this jet propulsion laboratory presents
the von Karman lecture a series of talks
by scientists and engineers who are
exploring our planet our solar system
and all that lies beyond
good evening ladies and gentlemen how's
everyone tonight excellent I really want
to thank you guys for coming out tonight
especially in that weather
we didn't expect a house anywhere near
this full so thank you very very much so
shall we
mission planning is a core strengths of
JPL engineering along with deep space
communications and navigation tonight
we're gonna take a look back at the
various scenarios and contingency plans
that the Cassini team made as they steer
the spacecraft into unexplored space
during its 2017 grand finale our guests
will discuss how the possible scenarios
some of which could have been Mission
ending compared to the mission as it was
actually flown along with sharing some
science highlights from the finale
tonight's guest was the mission planning
head lead for the Cassini mission
prior to joining Cassini he served as a
mission engineer and mission architect
for the Mars advanced formulation office
and four other various planetary mission
concepts
he joined JPL in 2005 fresh out of Cal
Poly San Luis Obispo where he received
his BS in aerospace engineering a BA in
physics and a master's degree in
aerospace engineering ladies and
gentlemen please help me welcome
tonight's guest mr. Eric Sturm
thank you very much and let me second to
the thanks for coming out in the rain
that's very impressive thanks to the
smarter ones who are online who are
watching from a nice warm dry area okay
so yeah as we said I'm Eric Sturm and I
am Cassini's lead mission planner and I
say am NOT was because while the Cassini
mission ended six months ago the project
is still going we're still archiving
we're still closing out we're still
writing reports it's not as fun as it
was but it's not past tense yet so I
have the honor of speaking you guys
speaking to you guys tonight as
Cassini's last mission planner but I'm
certainly not Cassini's only lead
mission planner and in fact I share that
title with five other people and you can
see them all here and so without the
hard work of these people Cassini
wouldn't have gotten off the ground
wouldn't have gotten into space wouldn't
have made it out to Saturn and it
certainly wouldn't have made it through
its tour and let me be here tonight to
talk to you about the end of that tour
Cassini's grand finale and as you can
see Cassini has been around a long time
he first got a lead mission planner back
in 1991 but the beginnings of Cassini
actually go back even before that back
to 1982
that's when Cassini was born and at that
time a working group was formed between
the European Science Foundation and the
National Academy of Sciences and they
were supposed to come up with a joint
concept for exploring the planets and
eventually what came out of that was a
concept that involved a u.s. Saturn
orbiter and a European probe that later
became Cassini and Huygens and you can
actually see Cassini and Huygens over
there that's our half scale model and
just to give you more of an appreciation
of just how long ago 1982 was that's me
back in 1982
so Cassini and I were born in the same
year so yeah it was a long time ago
okay so jumping ahead a few years
Cassini launched in 1997 and it followed
this nice loop-de-loop green path out to
Saturn that's actually two and a half
revolutions around the Sun it flew by
Venus twice Earth and Jupiter all on its
way out there and seven years later it
arrived at Saturn in July of 2004 and at
that time it began its prime mission
which is a four year mission to tour
Saturn and explore it and its moons and
the Rings it got a mission extension in
2008 to go an additional two years out
to 2010 that was the Cassini Equinox
mission and then it did such a good job
there it got a seven year mission
extension the Cassini Solstice mission
which was so named because it extended
the mission all the way out to the
northern summer solstice and that ended
six months ago so here you can see the
entire Solstice mission trajectory this
is from when it started in October of
2010 up until the end of November of
2016 and at that time Cassini began its
penultimate phase which were the ring
grazing orbits these are lighter gray
orbits that you see here and so this was
a series of 20 orbits the the farthest
they get from Saturn is out near Titans
orbit and the closest they get is just
outside the ring system Saturn's F ring
and that's their name the ring grazing
orbits so these 20 orbits took a little
over a week to complete they started at
the beginning of December of 2016 all
the way through the middle of April 2017
and here is the last of those ring
grazing orbits so it's the last time
that it's going to pass outside Saturn's
ring system and this time as it comes
back out towards Titans orbit Titan is
actually gonna be there and what we're
gonna do is we're gonna reform our last
targeted Titan flyby this is our hundred
and twenty seventh close flyby I have
tightened we came within 600 miles of
the moon and that changed our orbit just
enough that now as it comes back through
towards Saturn instead of being outside
the ring system it's inside
and it'll be between the planet and the
rings and so that dive happened on April
26 2017 so just about a year ago now
okay so that's what started Cassini's
grand finale it's a series of 22 and a
half orbits the dove between Saturn and
the Rings in a gaff about twelve hundred
miles wide and it did it at seventy six
thousand miles per hour which would get
you to DC in about three minutes or to
the moon in a couple days so it was
going pretty fast also periodically as
it came back out to Titans orbit Titan
would be there far away not as close as
the six hundred miles but it would give
it a little tug and change exactly where
we flew through the gap the last of
those distant flybys which is happening
right there was on September 11th and
that's where Titan gave us a goodbye
kiss some may call it a shove and it
actually made it so instead of passing
safely through the gap four days later
on September 15 2017 we are permanently
captured by Saturn okay so one of the
first questions I get when I say tell
people that we did this telecoil why did
she do that
Cassini was a cool spacecraft so first
why we had to do it at all was for
planetary protection the Saturn system
has two protected moons and protected
just means that they have environments
