How do space telescopes point themselves?

Daniel and Jorge talk about how our great space eyeballs know where they are looking and how they turn.

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Episode Transcript

Speaker 1 (00:08):
Hey, Jorgey, are you good at navigating? Depends on what
do you mean by navigating? Do you mean navigating the
complex issues of how to lead a good life? Then no,
I haven't figured that one out. But if you mean
like getting somewhere, I have a phone with GPS, so
I guess i'm pretty good. Well, what have you lost
your phone? Like? Or civilization crumbled? Do you know how
to oriente yourself in the woods? Well? I imagine I

(00:31):
could use a map and a compass, right, do you
mean like a basic old school compass or the compass
app on your phone? Okay, yeah, that's a good point.
I only have the compass on my phone. But I
guess you could probably find a low tech original you know,
og compass. Yeah, that would let you get low tech
original lost. I guess there's civilization crumbles were all lost.

(01:08):
I am morehammy cartoonist and the creator of PhD comics. Hi,
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I've never honestly been lost in the woods. Well, well, yeah,
I think that's self evident, because you're talking to us
right now. If you were lost in the woods, I'm
not sure we would have from you again. Maybe I'm
calling you from my secret woods hideout, or even I

(01:28):
don't know where it is, although if you have Wi
Fi there and are able to record, I'm not sure
you're that lost. Yeah, that's true. But I've often gone
on long backpacking trips and wonder if I really could
get myself out of the woods if I had to. Yeah,
it's pretty tricky because I guess it's hard to see
above the trees and know where you are right. I
can't see the forest for the tree. It's definitely a

(01:50):
particular skill of figuring out how the map represents the
world you're seeing around you, and how to figure out
where on the map you are. Well, I'm glad you're
not lost in the woods, Daniel, it's what of my
recurring nightmares. Welcome to our podcast Daniel and Jorge Explain
the Universe, a production of our Heart Radio in which
we try to avoid being lost in the woods of physics.
We try to navigate our way through all of the

(02:11):
confusing issues about this incredible universe, figure out how we
can actually understand it, what we can make sense of,
how big our map of the intellectual cosmos. We really
can illuminate it. So I think of this podcast as
your GPS for the entire universe, helping you know where
things are and how to get there. Because slowly, over

(02:31):
hundreds or thousands of years, we have started to build
a map of how the universe works. We have a
literal map of like what's physically out there in the universe,
but we also have a conceptual map one that tells
us how things work, how they explain the experience we see,
and what they predict about what is to come. Yeah,
because it is a pretty big universe and there's a

(02:52):
lot out there for us to explore and to check out,
and so having a map is a really good thing.
Do you know where we sit in the universe? And
it turns out that, we say it in a very
small corner of one tiny galaxy that's part of a
giant supercluster. And it's amazing we've been able to figure
that out just looking at the night sky from this
little piece of rock. Why do you call our galaxy tiny?
I think it's pretty impressive. Well, you know, it could

(03:13):
be bigger. You know, always use a bigger house, right,
I don't know. I have friends that moved into a
bigger house, and they found themselves just screaming at each
other from opposite ends of the house all the time.
I think they were happier in their tiny, little cramp department.
Sounds like they needed an intercom, which is like technology
from the eighties seventies. Yeah, exactly. And so if we
lived in Andromeda, we'd have an even bigger galaxy to

(03:36):
explore to find those aliens unless we had some sort
of like alien galactic intercom where we could just talk
to everybody. Yeah, you could have like a quantum warp
tunnel the intercom. But the universe is quite vast, even
beyond our tiny or large galaxy, depending on how you
see it. And it's incredible that we have been able
to figure out what's out there. Remember when you look

(03:57):
at a map of the superclusters or our galaxy, that
those are constructed from painstaking work to figure out where
everything is. We don't have cameras above the Milky Way
or outside of the galaxy. We've basically only ever observed
things from Earth or from very very close to Earth,
and those technological eyeballs we have built have allowed us

(04:18):
to piece together this concept of where we are in
the cosmos. Yeah, it's amazing what we've been able to
piece together just from our little viewpoint using basically like
two pieces of glass. Right, the original telescopes were che
tube and two pieces of glass. I mean they're a
little fastier now, but essentially the same thing. Yeah. I

(04:38):
think you're glossing over a couple of crucial details, like
the shape of that glass, but yeah, those are the
basic ingredients. Yeah, and so we've been able to look
at the stars and other galaxies from our point or
an Earth, but we've also been able to look at
the sky from the sky. We now have more than
a few space telescopes out there in orbit and beyond
orbit looking at the rest of the universe. Yeah, we

(05:00):
have two really awesome sets of technology ground based telescopes
that can get really really big tens of meters across
for the primary mirrors, but those can be obscured by
all the air that's between us and space. That air
wiggles and shimmys and makes a little bit unclear to
see what's out there. So we have this other awesome
set of eyeballs we built that are actually out there

(05:22):
in space above the atmosphere and can see much more clearly,
although they can't yet be quite as large, So it's
a complementary set of eyeballs. Now, these are not literal eyeballs, Like,
we didn't send eyeballs into space, did we. Well, it
depends on your definition of eyeballs. Right, they're not human
biological eyeballs, but they're more like cameras. Right. They take

(05:43):
pictures which are then transmitted to your eyes. Are they
in the shape of a ball at least there are
definitely some balls on them, right, we'll talk about it
in the podcast. But they have spinning wheels and spinning balls,
which are crucial elements of their operation. Oh all right, well,
so technically there are they are eye and balls. But
it is amazing that we have space telescodes. It's pretty cool.

