the shape is a really interesting
question right now because we thought it was related to the spin state
and material migrating down to the pole, but we had the question earlier
about the comparison to Ryugu. Ryugu is spinning much slower,
by a factor of about two, and it has a more pronounced
ridge and much sharper peak. And so that is counterintuitive to the rotation
state model, unless there's been changes in the rotation state of those two
asteroids. So I would say the team is having a field day right now because
it's an asteroid geologist's dream to have a mystery like this to go and solve.
So there's people throwing out all kinds of ideas about how these shapes and how
these ridges form. So I would say, "stay tuned," because I
think that the active debate is going to go on here.
-[Seth] And just numbers of the distance from here, from Earth,
and the spin rate for us please.
-[Dante] So, Bennu is on an eccentric orbit
that gets almost out to the orbit of Mars and actually
comes inside the orbit of the Earth. So it's an earth crossing asteroid.
It does a close approach to the Earth every 1.2 years.
I believe we have a one-way light time on the order of seven minutes right now.
So that's how we normally measure distance is how long it takes the signal
to leave the Earth and get to the spacecraft, and then come back.
The spin state is rotating once every 4.3 hours, and and we actually know it to
something like 10 decimal places,
but that's probably good enough for this.
-[Moderator] Okay, I think we have some questions on the chat.
Yeah we have a few questions. The first one is from Lisa Grossman with
Science News. The question is, "Does Bennu look more or
less boldery than Ryugu? Also, How did you feel
when you first saw Bennu up close?"
-[Dante] You're wanna take the boulder question?
-[Mike] I'd say offhand, the largest
boulders on Bennu are comparatively smaller than the largest
boulders on Ryugu, and that things scale pretty well, and that's actually in one
of the posters that you'll see on Wednesday.
It's less bouldery,
but they're comparable in some fairly
straightforward ways. And there's there's a whole presentation
on that on Wednesday.
-[Dante] And how did you feel when we first saw Bennu?
So I was pretty excited. It was like, "hey, we're there!
And we have an awesome asteroid to explore." I mean, it was...
it's a dream come true, and it's an honor and a privilege
to be able to lead a program like this for NASA, and for
the United States, and really for the world.
And we are... we can't wait to spend the next year mapping this asteroid.
It really is the culmination of a decade and a half of work on my part and
this team which has worked together for years and years and years, just an
amazing group of people. This is, obviously, a very small representation of
the hundreds to thousands of people who have worked hard to make this mission a
success. And I'm really proud of everybody and all the work that
we've done to get here. So we have an amazing spacecraft with a
world-class suite of instruments, and a fascinating world to explore,
and all of that rushed into my mind as I got those first images of Bennu.
-[Moderator] Do you have another one on the chat? -[Chat curator] Yes, we have a question from
Dan Bamberger, Freelancer.
"The radar modeling for Ryugu didn't show
the equatorial ridge, while Bennu's shape was known better.
still, this actual shape is more squashed at
the poles. How reliable are radar shapes in general?
-[Mike] So, we didn't have a radar shape model of Ryugu. We actually tried, but
for various reasons, mostly including funding, we weren't able to get
that shape model. I think that was in the early 2000s. Whereas, we had the radar
shaped model of Bennu and actually, specifically knowing the
squash, that was our biggest known uncertainty was how squashed it
would be. And it's well within the uncertainties we estimated.
So that's why we didn't come out
the same is because we didn't actually have the model to start with.
-[Chat Curator] Okay, one more question, from Leah Crane with New Scientist:
"Do we have any ideas of why Bennu
might have so much water?" -[Dante] Sure I'll take that one.
So, we think that Bennu, the mineralogy on the asteroid formed
basically at the dawn of the solar system
over four and a half billion years ago when material was accreting from the
protoplanetary disk, and the mineralogy is a remnant of that
early phase. So as I've mentioned several times at
this event, the asteroid is a fragment of a much
larger body that originated in the main asteroid belt.
And that body, when it formed, probably accreted, what we would say, anhydrous or
dry minerals along with ice particles and then very
early, within the first five to ten million years of the
formation of that asteroid, that ice heated up and melted and then
created, like a hydrothermal system in the interior of the asteroid, and that
hot fluid reacted with the anhydrous minerals and
formed the clays. And that is still preserved to this day.
So that's one of the most exciting parts of studying samples like this
is that you're actually looking at a rock that's four and a half billion
years old, that's older than the Earth, it's older than anything that
anybody could possibly touch or study.
And you're peering back to the dawn of the solar system and trying to
understand the chemical processes that played out,
that led to the Earth being the habitable world that it is today.
-[Moderator] Well, I think we have time for one more.
-[Questioner] So to follow up with that, the idea that you've got hydrated minerals,
the source of that water was not likely impacts with comets later, or
how much could that have been a contributor to your hydration, or
is it pretty firmly understood or expected that this was
proto solar system?
-[Dante] It's an interesting question, especially
your reference to comets. One of the things that we've learned
as planetary exploration has rolled out, and especially another
NASA mission, a previous NASA mission called the Stardust Mission, actually
went to the coma of a comet and brought back samples
of that material, and one of the things we learned from that is that
asteroids and comets are not like two distinctly different kinds of
populations. There's kind of this continuum
where you have really dry rocky and metallic stuff,
and on the other end you have nearly pure icy bodies.
And then you kind of can get any ratio of mixtures between the ice and rock
in-between there. So Bennu probably was, at least the parent asteroid that
Bennu came from, was comet-like at some point
in its early history, and that it was a combination of rocky,
metallic, icy, and organic material. And then all of that kind of accreted
together and you had some limited geology taking place to establish the
minerals that we have there. That's the working hypothesis, and I want
to emphasize that, right. One of the great things
is that we've laid out a hypothetical timeline for this asteroid,
and I published this back in 2015. We're going to test all of that, right.
We're not only going to test it with the asteroid encounter data, but we're going
to bring a sample back and the sample is going to tell that story
in great detail.
-[Moderator] Okay. I think we're just about out of time, so
that will conclude our press conference, and we'll reconvene at 3 o'clock.
-[Dante] Thank you.