that could possibly possibly be
habitable for life so one of those is
Enceladus Enceladus is pretty cool it
has a global subsurface ocean under a
thick ice shell and it's in direct
contact with an active rocky core very
much like environments found in our our
deep ocean vents and then there's Titan
with its thick atmosphere it has rivers
and lakes of liquid methane and ethane
and both of these are thought to be
environments where we could find life
and because of that it means we have to
absolutely ensure that Cassini would
never impact these and possibly
contaminate those environments with our
own life because what the last thing
we'd want to do is send a future mission
there and discover life only to find out
no that's life that we just brought when
we crash Cassini into one of them so
that meant Cassini needed to be removed
from the Saturn system one way or the
other
either through leaving it or through
crashing into Saturn the reason it
needed to happen now is actually because
of Cassini's gas tank we were running
low on fuel so here's our spacecraft and
this is not an engineering
representation but a good authorization
representation of our gas tank and we're
gonna fill this up with gas right now
and I should say when I asked my wife
what color is
Cassini's gas she said it's orange it's
definitely orange so we're gonna fill it
up now with definitely orange it's a
little like cerulean blue but definitely
orange so there it is and another nice
thing about actually being orange well
think of this as one hundred oranges
okay this is a hundred oranges worth of
fuel okay at lunch okay
now just getting out to Saturn we use 24
oranges so we're lot down to 76 okay now
we want to go into orbit about Saturn
alright well that's another 45 oranges
now we're down to 31 4 years after the
start of our prime mission use another
17 only 14 left ok two-year mission
extension we can do this
eight-and-a-half oranges okay seven year
mission extension we've got five and a
half horn just left all right well we
can use them all and okay now we have
the ring grazing orbits in the grand
finale for the last year and I think you
can see some propellant still in there
that's you know oranges have about 10 to
11 wedges that's a common knowledge
right okay yes that's that's one orange
worth of fuel or one orange wedge worth
of fuel that's how much we had left we
started with a hundred hundred oranges
we 899 opened that hundredth and eight
ten of the eleven wedges that's what we
were left with okay so that's what we
flew the last year of the mission on
okay and so why did we do it by going
into Saturn like I said we could have
tried to get out of the Saturn system
but by going into Saturn it gave us the
opportunity to collect a bunch of unique
science so we were going closer to
Saturn than ever before so we could get
lots of information about Saturn's
internal structure like gravity magnetic
field we're also going inside the Rings
for the first time which would let us
see there
possibly estimate their mass and get an
idea of how old they are also because of
how close we were we were flying through
the Aurora's through radiation belts
through ring dust so you're gonna get
direct measurements of all of those
things and then simply because we were
so close we'd get really high-resolution
images and these images are amazing and
these aren't even ones from the grand
finale we got better higher resolution
images than these okay that's a long
wind-up of the grand finale and why we
did it
as Cassini's mission planner for the
four years before the end of mission I
spent all my time worrying about this
gap minding the gap so the region
between the Rings and the planet and
here you can see where the 22 orbits
passed and you can see the final orbit
and orange that actually impacts Saturn
and you can compare that to an actual
image that was taken by Cassini and so
you can see we're flying right where
that D ring dust in the atmospheric haze
just sort of fade into the background of
the image so like really going right
where we think it's safest and so those
really were our two concerns for these
orbits how much dust was the spacecraft
gonna encounter and he'll think of an
atmosphere was it going to fly through
and that meant that each of the twenty
two orbits could be characterized by two
points the first was their ring plane
crossings so this is how high into the
ring plane did they go and how much dust
was potentially going to impact a
spacecraft the second point was how
close do they get to Saturn their
minimum altitude how much atmosphere
were they going to fly through and how
much drag and torque was going to be
imparted on the spacecraft you can get a
little better idea of how these are
distributed through the gap am I looking
at them down from above so now you can
seed out Saturn down below we got the
Rings up above and you can see the
orbits going through and what's neat
about looking at it in this from this
perspective these are actually in
chronological order so the first ones on
the left and the last ones on the right
and so you can see each of the ring
plane crossings
and the minimum altitude points you can
also see how there's the variations and
those are caused like I said by the
distant Titan flybys that we're having
periodically as we flew the grand finale
and to get a better idea of those
variations we can take all those points
and plot them up I'm an engineer I got
to put things into Excel I can't just
look at pictures and so now we have time
on the horizontal axis starting at the
end of April going to the middle of
September again
d-ring dust of above atmosphere down
below and we have distance from the
center of Saturn in kilometers on the
horizontal axis and I know everybody's
familiar with kilometers and you know
sixty-three thousand kilometres makes
perfect sense to everybody in the room
but just in case it doesn't we're also
familiar with how wide the earth is all
right so that's about five earth
diameters from the center of Saturn
another very common measurement so okay
so these are our ring plane crossings
and minimum altitudes you can see at the
end we get low in the atmosphere but for
the 17 orbits before that were actually
much higher up and we're more concerned
about the dust environment than the
atmosphere and so we're gonna start by
looking at that so what did we know
about the dust environment this is
literally everything we knew about the
dust environment this is our best
fixture of the D ring dust and so what