(06:05):
It's like, literally we build spaceships that are nothing but
or spacecraft that are nothing but a telescope. Right, that's
their only function, and they're out there in space doing
their job. They're sort of like robotic space telescope spacecraft. Yeah,
they're sort of like distant robot eyeballs that we connect
to our own minds. It is really incredible. And you know,

(06:26):
the telescopes here on Earth. That makes sense how they work.
You want to look at something, you can turn the telescope,
you point it at that thing that you want to watch.
But the telescopes that are out there in space, it's
a little harder to understand, like how those work, how
they keep track of where they are, how you can
turn and telescope in space? And a bunch of listeners
wrote in and ask me how does that work? So

(06:47):
do they end the program? We'll be tackling the question
how do space telescopes point themselves? Now? I guess, Daniel,
The question I guess is like, if you telescope here
on Earth, you're grounded to the Earth, so you sort
of know where you are and which way you're pointing.
But maybe the question that the listeners were wondering is like,

(07:08):
if you have a telescope out there in space, like,
how do you know where you are? And how do
you know which way you're pointing? Yeah, I think there's
two different parts to it, right, is how do you
know which direction you are pointing? And then also how
do you change your direction? Right? How do you actually
turn something that's up in space? Because here on the
ground you can push against the ground it's like connected
to something that you can push against. But up in space, right,

(07:31):
it's harder to move things around, especially if you wanted
to last four decades. M I see, because I guess
anything that you do, like if you have jets or anything,
then that means that you're expending energy. Yeah, and more
specifically mass, right, jets have to push out something. You
have to throw something out the back of the jet
in order to get the momentum. You mean, we count

(07:53):
to throw something at them from here, like to you know,
knock them into the linement. That was definitely one of
the plans, So I think it was pretty far down
on the list. Maybe zap them from Earth with lasers
also was pretty far down on the list. Oh, but
that would be pretty good, wouldn't it. That's what our
strategies for turning asteroids that are coming towards Earth. So
maybe we would also work for spacecraft. Yeah, you know,

(08:16):
it would indowd used lasers. Actually, I think that would
work if you had like sales on the telescope and
you could just push it from Earth with lasers. That
would be really cool. I can't imagine what could go
wrong or why there might be an issue with building
an enormous space laser. They should high NASA obviously because
we have good ideas. I'll be expecting an email as

(08:39):
soon as we're done with this podcast. Well, as usually,
we were wondering how many people had there had thought
about the space telescopes out there in space and how
they turned themselves to point at different stars. So thanks
to everybody who answered these questions for the podcast. If
you would like to participate for our future episodes, please
please please do right to me two questions at Daniel
and Jorhey dot com. We'd love to hear a huge

(09:00):
variety of voices from all over the world. So think
about it for a second. If you earned space pointing
a telescope, how would you turn yourself. Here's what people
had to say. I haven't thought about it. Maybe by
using some geroscopes, either this cameramount that you pointed at
the North Star and then it's basically calibrated to turn

(09:29):
to compensate for the rotation of the Earth, which is
like very consistent. So I'm assuming that space telescopes would
do the same. I would guess that the space telescopes
point themselves the same way that Alon Musk's SpaceX rockets
do with the air pressure thing, I don't know, maybe

(09:50):
either that or like a ion engine, I don't know.
I learned that the James Webb telescope has a set
of wheels that spin and apply some torque, so the
whole thing making the twist a little okay, I think
I actually remember this one from a previous episode in
which we said that it was actually very hard to

(10:10):
orient yourself in space, with the exception of being able
to use pulsars, which you described as sort of like
celestial guiding points that flash very consistently and can therefore
somehow be used to triangulate your location, assuming that you

(10:31):
already have the known location of two or more pulsars.
I believe they use gyroscopes in order to orient themselves,
or perhaps they off gas, you know, shooting little jets
in particular directions in order to orient themselves in order
to point themselves in a particular direction, and they use

(10:52):
the background stars to orient themselves correctly. All right, pretty
technical answer here, but pretty imaginative. Yeah, our listeners have
thought about flying through space, how to get around, how
to turn, how to know where you're pointing. We got
some pretty smart folks listening to the podcast. Yeah, let's
flatter our audience. You guys are awesome, beautiful and brilliant.

(11:16):
But I feel like the answer is here, and we're
also a little confused about what we're asking in the question,
like are we asking like how does a space telescope
orient itself? Like how do we how does it know
which way it's pointing? And also how does it turn
to point at something it wants to look at. Yeah,
I think we're asking both questions and have different answers,

(11:37):
both of which are really fascinating. So I think all
of that is involved. I mean, you have your eyeball
out in space, you wanted to look at something in particular,
you got to solve both problems. You gotta know where
it is now and how to change its position. M
do you think there's a there's like a joystick somewhere
in NASA or Houston Control Center that points these telescopes?
Like who gets to move that joysting? And I wonder

(11:59):
if there's a red button on the top of that
joystick and if it actually fires something or if it
just has a little like sound effect peo pew, or
maybe if you press the button like a flag pops
out at the end of Hubble boom, or I wonder
if anyone that NASA has never been tempted to turn
the telescope around and point it at Earth, like, what
could it look at? What could it see? You could

(12:21):
take a selfie with Hubble? Right? Yeah, oh man, do
you probably find all of NASA selling those selfie opportunities.
Hubble is quite delicate, and if too much light enters
its aperture it could damage it. They have to be
very careful about not pointing it, for example, towards the Sun.
And I wonder if even the Earth might be too
bright a source for Hubble. Well, I guess it would

(12:41):
have to be night selfies. Then all right, well, let's
dig into this question of how based telescopes orient themselves,
how they know which way they're pointing at, and then
if they want to point somewhere in particular, how do
they move themselves to point in that direction. So, first
of all, Daniel stepped us through this. Why this is
important and heart wells important because we want to choose

(13:02):
what we are seeing. Remember that the telescopes don't see
all of space, right, It's not like when you look
out of the night sky and you stare up and
you basically see the whole sky, or at least the
part that's not blocked by the earth. A telescope is
very very narrow aperture in comparison, and so you're only
really looking at a small portion of the sky, and
you want to get to pick which portion of the

(13:23):
sky you are looking at. Are we studying this galaxy,
are we studying that star over there? Are we tracking
something that's moving? So you definitely want to have control
over where your telescope is pointed. Yeah, it's sort of
like you say, it has a very narrow field of view.
I imagine it's sort of like walking around your neighborhood
looking through a straw or something like that. Right, that's
what it means to have a narrow field of view.