we did is we took this picture and we
overexposed it and then you could draw a
line and say hey outside that line
there's dust inside that line we're not
really sure but we think there's not
dust and if we go back to and compare
that to the range through which we flew
the orbits you can see that our highest
orbits actually went above it they went
into a region that we knew there'd be
dust and so going back to our corridor
chart and adding that line we can see it
was actually our four highest crossings
so those four crossings flew through a
region that we knew there would be dust
and so what does that mean well it
doesn't particularly mean a whole lot
because Saturn is a dusty place we flown
through dust a lot before and so
whenever the dust was such that it could
have posed a risk to the spacecraft what
we would do is we'd take the entire
thing again you can look at it over
there we take that huge antenna and we
turn it into the direction that the dust
was coming from and you can see that
that shields all of the spacecraft
behind it so all of our sensitive
electronics
instruments were protected behind the
high-gain antenna so good we have a
shield we will use it so there we go
our four highest crossings were shielded
but then the question is well what about
those other 13 crossings like I said if
we go back to this picture we don't
necessarily know what's happening inside
that that Purple Line it could be
there's no dust there but it could be
the nature of the dust is changing and
reflecting this light as an example
think about if it's nighttime and if and
it's foggy and you're driving you have
your headlights on the fog is just
reflecting everything back at you all
you can see is fog you can see the fog
very easily but then if it's raining
instead of foggy you have still lots of
droplets in the air but they're much
bigger and they don't reflect as much
light back to you so we couldn't know
for sure if it was foggy outside and not
foggy inside or if instead of foggy it
just started raining and if it just
started raining that meant there were
large dust particles there that could
pose a risk to the spacecraft so given
that information and I know what you're
all thinking okay well we have a shield
let's just use this we're gonna Captain
America this sucker and we're good we're
done and if you thought that great you'd
probably make a very good spacecraft
engineer but you'd probably upset a lot
of scientists the reason being
everything on Cassini is a body-fixed it
can't point in two directions at once so
if we're using our antenna as a shield
it means the cameras can't look over
here at this very interesting thing and
so while that may not seem like a big
problem because hey this is only for
ring plane crossing that's just this
small little piece of the orbit Cassini
is the size of a school bus and it's not
very nimble and it doesn't have a very
large dusters and so what that means it
takes a lot of time to turn the
spacecraft and when you factor that in
the time to turn to that shielded
direction and then the time to turn back
to our nice science friendly attitude
suddenly we wipe out all of our
high-priority science areas so okay we
know this is a bad idea but it's
possible this is a bad idea too because
we don't actually know what's there so
what we did was
you came up with a contingency plan that
said hey we're gonna shield that very
first crossing and when we do that we're
going to use an instrument the radio and
plasma wave science instrument to detect
the dust particles that are there when
the dust hits the spacecraft it creates
plasma waves and that instrument can
detect those waves and from that we can
see if the environment is safe or not
but the catch here is that those that
first orbit and that second orbit are
only 6 and 1/2 days apart so we had to
get the data back to analyze it and
decide what to do with the spacecraft in
6 and 1/2 days if we were going to keep
everything else unprotected and so
that's this is what this looked like so
here we have our first ring plane
crossing in green coming around to the
second one in red and then we have all
of our communication passes in blue out
on the far side and so after we came
through that first crossing it took a
whole day before we even got to talk to
the spacecraft so that wiped a day out
we only have 5 days left the spacecraft
gave us back all of the radio plasma of
science and stern data and then we
wanted to opportunities to tell the
spacecraft if it needed to go to the
safe attitude and so that's what you see
they were those next two opportunities
and had it seen how's it as dust it
could have turned the spacecraft and
gone in the next one shielded okay so
that was the plan that was a contingency
plan and what actually happened well in
order to understand what happened what
happened let's take a step back to our
ring grazing orbits okay these are the
20 orbits before the grand finale
during the third ring grazing orbit the
radio and plasma wave science instrument
was on and it took data for us and the
data it gives us is this and what this
is is this time on the horizontal axis
it is dust particle size on the vertical
axis and it is the density we see at
that time at that size of that dust
particle so this is how how thick of a
dust cloud do we fly through and so you
can see on this third ring raising orbit
the ring crossing should probably jump
right out at you it happened happened
right there and on this scale of less
dense to more dense though the red that
we're seeing here actually isn't even
dangerous to the spacecraft so this is
what we saw when we went through the
first time
even have to shield it they would have
to go into a nice deep dark maroon for
us to be worried about it so what did we
see we saw this we almost literally saw
nothing you can't even tell where ring
plane crossing happened it happened
there but there's just there was just no
dust and so we didn't have to invoke the
contingency the scientists there as much
for joining me and so this is what it
looked like and in fact we didn't see a
noticeable amount of dust until that
first orbit above the line and there the
dust wasn't hazardous and so it turns
out we did use our contingency but we
used it in reverse and we took we were
able to take that third orbit and
unshielded and what that meant was on
the Left we have the original design on
the right we have what we did and so
instead of turning and pointing straight
down to look at the dust and shield us
as we came in we kept rotating past and
this led let our cosmic dust analyzer
peek out from beside the antenna and
directly sample the diamond dust
particles so we were actually able to
get more science than we thought we were
going to okay so that brings us back to
our corridor and now now we're done with
dust we're done with us first seventeen
nobody's worried about dust anymore
right okay good good no one's worried
about dust uh-huh
now we're worried about the atmosphere
for those last five and this is not the
first time Cassini flew through
atmosphere Titan has a very thick
atmosphere and we flew by Cassini I
would say lots of times
so again 127 times we flew by Cassini
and during those low Titan flybys we
switched from reaction wheel control to
thruster control and the reason we do
that is the thrusters can by provide
about 10 times the control authority
that the reaction wheels do and that
means that we could fly through a ten
times thicker atmosphere and still point
the spacecraft in the direction we
wanted to point it now if the atmosphere
was so thick that it would overwhelm our
thrusters what we could actually do is
perform a maneuver change the trajectory
and fly through a less dense
part of the atmosphere so these are our
two options when it comes to dealing
with Saturn's atmosphere so what did we
have to do well first we need to know
what do we know about Saturday's
atmosphere at the time well everything
we do came from solar and stellar
occultation z-- which is having Cassini
watch the Sun and stars set into
Saturn's atmosphere and see how their
light gets filtered Durrett and from
that the scientists derived a density
model so this is this gives us density
as a function of radius and latitude on
Saturn and so we can use that model and
go back to the proximal corridor and
start adding some boundaries so the
first is our spacecraft capture boundary
so this is the region that we had to get
to in order to ensure that Cassini would
not come back out of Saturn again and
you can see the very last plus on
September 15th is well below that region
above that we have the tumble boundary
for thrusters so this is the point where
the density of the atmosphere is such
that if we were on thruster control we
couldn't point in the direction we
wanted to point and we'd start to drift
and then above that we have that same
boundary but for the reaction wheels so
this is where the atmosphere is 10 times
less dense than the boundary for the
thrusters and you can see that our final
5 are right between those two so what
that meant was we flew those last five
on frustr control and you also see that
two others were on thruster control
earlier than that that wasn't because of
atmosphere is actually because of
science observations they wanted to take
advantage of the higher turn rates that
were allowed thanks to the thrusters
higher torque okay so to get an idea of
just how close we're coming to not just
having to use thrusters but potentially
having to use a maneuver and get out of
this region we can zoom up on the final
5 and so here you can see about how
close we're getting to the tumble point
and so we thought we had about 120 miles
of margin between where we were going to
fly and where we would come
and to get a better idea of just what
this meant for the spacecraft in terms
of the thrusters we can convert that
altitude margin into a thruster duty
cycle margin and so here frustr duty
cycle is the percentage of time that the
thrusters are on while they're in the
atmosphere so if they're on 0% it means
they didn't really encounter atmosphere
if they're on a hundred percent it means
they're flying through more atmosphere
than they can handle and we're about to
lose pointing control and you can see
that our worst case scenario had us at
just over 30 percent and as a point of
reference Titan flybys low Titan flybys
in the past were designed to have a 70%
worst-case so we were even less than
half of that so we felt pretty
comfortable going in but again we were
prepared to be surprised so just like
with dust
we had a contingency plan for the
atmosphere we had a contingency plan so
the plan was to fly that very first one
just as planned it has the lowest duty
cycle of all of them it was the highest
in the atmosphere it was the least
likely to lose control but if what we
saw there was more atmosphere than we
thought we could perform a maneuver in
between the two passages and pop up into
a safer part of the atmosphere we could
then fly this blue trajectory to the end
all by performing one maneuver
okay so again all of you wanted to
shield absolutely every crossing so now
you're all just ecstatic hey we have
lots of margin here and we have a
contingency plan so we're good we're
gonna sit back relax and enjoy these
last five orbits again you're gonna
upset a lot of scientists the scientists
came back and they said well you know
that's only 25% you can go to 70 so you
can go lower right well that that really
depends on how much fuel we have and
what actually have a saw in the
atmosphere but let's just go ahead and
and take a look at it we can go lower
and specifically they want to go lower
for those last two because those are the
ones we're doing a direct sampling of
the atmosphere and so looking at the
higher of those two the Rev 291 now
we're changing the horizontal axis now
the horizontal axis is the size of the
burn we'd need to perform
to get lower into the atmosphere and to
hit different duty cycles it's in meters
per second which is speed its how how
much do we have to change the speed of
the spacecraft to get lower in the
atmosphere four and a half meters per
seconds about ten miles per hour and so
as we start to change the speed of the
spacecraft out near Titans orbit we get
lower in the atmosphere and we find that
it would take about two and a quarter
meters per second about five miles per
hour of speed change in order to get low
enough in the atmosphere that we'd hit
that 70% which also corresponds to a
best guess and best estimate of a forty
percent duty cycle to give you some idea
of the size of that burned we're going
3,700 miles per hour out at Titan and we
need to change the speed by five miles
per hour so it's kind of like sticking
your head out of the car window and
blowing backwards to see how much speed
that adds to you it's so not a lot we
are one wedge or one orange wedge of
propellant was going to be enough to do
this so this in fact became a viable
option and what it meant for the the