(13:45):
Like if you close one eye and the other eye
could only look through a drinking straw, your field of
view moop be super narrow, and it'd be pretty hard
to know where you are. And anybody who looked through
a telescope has that experience. Your telescope sort of towards
the thing you're looking for, and then you look through
the telescope and you don't see it and show wiggle
the telescope around and try to find the object. It's

(14:07):
not easy when you're looking through a telescope to find
that particular object has to be pointed very very close
for you to even see it. And a straw is
a great example, but it's actually not even dramatic enough.
Some of these telescopes, their field of view is so small.
It's more like looking at a grain of saying you
hold at arm's length, right, that's the fraction of the
sky these telescopes can look at at one time. It's

(14:29):
like looking at a straw. That's the width of a
grain of salt, and a meter lungs what you're saying, Yeah,
that's exactly right. So some recent images, for example from
James Webb, where they focus on the deep, deep sky,
they point at one particular place in the sky and
they take a bunch of pictures of that one spot.
And the reason you want to hold it there for
a long time is that the things that they're looking

(14:50):
at are quite dim. You know, these distant galaxies don't
send a whole lot of photons per second, so you
want to build up a crisp image of them. You've
got to wait as many seconds as possible to get
as many photons as possible. So you have to keep
pointing in the same direction for as long as possible. Yeah,
and I imagine that's extra hard because first of all,
like that thing that you're looking at might be moving,

(15:12):
but also like the space telescope is moving, right, and
like these space telescopes are usually an orbit around something,
either the Earth or I guess mostly the Earth, but
either a near orbit or far orbit. Yeah, we're always
moving relative to the Sun. And even if these distant
objects aren't effectively moving relative to our galaxy, You're right,
our position is moving, and so you have to do

(15:33):
something to stay on target. You can't just turn it
and point and take pictures. The things you're looking at
will change as you orbit the Sun, and so you
have to do work. I have to do something to
keep pointing in the same direction. M okay. So then,
and that's hard to do to move your space telescope
because basically there's nothing to push against in space. Exactly,

(15:54):
if you're swimming in a swimming pool and you want
to turn, what do you do. You hold your arms
out and you push against the water. Right, You're pushing
against something, and so you turn. But in space, what
is there, right, There's no air, there's no water, there's
nothing to push against, and so turning yourself is much
harder because there's nothing immediately there for you to push against,
for you to like boost off of right. And but

(16:16):
usually satellites and spacecraft the way they navigate and turn
and move around, they have rockets, right, or at least
some sort of like a listener suggested, like an ion engine.
And the crucial thing here is conservation of momentum. If
you're stationary and you want to get moving, then to
conserve momentum, you have to throw something going the other direction.

(16:38):
That requires mass right the same way that like if
you fire a bullet, you feel a recoil. If you're
out in space you turn on a rocket, then basically
the motion of your ship is the recoil from firing
the rocket, because it's basically shooting a bunch of tiny
bullets out the back of the rocket. The rocket is
not just flames, it's throwing mass out the back of it.

(16:58):
So you don't just need fuel to run the rocket.
You need some sort of propellant something to throw out
of the rocket to move your ship. And that's true
both for motion and for rotation. And so if you
need a mass to do it, then eventually you're going
to run out because you can only bring a limited
amount of mass. So the goal is to figure out
a way to turn your telescope without using some kind

(17:20):
of propellant, right, because I guess if you're using a propellant,
even if there are like ion atoms or molecules, you're
going to run out eventually, right, You are going to
run out eventually. And if you spend billions of dollars
and decades to develop this thing, then you want it
to last as long as possible. So you're going to
try to avoid at all costs having things that run out.
Can you make it electric like an electric car? Yeah,

(17:43):
you can make it electric, and an ion engine essentially
is electric, but it still has to throw something out
of the back, right, it's throwing ions which have been
accelerated by electric fields. And just bring like a really
big gas tank, like one that will last one hundred years, right,
Because these missions usually don't have like an unlimited lifespan, right,
They usually come with like an expiration date. You can't

(18:05):
do that. But then the gas tank is big, which
means it's heavy, which means you need more gas to
launch it. And usually you want to use all of
your available space and mass to design it for science
rather than having a huge fuel tank on the back
of it. I see. So I guess if you can
figure out a smarter way to turn out there in space,
then you could have more science than your rugget bigger
telescope exactly, more science and more years of science because

(18:28):
you wouldn't run out of something. Then you need to
turn the thing also be greener, I imagine, right for
the space ecosystem, you'd be less pollution. That's true exactly,
and so for all of our neighbors out there, we
should be consider it. All right, Well, that's why it's
important and hard to turn a space telescope and orient
it out there in space. And so let's get into

(18:49):
how you would actually do this and how you would
find yourself if you were lost in space. So let's
dig into that. But first let's take a quick right,

(19:10):
all right, we're talking about space telescopes, which are telescopes
in space basically finally a well named physics object. Well,
I know, right, and we're talking about how they point
themselves out there in space. So let's tackle maybe the
first question is, if you're on telescope out there in space,
how do you know where you are? And how do

(19:31):
you know where you're pointing? So these telescopes typically have
multiple ways to figure out where they are pointing. First
of all, they just have a bunch of sensors. Like
the Hubble, for example, has several different kinds of sensors.
It has a sensor that tells it where the Sun is,
which helps it know where it's pointing, but also helps
it avoid pointing into the Sun accidentally. It also has
sensors for magnetic field, so that you can use the

(19:52):
Earth's magnetic field to help figure out where it is.
And then there are sensors that look at stars, and
there's like a known star map and helps it get
an orientation roughly for where it is, So to get
a rough idea for where it is and orient itself,
it has essentially maps the Sun, the magnetic field, and
the stars that give it a sense for where it is. Yeah,

(20:13):
that's usually how they do it in science fiction, Like
if you're in a spaceship and you land in a
place you're not quite sure where you are. Usually the
way you orient yourself is by looking at the stars
around you, and if you sort of know where they're
supposed to be, you can figure out where you are
relative to them. That's the idea, right Basically, they're looking
at the constellations. They're looking at the constellations, and Hubble
is not a traveling spacecraft, so it will never appear

(20:35):
in Andromeda and have to figure out where it is.
It's always going to be orbiting the Earth, and so
we know what the stars look like when you're orbiting
the Earth, and so you just need a few examples
of particular known stars and you can roughly figure out
where you are. So those are the sort of lower
precision instruments, sort of baseline that Hubble uses to figure
out where it's pointing. But it also has much more

(20:57):
precise way to measure how it's so not just like
look at the map and figure out where you are,
but also understand how far you have turned right. And
so in internal to Hubble and almost all of these spacecraft,
they have gyroscopes. Gyroscopes are these balls that's been really
really fast, and so they're insensitive to the motion of Hubble,
and they can measure sort of how far it's turned.

(21:19):
M Yeah, that's pretty cool. We use gyroscopes here on
Earth all the time also to measure how things turn.
But I guess you know as an engineer, the trickling
thing with gyroscopes is that they tell you how much,
if whether you've turned and how much, But over time
they're sort of not calibrated to something fixed like the
sun for example, exactly. And so if you're holding a

(21:40):
gyroscope and you turn, the gyroscope stays pointing in its
original direction, and so you can measure I turned thirty
six point two degrees. So it's a relative measurement, as
you say, it tells you how far you have turned,
doesn't tell you where you're actually pointing. That's why Hubble
has this combination of having the rough sensors to tell
it the absolute measurements like on pointing and is part
of the sky or that part of the sky or

(22:01):
this part relative to the sun, plus these gyroscopes to
measure very precisely how far it has turned. So it
needs a combination of these sensors to get an absolute
sense for where it is pointing in the sky. Because
I guess if you're using a sensor to track where
the sun is. You're basically talking about a camera, right,
and so maybe a camera is not that accurate. Yeah,

(22:23):
it's basically a low tech camera, and the precision of
that is limited by like the pixels of the camera
and also basically the width of the object you're looking at.
And so the gyroscopes give you the most precise measurement
of how far you have turned. And these things need
to be again, super duper precise. Like when Hubble is
focusing on something and trying to keep it in its

(22:43):
field of view, it's like holding a laser beam focused
on a dime two hundred miles away. That's how precise
we're trying to be. You mean, like how stead of
your hand needs to be basically right, Yeah, exactly, And
so you're focusing on a dime that's two hundred miles away,
plus you're moving relative to that dime, and so it's
not just about being steady, it's about slowly tracking, it's

(23:05):
about turning your telescope so you can keep on it.
So these gyroscopes are super duper important to the operation
of these based telescopes, and Hubble has been going for decades,
and because these things are so important, they actually went
up in two thousand and nine and replaced all six
of them. Hobble has six of these things, six gyroscopes.
I met six gyroscopes. Yeah, and each one spins at
like twenty thousand rpm. Why do they need to be replaced, Well,

(23:29):
eventually they degrade. You know, there's always some amount of
friction in those things, so they'll rub against each other,
they'll slow down, they'll heat up, and nothing is a
perpetual motion machine, right, and so eventually these things do
need to be replaced. No, when you say it needs
to be accurate to the point where you can spot
a dime two hundred miles away, is that when you're

(23:50):
tracking something, you know, when you're trying to stay focus
on a star? Or is that more for finding stars
and things like that? I imagine in the gyroscopes maybe
don't really help you to find a star. Yeah, the
gyroscopes don't tell you what's out there at all. They
just tell you how far you have turned. And the
scientists need to decide where they want to look. So

(24:11):
maybe they've seen something already in the sky near another object,
then they want to appear more closely, or they've seen
it maybe in the infrared using Spitzer, and now they
want to get optical images of it. So they have
to already know where in the sky to look. So
they have like galactic coordinate systems they used to orient
to say where something is in the sky relative to
the plane of the galaxy, for example, And so you

(24:33):
have to know basically where something is and then go
look at it. Is there like a galactic coordinate system. Oh. Absolutely.
When you look at the maps, for example, of the
cosmic microwave background, those are relative to the plane of
the galaxy. So the galaxy runs through the middle of
those like a line through the middle of that oval.
And then you go above and below the galactic plane.
It's arbitrary, right, You could pick an access anywhere in space,

(24:56):
and so we pick it relative to the Milky Way
center to the like basically the main axis of the
Milky Way. Yeah. And if you are out camping and
lost in the woods and you look at the sky,
you see the sort of Milky Way of stars across
the night sky, and that is the plane of the galaxy, right,
if you're looking above it or below, you're looking out
from the galaxy, because remember, our galaxy is kind of

(25:17):
like a disc, and if you're looking at that line
and you're looking through the galaxy, which is why it
looks so milky, because there's so many more stars and
gas and dust and all that kind of stuff. So
that's the galactic coordinate system. We used to talk about
where things are in space. Well, that gives you the direction,
but like, where's the origin of this coordinate system. It's
at the center of the Milky Way. If you look
at that oval, for example, and you put a dot

(25:38):
in the very very center of it, that's where the
black hole is. But then when we look at our
night sky, it's going to be a little different than that. Right,
that's right, we don't see that entire thing. But you
can map the sphere of things that we can see
onto that coordinate system. But you have to like a
little bit of an angle change because we're not at
the center of the galaxy, right exactly, we're not at
the center of the galaxy. And also our solar system

(25:59):
is filted a little bit, so you have to know
where the Sun is relative to the center of the
galaxy in order to map that on cool But then
you said it uses sort of a cameras to see
the constellations in a way or a map of the stars.
Does it actually do that, like does it actually like
track certain stars or constellations? And is that one of
those maps you can buy in Hollywood Boulevard to the Stars. Yeah.