scientists was now if we're looking at
change and altitude on the left and
change an orbit period on the right or
the amount of time it takes to get back
around to the same point in the orbit as
we do different burn sizes what it meant
was they got a little over a hundred
miles deeper into the atmosphere
176 kilometers and it only changed our
orbit period by two minutes so all the
other science observations we were doing
during the orbit other than right there
where we're at the lowest point I really
weren't affected because two minutes
over six and a half days not too bad and
so this became our third viable path
through these final five all right so
now question is what what really
happened so we flew through that first
time and it came in right there
30% and doing some quick math that is
two to three times thicker than we
thought the atmosphere should be and if
you use that data point to update our
model the other predicts do this so wow
that's 70% line we didn't fudge the
numbers for that that second one is
actually sixty nine point seven percent
so it came in just under it but needless
to say the scientists were like okay we
I need to go deeper um and the project
manager said hey you know what we were
still below the line and that's our
worst case so we're gonna stay the
course and we're just gonna not perform
any maneuver and so turns out that was
the right call the next four came in
here so you can see they were actually
below what the updated model would have
said so we flew through some thick
clouds that first time and scared
ourselves but they were still above the
original estimates so and this is still
a mystery we're still trying to figure
out exactly why our stellar occultation
x' and the direct measurement of the
atmosphere didn't quite line up so going
back to our final five chart we can
update this for where the atmosphere
actually was and we think it was about
there and so that was our as flown final
five and then we can back out again now
we've done dust we've done atmosphere
and here's the as flow and grand finale
so for shielded crossings instead of
five and no maneuvers in those final
five okay so that's the end of my talk
as a mission planner now I'm gonna try
and play scientist which can be
interesting or maybe just space
enthusiasts that's probably a better way
to put it
okay so some fun science stuff so what
was the result of all this well first
before we even got to the grand finale
one of my favorite images was taken
during the seventh ring grazing orbit
and here it's Deafness orbiting in in
the Keeler gap yeah you're having some
trouble seeing it huh yeah here's the
actual image that was taken and so here
you can see Deafness one of Saturn's
moons orbiting in the Keeler gap between
the Rings in the main ring system and
you can see as the the ring particles go
by Deafness it actually creates a wake
and you can get an even better view of
this by applying some false motion and
so you can see that as daftness orbits
to the right the Rings below are
orbiting slower and appear to move to
the left the Rings above or orbiting
faster and go to the right
and so you have a wake on the outside
ring going out to the left and on the
inside ring you may just be able to make
out the start of a wake going in the
other direction because of the relative
motions so very analogous to that is
this other fun image of a ring propeller
so ring propeller is very much like
deafness only on a very very small scale
it's orbiting in the main ring system
it's a very small movement and it
creates a similar wake where you have
these wakes coming off the opposite
edges what's really neat about this
particular set of images is because of
the way the ring grazing orbits came in
over the top and then exited down below
we were able to photograph the same
color in a relatively short amount of
time from both the left side and unlit
side of the Rings and so that's what
you're seeing there and scientists are
interested in these because they think
that they are analogous to a baby planet
and a protoplanetary disc and how it can
start to a creep mess in an early solar
system okay so now we're actually at the
grand finale this is grand finale orbit
number one were a day out from the first
dive through the Rings and we're over 70
and so we got this series of images of
Saturn's hexagonal jet stream and
hurricane and it's North Pole and you
can see it's really neat what's even
more interesting is we have a similar
set of images from four years earlier
and these are true color images and you
can see that the hexagon has very
clearly changed from blue to this nice
yellowish orange and the reason for that
is in 2013 the hexagon had only just
become exposed to light and 2017 now
we're at the northern summer solstice so
Sun lights been hitting this region for
a long time and this region is full of
photochemical aerosols and so when the
ultraviolet light from the Sun hits them
they form a smoggy haze that turns the
hexagon from blue to yellow ash orange
as for why that hurricane in the center
stays blue there's a couple thoughts one
is that that hurricanes actually had a
lower altitude than the surrounding
clouds
and so they're shading it from the Sun
so given enough time they could
eventually form a haze however if it
acts at all like Earth hurricanes it
actually creates a downwelling that's
right there and so it's sucking any haze
particles that are formed back down into
the deep atmosphere and so that keeps it
that nice pristine blue color okay so a
day after this we did our first dive
through the gap and while we were doing
it we were taking images with our camera
from the North Pole all the way down to
the equator and you can see the images
in the lower right there at the end of
this animation the spacecraft will
actually turn because we had to do I
should go to our shielded attitude and
so you can see it turning in order to
protect itself during that first
crossing we still didn't know that there
was absolutely no dust there so but you
can take all those images and you can
stitch them together and you get a
Saturn noodle so this is a thin little
strip all the way from Saturn's North
Pole down to its equator on our second
dive we were in a high rate spin to get
high-resolution magnetometer our
magnetic field data and here you can see
the spacecraft diving through the graph
and that colored line is Saturn's
magnetic field line that passes through
the spacecraft and so you can see where