(26:23):
So Hubble has a bunch of these different systems, right,
has the course sun sensors, has a magnetic sensing system.
Then it has star trackers, right, and the star trackers
determines Hubble's altitude by looking at the location and brightness
of stars that it sees, So it has a broader
field of view than Hubble sort of main camera. And
this lets it like identify unique patterns throughout the sky,

(26:45):
which a computer then maps to star maps internal to
Hubble and lets it figure out like if there's a
correction or if it's slightly pointed in the wrong direction.
And then the fine guidance system uses the gyroscopes and
everything else to sort of fine tune everything. Now that's interesting.
They had to go and replace those gyroscopes. Is that
something we can do pretty easily? Like how do we

(27:08):
do that? We need to send a rocket with people
or do we send robots it's not something we can
do very easily. We have to send astronauts up there
because it's a complicated job, and so it was done
in two thousand and nine, but that was the last time,
and it's not something that we can do for James
Webb for example. James Webb. Remember it's not in Earth orbit,
it's out at a Lagarrange point. It's much much further away,

(27:28):
and it's not a place where we can send humans.
So either we have to develop robotic repair people or
we just can't replace it. So James Webb actually has
a slightly different technology than Hubble does. M what does
the James Webb telescope do? So Hubble has these spinning balls.
They're like mechanical, right, But James Webb tried to look
for something that was less mechanical, that didn't require something

(27:50):
spinning a really high speed, because that seems like sort
of easy to mess up, like a little grain in
there can really mess it up. So James Webb actually
uses this weird technology. It's a quartzemisp fear that resonates
in a particular way, sort of like if you have
a wine glass and you rub your finger around it.
It resonates and it makes like a ringing sound. That's
that wine glass like flexing a little bit. You can't

(28:11):
see flexing, but it's actually shaking a little bit. And
if you like rotated the wine glass, then the sound
would rotate with it. So what happens in the gyroscope
inside James Webb is that the quartz hemisphere resonates in
this very particular way. It's surrounded by electrodes that are
like driving the resonance. They can also detect any slight
change in its orientation, Like if James Webb rotates around

(28:34):
this quartz hemisphere, they will hear the resonance impacting the
telescope at a different location. Well, it's pretty fascinating and
so I guess those don't wear out. The hope is
that they don't wear out as fast. Right, everything will
wear out eventually. This is still moving. Every time James
Webb moves, it moves relative to these gyroscopes, and so
there's a potential for friction there. But you don't have

(28:55):
a spinning mass, right, and so it's less kinetic energy,
it's less mechanical, and so the hope is that it
will last longer. And so that's how it orients itself.
And so if you wanted to point to like a
particular galaxy out there that you know about, um, do
you still have to kind of like tan around you think, like,
do you think there's someone and NASA with the joystick
going back and forth, back and forth, opened out, Oh

(29:17):
there it is. Or do you think they can just
go like point to here, boom, it's pointing there. I
don't know the details, but I'm pretty sure it's not
a joystick. I think they type in the coordinates and
Hubble like pans over. This thing happens very slowly, like
when Hubble turns, it turns about as fast as a
clock does. It's a hubble, for example, can turn ninety

(29:39):
degrees and about fifteen minutes. This is not something you
want to spin around very quickly. I see, So it
just takes a while with the joystick. I hold the
joystick for a while. Yes, it takes patience with a joystick.
Probably they do have a joystick that's not actually doing anything.
It's just connected like the large change on collider and

(29:59):
the the center. They have a big red button you
can press that sets off lots of alarm bells and
flashing lights, but doesn't actually shut anything down. Wow, that
sounds like something the fire department did not approve. All right, well,
that's how space telescopes orient themselves. How they know where
they're looking at in the night sky or I guess.
And if you're if you're in space, every every night
is the night sky. It's always night in space. Yeah,

(30:21):
unless you're looking at the Sun, I guess. But now
let's talk about how space telescopes move, how they actually
turn to look at a particular star or galaxy or nebula.
So let's get into that, but first let's take another
quick break. Or we're talking about how space telescopes point themselves.

(30:50):
That's that seems very like self accusatory. I mean, like,
what's the point of space telescopes? No, like they have
to point at them they're pointing themselves at themselves. I mean,
some you've got to do it, right. How introspective are
space telescopes? I guess they're not really pointing themselves. We
are pointing them, right, somebody is doing it. Yeah yeah, right,
the joystick. It's not like they're up there just deciding

(31:12):
on their own. Hey I'm going to look at Andromeda today.
Yeah yeah, I'm sure there's the NASA joystick person listening
to this brain now going, hey, I point the space
tells cooes. Do you think space telescodes point themselves? That's right?
What do you think the garbage takes itself out just
because you're not doing that well? We talked about how

(31:33):
space telescopes can know which way they're pointing out out
there in space, because I guess it's pretty disorient can
be disorient thing. If you're out there in space, you're
sort of it's hard to know which way is up
and down exactly. And so the second question now is
how do they actually turn? How do they like if
you're looking in one way looking at a star and
you want to look at the star over there, how

(31:53):
do you make that term? Because, as we talked about,
you don't want to rely on propellants or rockets or
ion engines because those are kind of costly. They maybe
you might run out at some point in the future. Yeah,
and those would be nice, right, you'd like to do that.
It's sort of an easy solution because it lets you
have a net force. Right, you have your space telescope,
you throw something off the side, you're applying a force

(32:15):
to that object. That object applies to force back to you.
You turn or you move and make some sort of sense.
But as we said, that requires some mass, and so
now we need a solution that doesn't have any net
force or no net torque on the object. Right, you
have to figure out how to turn the telescope without
applying an overall force to it. Oh, I see what
you're saying. Because if you are applying an overall net

(32:38):
force or torque, that means you're expending energy in the universe, right, yeah,
and not just energy momentum. Right. So if you're going
to turn this thing from the outside, you're like, put
your hand on it and turn it, then you're applying
a force to it, right, Or if you're on the
telescope and you're throwing a rock off the side of it,
you're using some mass. You're expending momentum. So what we

(33:00):
want is a way to turn the telescope without changing
its total momentum, because changing its total momentum, by Newton's laws,
requires something else to balance that momentum, which means something
else with mass, and there's nothing else out there. It's
just floating out in space. How do you turn the
telescope without applying some overall force to it. That's the
physics puzzle, Like how do you change your absolute orientation

(33:22):
without changing your overall angular momentum? Kind of Yeah, imagine,
for example, you're on ice skates and you're on a
super duper slippery surface. How do you turn? Or you
can't push against the ice because you're on ice skates
in a super slippery So how do you turn your direction?
How do you change which way you are pointing? That's
basically the puzzle, right, So if you could push against

(33:43):
the side, that'd be great, but there is no side.
If you could like throw a rock, then that'd be great,
but you can't do that. So the question is how
do you turn on this slippery surface? Right? Or I
was thinking it's more like, you know, if you were
stuck out there in space, like if you're an astronaut.
So imagine you're an astronaut and your space suit and
you're out there and base whether you're looking away from
your spaceshipe or away from the Earth, and you want

(34:03):
to turn around to look at your spaceship or Earth,
but you've run out of fuel and maybe in your jetpack,
how do you turn yourself around, Like, you can't just
like grab something and pull yourself to look the other way.
And you can't just like flail your arms because it
would be hard to sort of change your orientation. Yeah,
even just flailing your arms won't do it, right. You
can't by flailing your arms apply any overall force to yourself.

(34:26):
So this seems like an unsolvable problem, and the way
to solve it is to find a loophole is to say, well,
what if I don't want to turn the whole telescope.
What if I only want to turn part of the telescope.
So imagine like an invisible dividing line. You say, this
part of the telescope I want to turn because it's
got the cameras on it, and this other part of
the telescope has the electronics and all the other stuff.

(34:46):
They can't see anything, so I don't really care about
that one. So instead of turning the whole telescope, what
if you just want to turn part of the telescope
one way? You can do that by turning the other
part the other way. I imagine, for example, having two
ice skaters that are ski getting together. One of them
can start spinning if they push against the other one. Right,
So instead of turning the whole telescope, just turn the

(35:06):
part of the telescope you want to actually use to
look at the universe by pushing it against another part
of the telescope, or maybe instead of iceicators. You can
imagine our stranded astronaut out there in space. You know,
they can't look in a particular way by themselves, but
if they had a buddy or a friend, like, one
of them could push against the other one and at

(35:27):
least one of them can look back at Earth or
at their spaceship. Exactly. If you don't care what your
buddy gets to see, then you can turn in one
direction by pushing against him or her. And that's exactly
what they do. On the space telescopes. They have a
little part of it called at a buddy. It's got
a little space telescope. Buddy, it's got the important part
and the not important part, and the non important part

(35:49):
is just there to help the other part turn. It's
at the buddy, the sidekick, right, and so on a
space telescope, this is called a reaction wheel. Essentially, it's
a little piece which turns the opposite direction that the
spacecraft does, so spacecraft says, I want to go that way.
Then the reaction wheel turns the other way in order
to balance it. So you're not changing the overall angular

(36:12):
momentum of this thing at all. You're only changing the
angle momentum of the part that you care about and
the part you don't care about. The sidekick gets the
opposite angular momentum, so physics is happy and you get
to point the part that you want in the right direction.
M So I'm imagining like inside of the space telescope,
there's basically like a just a big disc maybe right,

(36:33):
or like a big donut or cylinder that's that you
can spin. Is that the idea? That's exactly the idea.
So if you want to turn like clockwise, you would
turn the donut or the disc counterclockwise. M Imagine you
two astronauts. One of them ones to turn clockwise, so
he pushes against the other one and one of them
turns one way, the other one turns the other way.
Now on the space telescope, you don't want like a

(36:54):
second telescope to push against, so you shrink the other
part down as much as you can make it massive
and make it spin really, really fast, so it can
store a lot of angular momentum. And so the space
telescope has one of these for each direction it might
need to turn interesting like up and down, the side
to side, in front, the back exactly. So you need

(37:15):
three of these to control your direction complete the in space.
Usually they have extras just in case one of them breaks.
But they're called reaction wheels or momentum wheels, and they
are fixed in place on the sort of on the
side of the telescope. They spin many many times, like
a thousand or four thousand times a minute. M Now,
I guess maybe I have two questions. One is okay,

(37:36):
so I'm out there and floating a space, and I
want to turn clockwise, So I spin my little wheel counterclockwise,
and that gets me to turn clockwise while the spinning
wheel is spinning inside of me. Now, let's say I
want to stop because I certainly I got some angular
momentum turning. How do I stop turning? Do I just
spin the wheel the other way? Just spin the wheel

(37:57):
the other way exactly, And so you can apply whatever
torque you want to yourself as long as you're applying
the opposite torque to the wheel and that works in
both directions, and so the wheel isn't like ever stationary.
What you're doing is you're speeding the wheel up or
slowing the wheel down, and I do that with a
little electric motor which is solar powered. So it is
sort of like your Tesla as you said earlier, yeah,

(38:18):
or like the Prios, right, or any any car with battery.
Like when you break, you're putting energy into the battery,
and then when you need to accelerate, you take energy
from the battery. So basically the same concept, right, Basically
the same concept exactly. So you want to change your orientation,
you have to change the speed of the wheel to
create a torque on the rest of the object. And
so this thing spins really really fast, so it can

(38:39):
store a lot of ing the momentum, but it's still
really small and low mass compared to the actual telescope,
which means you can't turn the telescope very quickly. But
that's good, right, you don't want this thing jerking around.
They're not super dupe or powerful, but you don't ever
need to ever change the telescope's direction really really quickly.
It sort of feels like you got something for free

(38:59):
or something for nothing, you know, do you know what
I mean? Like that was pointing one way and then
I did something, and now I'm pointing it another way.
But I didn't lose really any energy. Yeah, there's two
different aspects of this, energy and momentum. So momentum conservation
is satisfied. Because part of U spunum one way, the
other part supund the other way, so it adds up
to zero. Just like your two astronauts, they could also

(39:20):
split apart if they push against each other, right, they
could float away in space when you could get back
to the spaceship, and the other one could be lost
to infinity, and that would satisfy conservation momentum. There'd be
no net force on the pair of them, even though
there is a force relative between them, So momentum is satisfied.
But you're right, we are using energy, so this is
not for free. You need to speed up that reaction

(39:40):
wheel or slow down that reaction wheel. That requires some energy,
and so this thing is not for free. It does
use some energy, but it doesn't need any propellant. Right.
A rocket uses both energy and propellant, has to have
some math to throw at the side. This doesn't require
any propellant, though it does use some energy. Yeah, I
guess what I mean. Like in the two astronaut example,