that field line both hits Saturn and
intersects the Ring plane and that can
give a lot of interesting results as far
as there are particles that travel along
that field line and this kind of shows
you what the source of those particles
may be may be as far as the latitudes on
Saturn or the exact distance into the
ring plane that they that they are on
our third dive I'm not going to go
through these one by one but the first
three were pretty cool
on the third dive we were on earth point
and communicating with earth as as we
drove through and earth is above the
ring plane relative to Saturn and so you
can see here we're pointing up above the
ring plane but as we dive through it
what that means is the signals actually
going to pass through those rings and
from that we can look at what that does
to the signal and get some information
about the Rings it looks a little like
this which is a messy plot but it's it's
basically the the transmission strength
of the signal as it goes through and it
lets us see how
think the rings are how sharp those
edges are between the ringlets okay so
jumping ahead now to the twelfth orbit
here we got a really good look a really
high-resolution picture of Saturn's B
ring which the Rings are made up of
mostly water ice and if there were pure
water ice they'd probably look like this
but in this particular opportunity we're
actually able to get this image in color
and here the color comes from impurities
in the water ice and the source of those
impurities is actually still debated by
the scientists it could have been rocks
and minerals that were part of the
accretion disk of Saturn as it was
forming or it could have also been
meteorites that are coming in and
impacting the ring system and getting
obliterated and orbiting with it okay on
the fourteenth orbit a day or two after
we passed through the gap for the
fourteenth time we look back at Saturn
and specifically at the South Pole and
we got to watch the Aurora this is a
false color image but it's reproduced
with what should be a somewhat natural
color for the Aurora and so here you
have Saturn is the big black body up top
you can see the starry sky sweeping by
in the background and the Aurora
orbiting the South Pole if you look
closely you can also see that the stars
sort of take a sharp right turn just
before they set behind Saturn that's
because their light actually is
refracted by Saturn's atmosphere as it
travels back to Cassini okay so next is
one of my favorite observations because
I actually had a hand in helping Planet
which is a mission planner I don't
normally do that's science planning's
job and well what we did is we're
actually able to have the spacecraft as
it was diving through take pictures of
the Rings from the inside out and create
this movie where we got to see the lit
side the unlit side and even neater we
got to see the entire ring system in one
frame because of the foreshortening see
as you watch it go by again so their
entire ring system in in one camera
frame pretty
okay Norbit later we've got to watch one
of the percent protected bodies of the
Saturn system so this is Enceladus again
it has that global subsurface ocean in
contact with an active rocky core and as
it orbits Saturn squeezes it and it
creates these geysers that shoot out of
its South Pole so these are water
geysers shooting out of the South Pole
this this is about 14 hours of
observation and it was taken from a half
a million miles away so and this was our
last dedicated observation of Enceladus
okay grand finale orbit 21 so just one
and a half to go we got some really
great pictures of the down side of
Saturn's atmosphere so here you can see
some of the structure in the clouds and
once again like with the B ring
we got the images in color and it's a
little hard to make out but you actually
have these multi-hued bands of green and
red in the clouds and again this is a
true color image that is actually what
you would see if you were there alright
getting close to that the end now on the
final plunge so this is our last half
orbit last trip from Titans orbit down
into Saturn we took a series of seven
image sets of different objects in the
Rings and so these are the final the
final seven sets of images the Cassini
took and this was about a day before we
plunged into Saturn also at the end of
that we had Cassini image what would
eventually become its impact site which
was a little bit morbid but very
scientifically interesting so this gave
us some context for what we would later
be seeing with our other instruments as
we were plunging into Saturn's
atmosphere okay so a little before 5:00
a.m. on September 15th 2017
Cassini entered Saturn's atmosphere and
within a minute or thruster saturated
and we couldn't point the spacecraft
anymore it only took 20 more seconds for
s-band signal which you can see there to
completely dissipate
and within the next minute Saturn's
atmosphere destroyed the spacecraft and
Cassini became a part of Saturn so end
of mission was called at 456 a.m. JPL
time September 15 2017 when we lost the
s-band carrier signal okay so with that
I just want to say thank you for coming
I would also like to thank all of my
Cassini family all of my predecessors
and also everybody here who helped put
this on it takes a lot of work and I get
the easy part I just come up here and
talk to you guys so that they all did a
lot of work so give them a round of
applause
okay so with that I have time for
questions however if you want to ask a
question I ask that you go up form a
line at that microphone there that way
everybody in here can hear you and also
everybody on line can hear you
and also I will preface this with I will
answer these to the best of my knowledge
if you remember those dates at the
beginning 2013 to 2017 does my reign of
terror and it went all the way back to
1991 with mission planners and even
farther than that you know I like I said
I was a baby when it was born so I will
do what I can I may rely on some
friendly faces in the audience to help
me out well the first one is actually
kind of a remark not a question the
Bertha Cassini goes a bit back before
1982 in the mid 70s Donna parado noticed
that essentially every Mariner was
different and she said we had to
construct some building blocks and she
proposed a project called Mariner block
2 and then they stop calling it Mariner
because all the Mariners were
solar-powered and then later on they
were nuclear-powered but Mariner block 2
is where Cassini got started and of