(40:02):
if you and I are in space and I'm like, Daniel,
save yourself. I'm going to push you towards the spaceship
to save yourself, and I push you. You're moving towards
the spaceship, but I'm not. I'm moving away from the spaceship.
But then I'm what if? And then but then suddenly
it's like I changed my mind. I'm like, wait, wait, wait, no,
that was a terrible idea, and I pull on the
rope that was attached between us to bring us back together. Technically,

(40:26):
we would not, like, our center of mask would not
have moved. That's right, right. Our center of masks cannot
move without some external force. Right. So even if you
don't change your mind and I drift back to the spaceship,
you're drifting away from the spaceships. Our center of mass
is not changing, right, right. But on the spinning example
with the space telescope, I kind of it sort of
feels like you did get away with something, right. It's

(40:48):
like you spun the mass one way and then you
spun it the other way and now you're you're in
a different spot. Your total orientation change direction. Well, part
of the spaceship changes direction and another part changes direction
in the opposite way. So the total angle momentum hasn't changed. Right.
But then when you slow down to stop, you spin
it the other way, and presumably it's the same amount
of momentum that you need to take out or put

(41:10):
back in. And so you and the wheel are in
the same spot you started with, but both of you
are pointing in a different direction. Though you're both pointing
in a different direction, but the angular momentum hasn't changed.
You've expended some energy, but the angular momentum isn't different. Yeah, right,
it sort of feels like you're getting something for free. Well,
it's sort of like if the astronauts push against each
other and they're further away, it costs some energy to

(41:30):
change that configuration, but it didn't change the overall momentum. Yeah,
but in the astronaut example, they didn't move if they
come back together. But in the wheel case, do you
do sort of like move, You're not pointing in a
different direction, right, Well, in the astronaut a case, imagine
we're connected by ropes and you push against me, so
that I drifted back towards the ship, and you drift
away from the ship. And then you change your mind,
and so you tug on the rope to stop my motion,

(41:53):
which also stops you. Now we're further apart than where
we were, but we have no change in our center
of mass, no change in our overall momentum. We've lost
is you spend some energy pushing me away and then
pulling me back, So in the same way, when you're
orienting the telescope, you've changed its overall configuration, but there's
no change in its overall angle momentum. That you have

(42:13):
spent some energy to change the directions of both parts,
the telescope and the reaction wheel. Interesting, so well, well,
I feel also that the other part question I had
is it isn't spinning a little wheel basically the same
as flailing your arms, Like if I was stuck out
there in space, could I also just like gonna spin
my arm and that would reorient myself? If you could
turn your arm effectively into a reaction wheel, then yes,

(42:37):
I don't know if you really could get your arm
to spin independently along the same access though I had
to think about the biomechanics of it. Actually, you're an
expert in that, aren't you. I'm not sure if you
really can have it spinned independently, or if when you're
moving in or way, if you're moving in a circle,
if you're effectively pushing back on your body. But yes,
if you, for example, ripped your arm off and attached

(42:58):
it via mechanical axle to your body, then by spinning
it you could change your direction. That seems a little dramatic,
but I think the answer, since you say that I'm
the expert, I think the answer is yes, I think
you could do that. It's kind of the reason why
when you jump off a cliff into the water, for example,
or of a diving board, people flail their arms. They

(43:19):
sort of like moving like a windmill, and that because
they're trying not to fall on their face in the water. Yeah. Well,
I'll trust you whether that's possible. I prefer the cleaner
physics but more gory example where you actually pull the
arm off, but I trust you that it's possible even
without filling your arms. All right, we're in space. You
can rip your arm out, but they don't have to

(43:40):
look back at the spaceship, although I'm not sure what
you're going to do once you get the spaceship. Are
you going to open the door? And I'll do my
way and we'll see how that goes? All right, Well,
we'll see if the door was designed to be opened
one handed, just space was designed for arm removal. I'm
not saying it's more practical. I'm just saying the physics
of it is clearer. I see, I see, And that's

(44:02):
more important than your arm. I guess in this scenario,
if it's just hypothetical and I want to give the
accurate physics answer, then yes, I prefer the more gruesome
but clear physics scenario, right right. I think as an engineer,
I would try my way first if it works, rather
than getting to the physics dogma here. All right, but

(44:23):
you can be expending valuable oxygen as you do your experiment.
All right? So then is this how the James Webb
space does coupe orient itself? Do they have? Does it
have these spinning wheels? Do the Hubble also do this? Yeah?
So basically every spacecraft does this. James Webb has six
of these reaction wheels that are spinning that help it
turn Hubble has these things. Kepler has these things, and

(44:43):
Kepler is a fascinating story because these things failed on Kepler,
which made it very, very difficult for Kepler to do
its mission. What happened? So Kepler launched two thousand and nine,
had four of these reaction wheels you only nearly need three,
but it had a spare just for good measure. And remember,
Kepler is a telescope that's looking for planets to eclipse

(45:03):
their stars. So you got to watch a star for
a while, for a long time to see it's one
ten thousands drop in brightness as a planet goes across
the star, so you really got to be focused on it.
A few years into itsmission, in two thousand and twelve,
one of these things failed and they didn't understand why.
But that's okay. They were had four, so they had
one spare. They're okay with three, and then the next

(45:24):
year they lost weight. I have a question, like, you
need one for every direction, right up, down, left, and
right from the back. Which one is your spare? Like?
Can you spur point in all three directions? Yeah? Good question.
I don't know the answer. I guess the engineers have
probably figured that out. Okay, So then Kepler lost one
and they activated to spare and then what happened, And
then they lost another one in twenty thirteen, so now

(45:46):
they only had two, which limits how the spacecraft can
turn right. And this thing has to be able to
turn in three D to track an arbitrary star. So
people were pretty bummed. They spent a lot of time
and money on this spacecraft, and it also cost money
to operate. It's not like once you have it up
there in space it's free, it's saying, costs millions of
dollars to operate the deep space network and the people

(46:08):
and all electronics and everything. So it's a real question
of like you just shut the thing down or do
you try to figure out another way to operate this telescope.
I wonder I'm guessing the answers no, because otherwise it
would have figured that out. But I wonder if you
can just use two to orient yourself in any direction
in space? You know what I mean, because orientations in

(46:28):
space are these kinds of weird transformations where you can, like,
if you wanted to point to the right, you could,
but you don't have something that turns you to the right.
You could maybe point down, turn left, or you know,
turn the other way and then switch back and do
some weird complicated maneuver to get you to point right. Well,
these things are orthogonal from each other, and so having

(46:49):
only two basically only lets you map out a plane
in a three D space. But like it, used one
to turn one way, then that reorients the other one,
doesn't it, So you essentially kind of can point in
any direction. No, yeah, that's a really good point, and
I think that that's essentially what they tried to do.
But you still need help in that third direction because
you don't want to drift right. You don't want to

(47:10):
drift in that third direction. And once you've turned and
pointed at the star, now you've used your two reaction
wheels along those two planes, which means you're susceptible. You're
always susceptible to moving in that third dimension. And so
in order to correct, you would then need to turn
twice basically in order to correct, which you'd bring you
off of the star. So they actually came up with
an ingenious way to try to prevent that from happening. Oh,

(47:33):
I see what you're saying is that if you could
point anywhere you want with maybe two reaction wheels active,
but you wouldn't be able to maybe track a star smoothly. Yeah,
you might have to take like zig zags, right, and
which means you couldn't keep it in your field of view.
So then what did they do? So they came up
with this really cool scheme to use the sun. Right,

(47:54):
the Sun is actually pushing on these things. Remember our
conversation earlier about like zapping a solar sail attached to
telescope with lasers from Earth. They basically are doing that,
except they're using sunlight instead of lasers from Earth. So
as it moves around the Sun, the solar wind and
the photons push against the solar panels on Kepler, and
so now instead of compensating for that, they're using that

(48:15):
to help keep it stable. Interest in using the solar wind, Yeah,
they're actually using the photon pressure, right, not just the
solar wind, but the actual photon pressure. It's like a
solar sale. So the solar panels are in sort of
like a hexagon around Kepler. And if the pointy part
where the solar panels meet, if that thing is oriented
right along the direction of the photons, then it sort

(48:36):
of stays stable and it's turned a little bit, then
it's unstable. So they can use that orientation to help
either push on the spacecraft or to keep it stable,
but would that help it track a start? It really
limits what they can do. They can only look at
sort of a couple different places in the sky, but
for a couple of spots and its orbit around the Sun,
they can use the Sun to compensate for the lack

(48:59):
of the third reaction wheel and keep it stable and
keep a tract on a planet for a little while.
So it's not a complete recovery of its abilities by
any means, but it's a partial recovery of the science
mission cool. Well, that's a pretty clever technology, I guess,
although I feel like they should change the name from
reaction wheels to flailing arms. It's a really big bummer

(49:21):
that these things went bad. They've been trying to understand
what happened, and in twenty seventeen there's a paper that
came out that suggests that it's due to geomagnetic storms
from the Sun. Basically, the Sun has like some big
energetic event, it dumps out a bunch of plasma and
a coronal mass ejection and as this passes through the spacecraft,
it interferes with the operation of the reaction wheel. Wow. Yeah,

(49:45):
that's pretty cool and also a pretty convenient story to
make up for the fact that era the thing you
design did not last as much as you thought it would. Yeah,
and these reaction wheels are very specialized technology. This one
manufacturer that has been putting these things out it's called Ithaco,
and their reaction wheels have failed not just on Kepler

(50:06):
but also on other spacecraft. So James Webb actually went
to a different manufacturer to produce these things. So we're
hoping that James Webb's reaction wheels lasts a lot longer.
And so that is a pretty clever way to turn
yourself in space to have these reaction wheels. And so
basically the space teal skills use them. Do other spacecraft

(50:27):
use them like the voyage you're used at, or do
some of these like the Parker Solar Probe does it
use that too? Some other spacecraft do use these kind
of things, But remember they're very slow, so they're not
great for navigation. They're really just great for like very
gentle orientation. Another example is light Sale Light Sale is
one of these things that's testing out the ability to
sail on sunlight. There's a huge solar sale that it's

(50:49):
using to gather momentum and navigate around the solar system.
But they also want to be able to steer this thing,
and so they have a reaction wheel on it to
try to turn it sort of towards an away from
the sun to change how it's sailing. So then it
only needs one wheel. It only needs one wheel. Yeah,
though it's also sort of experimental craft, and so I
think they're trying to be simpler and cheaper. Everybody would

(51:10):
love to have more of these wheels, and a lot
of the spacecraft have a combination of reaction wheels and
chemical thrusters. Chemical thrusters are for when you've like saturated
your reaction wheel you can't turn anymore because it's already
spinning in it's max rpm, or when you need to
turn faster than you can with your reaction wheels, that
you want to use your chemical thrusters very sparingly because
you just use up the mass and then eventually you're

(51:33):
run out cool Well overall, a pretty clever solution to
move yourself, at least in orientation in space. Yeah, it's
a very clever idea. And when I think we'll be
using for a long time in the future, if we
can make these things more reliable and if they don't
require tearing your arm off, yes, let's try that solution. Second,

(51:54):
So first zapping with lasers, second tearing your arm off.
Kind that's right, I'm gonna be up up there is
space going yes, season, go ahead and shoot the lasers
at Daniel and let me know if that works. And
if it does, then you can shoot them in me.
But I'm going to be flailing my arms out here

(52:14):
and I'll see you back at the space ship. I
wonder if that big earth laser for zapping astronauts also
has a joystick? And who gets to run that one?
Oh man? Yeah? Yeah, And what kind of training the
prison needs to do? You know, play a lot of
asteroids maybe, or a lot of Halo. Perhaps you want
someone who can get a good head shot the first
try Fortnite experts. All right, Well, hopefully you did not

(52:36):
get lost in this discussion, and we navigated your brain
to understanding how space tells coopes move and orient themselves
to look at the universe out there and This is
crucial to our ability to understand what is out there
in the universe and to continue to build that physical
and conceptual map of how the universe works. Thanks for
joining us, See you next time. Thanks for listening, and

(53:06):
remember that Daniel and Jorge Explain the Universe is a
production of iHeartRadio. Or more podcast from my heart Radio
visit the iHeartRadio app, Apple Podcasts, or wherever you listen
to your favorite shows.