course Ksenia was supposed to have a
twin called craft for the comet
rendezvous asteroid flyby and that got
axed long before long before Cassini
actually got going along so Cassini is a
bit older the 1982 it goes back to the
mid 70s okay we'll have to update a
couple books and I'm going to keep
telling that story because my picture
before 1982 is nice the question I have
in the in the mid 90s before the Neptune
flyby for Voyager I asked Fred
Billingsley with such a long baseline or
the Voyager cameras good enough to do
any decent astrometry to measure the
distance to nearby stars using parallax
and he said the cameras on Voyager
aren't good enough for astrometry even
with that enormous baseline where the
Cassini camera is good enough to get
better parallax measurements with an
enormous baseline that we can get with
just using the earth baseline for
astrometry
I don't know that we ever did that with
Cassini I'm gonna look at a friendly
face here
yeah I think we were too busy with
Saturn science but yeah so I'm not sure
if the cameras were good enough or not
to do parallax no star images so we can
get back to you thank you yeah
I have an attitude control question it
looked like the tumble boundaries that
you showed for RCS and thrusters were
not flat why is that so the the tumble
boundary is not just a function of the
density of the atmosphere it's also a
function of how fast you're going and
the attitude of the spacecraft a good
way to think about this is if you're
driving in a car you put your hand out
the window and put it face on to the
wind and start driving faster it's gonna
push you harder also if you change the
pointing of your hand so that it's like
this it's gonna push you less so in that
one where it bumped up we weren't
necessarily going any faster they were
all about the same speed but we were at
an attitude that was more like having an
open palm out the window than a closed
palm out the window and so the
atmosphere turned us harder which meant
the thrusters had to fight it more thank
you yep
so what were the most so what were some
of the most difficult decisions that you
had to make as a planner um I think the
the first one was just coming up with
the fact that we could have a
contingency plan and you know not just
Captain America and just say hey shields
shields everywhere but to actually have
to prove that you know we could take
data analyze it and make a decision and
uplink a contingency to the spacecraft
all within six and a half days because
these orbits were much much shorter than
anything we'd ever done before the the
ring grazing orbits were just over a
week these were just under a week the
closest we got to that before was nine
days and we weren't inside the the ring
system so proving that we could turn
something around fast enough I think was
the biggest challenge and as far as
mission planning goes thank you yeah
thanks so much for the awesome
presentation how do you know how many
orange peels you have left in the tank
on a spacecraft
that's a good question I actually didn't
say we have some uncertainty in that
estimate right the uncertainty in that
one orange wedge was about plus or minus
one and a half orange wedges so we
really needed to we couldn't go any
longer and so it really all comes to two
things first it's very accurate
measurements on the ground when we load
up the spacecraft and take its weight
and so we have some idea with a
relatively small uncertainty of how much
fuel we put in it then that's small
uncertainty actually just grows and
grows and grows because from there each
time we perform a maneuver we have a
model on the ground that says about how
much fuel we thought we burned but that
model has some uncertainty so each time
you use that model you know what may
have been you know a fraction of an
orange wedge of uncertainty is just
growing and growing and growing such
that by the end not only is our
uncertainty bigger but the amount of
fuel we have left is smaller and so
things start to look a lot scarier thank
you yep I guess my question is there
must been a lot of discussion about what
to do you know what and there must have
been some observations that would have
been nice that you couldn't do could you
tell us about some of those yeah I can I
can tell you a little so that's all
science planning which is right after
mission planning happens I'm the bigger
picture and then science planners go
down and divvy up the individual orbits
but what I can say is that science was
split up into different disciplines and
each of those disciplines and
instruments were given the opportunity
to say what they thought they needed in
the grand finale in order to get this
really great science that we were
promising everyone and luckily when we
added all those orbits up it came out to
35 out of 22 and so that's a little bit
over but at JPL we know how to make 35
feet into 22 we we were able to get this
the instrument scientists together to
talk things out and figure out where we
could share orbits where we could have
more than one instrument on at the same
time and both get the same science and
then we also had to push back a little
bit and say do you really need that many
orbits could you do with one less
and so it was just a series of
negotiations with the project and the
science teams that went back and forth
like that to get it all fit into the
twenty two and a half orbits thank you
yeah hi my question would be for for
mission for future missions what sort of
guidance do you have for people that
will be in your shoes five years ten
years and fifteen years from now what
sort of maybe you can think of like one
or two or three things that what sort of
guidance do you have for them learning
what you learned on this mission where
it seemed like there were a lot of
options that you may not have thought
you that you had that you did have yeah
so the first one is a plan for what you
think you're never going to do because
you're gonna do it we never thought we'd
fly inside the ring system and so being
able to do that was actually a pretty
big deal it took a lot of work we had to
change a lot of tools in order to
actually get them to function inside the
rings because we never thought we'd be
there but what that really gets to is
I'd say a couple other lessons it's you
got to keep the spacecraft simple and
robust and that makes it so that you
have something that lasts a long time so
that you can just keep doing this great
science and also you need to keep your
system flexible so that you can react to
new discoveries and going in places and
so I think those are the two key things
about the Cassini spacecraft and the
project that made it so that we could do
this thank you so much yeah how come
some of the pictures that Cassini took
were black and white and others were
color yeah so Cassini's cameras actually
are just black and white and the way it
works is we put different color filters
in front of them so that just that color
of light comes through and depending on
the science we're doing and how much
time we have that determines how many
filters were able to put in front of the
camera when we make a given observation
in order to get one in color as we see
it we need three filters the red the
green and the blue
and so for those particular ones we were
able to get all three filters in front
of the camera and reproduce true-to-life
images hello thanks for your lecture
tonight
you showed a beautiful picture of the
moon Daphne going through the ring plane
yeah how big is that moon in comparison
to ours or something else that we can
reference oh okay my friendly face says
that it's about 150 miles across so it's
tiny it's tiny yes yeah okay I'm getting
some blue card questions from online but
yeah so as Cassini gets extended over
all these years
does it have an arc like did you know
you were gonna dive into the Rings this
you know all these years later as it
gets passed the mantle gets passed they
making up new missions as they're seeing
what kind of science they're gathering
to talk a little bit about that yeah so
I think it was 2008 of 2009 when we
first thought of doing this grand finale
so you know eight or nine years before
it as far as how a Cassini kept getting
extended we would we'd submit a proposal
and go through a review process every
two years and that would say hey here's
what we're doing here's our status
here's all this great stuff we did and
if you give us this much more money
here's this much more great stuff that
we think we're going to be able to do
and so it was sort of on this rolling
two-year basis that we were able to try
and plan out what the next two years
would be thank you just in awe of what
humans can do
okay so now we have questions from
online so this one's from YouTube from
one heart how small could Cassini be
made with today's technology actually
interesting me I don't think it could be
made to much smaller yeah that that
graphic that I showed you where it was
filled up with propellant that wasn't
accurate but it was a lot closer to
accurate than you may think it was most
of that spacecraft body that you see
there is filled with fuel and so that's
really where it gets its size the other
thing that sizes it I'd say is the
antenna up on top and Saturn hasn't
gotten any closer to Earth as far as I
know
and so this antenna would need to be
about the same size in order to
communicate with earth so the part that
would get smaller if you look at the
spacecraft the very this ring going
around the top below the antenna but
above the probe that's where all our
electronics are and those could probably
get a little bit smaller okay also from
YouTube from astronomy nation was the
contamination of Titan by the Horgan's
probe taken into consideration yes so we
knew we knew that the Huygens probe was
going to Titan and so we built it with
that in mind and so it went through a
much more stringent planetary protection
process than the orbiter did and so that
made it safe to go down to Titan but
Cassini it's very expensive to do that
and so Cassini was not built to those
standards it wasn't going to go down to
Titan and so that's why it it could not
be crashed on on Titan
okay YouTube from Eric Lam planta what
was the biggest surprise that came out
of Cassini's discoveries I think this is
an opinion but in my opinion the biggest
surprise was Enceladus I don't think
anybody expected to find this global
subsurface ocean that actually had like
activity where you have these Jets
coming out of the South Pole and then we
later find out hey it's in contact of
the rocky core oh and guess what we
think there's actually evidence that
there could be the chemicals in that
ocean necessary to support life and so
that that is definitely by far the
biggest surprise also because the
Enceladus at Saturn is very much like
Europa at Jupiter it's almost identical
in that it's a nice icy moon with a
subsurface ocean in contact with the
rocky core and discovering two of these
in one solar system means that these
potentially habitable environments
outside of the sun's habitable zone is
are not rare if you have it twice in one
solar system it means in the whole
universe that it could be occurring a
lot so it really increases the
probability that we could someday find
life beyond Earth okay and YouTube from
Claudia what kind of Oh what kind of
cameras were on Cassini so the cameras
on Cassini were basically telescopes so
like I said that one picture of
Enceladus was taken from half a million
miles away so the cameras are digital
cameras the the highest resolution one
is like one megapixel yeah one megapixel
camera it was launched in 97 you know
give us a break
yeah but then it has this huge lens put
on it so that the fields of you are
really really small like you could not
use them in this room you know if you
like using binoculars in your bedroom
you're not going to see a whole lot so
that's what it was it was a 1 megapixel
telescope alright any other questions
from the room from online oh yeah I'll
repeat it yes the data was transmitted
yeah okay so he asked what what the the
baud rate of the the data rate that
Cassini was able to transmit at and
again I'm gonna look at my my friendly
face yeah oh yeah yeah I have another
friendly face you want to come to the
microphone so the high stay rate we
could transmit out was 142 thousand bits
per second from a billion miles away
yeah it may not seem like fast but from
a billion miles away it's doing pretty
good okay if there's no more questions
you're still welcome to to stay here
you'll be warm and dry
how big was your Cassini team and I'm
talking your dates when you were planner
people when I was the planner I think we
had fifty to a hundred part-time people
we didn't have a whole lot of full-time
but oh sure yes sorry yeah the prod yes
the project people at JPL sorry the
project people at JPL I believe there
are about 100 part-timers that all fit
into about 50 full-time equivalents all
right you can call it again if you want
