October 6, 2021 • 61 mins
Listener Terje wants to know about space navigation. We look at how orbital velocities, special and general relativity and even stuff like solar radiation can make navigating from point A to point B a big head scratcher once you go into space.
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Episode Transcript
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Speaker 1 (00:00):
Hey there, listeners, this is Jonathan. Before we get into
the episode, I want to acknowledge something. Uh. Twitter user
Charlie Tango Bravo helpfully pointed out that I made a mistake.
When we first published this episode, I completely misrepresented the
inverse square law, and so later in this episode you
(00:23):
will hear me kind of interrupt to give a proper explanation.
My apologies for that. Uh and um, yeah, I mean,
this one's totally on me. I made a mistake, and
I feel real dumb about it. But you know, mistakes happen.
It doesn't. It just tells me I need to be
(00:45):
even more careful in the future to make sure that
I'm not misrepresenting something, which I totally was in the
original version of this episode. Anyway, let's get to the episode,
and when we get to the part where I'll correct it,
I'm pretty sure we'll be apparent. Enjoy. Welcome to Tech Stuff,
(01:08):
a production from I Heart Radio. Hey there, and welcome
to tech Stuff. I'm your host, Jonathan Strickland. I'm an
executive producer with I Heart Radio and I love all
things tech and listener Terry A. Carlson, and I apologize
for very likely Butchering your name, asked if I might
(01:32):
cover how space vehicles navigate. So we're going to talk
about navigation, and this is one of those things I
find really fascinating and also sometimes frequently actually a bit confusing.
Now I blame part of that confusion on my own
fascination with stuff like Star Wars, which some folks call
science fiction. I think of it as fantasy that happens
(01:53):
to be set in space, or even Star Trek, which
is closer to science fiction than Star Wars, but can
play a bit fast and loose with science and technology.
And these kind of properties gave me a really cool
but an accurate feel for how space navigation works. Terry,
you asked that I cover what references and methods do
(02:15):
space programs used to actually do space navigation? And that's
a great question, because all of this really does rely
on reference or relationships between a vehicle and something else,
Like it's all relative. You know, we're going to talk
about that a lot in this episode. And I guess
(02:35):
on some level this is intuitive, but I had not
really thought about it in concrete terms before. So, for example,
here on Earth, if you're giving directions to someone, you
would tell that person how to get to a place
relative to where they are right now, Right, I mean,
the same thing holds true for space vehicles. But we've
(02:56):
got to remember that everything in space is moved ing
all the time. So here on Earth, if you need directions,
you can start from a position in which you know
you're not moving relative to the Earth. You're standing still.
On Earth, You're still moving, but that's because the arts moving.
We're going to get to that. And you know you're
(03:18):
standing still with reference to the Earth. But in space,
everything is moving in reference to everything else. Now you
could be moving at a similar velocity relative to your surroundings.
So from your perspective, it might seem like you're not
all really moving together, but trust me, you totes are.
Now what I'm about to go into it really matters
(03:39):
when we start thinking about the possibility of traveling beyond
our solar system to another. But it's it's, you know,
something that we have to take into consideration, even when
we're talking about travel within our Solar system. So let's
start with the easiest stuff first and then work our
way up. The Earth travels in a nearly circular orbit
around the Sun, right, I mean, this is not news
(04:02):
to you. I imagine. So the Earth is moving, it's
moving in an orbital path around the Sun. The orbit
moves at a speed that's around sixty thousand miles per
hour if you prefer, that's about or almost thirty kilometers
per second. Now, if we're talking about going to space
to do stuff in low Earth orbit, we're pretty much
(04:24):
thinking to Earth, right, We're not too worried about everything
else that's going on. The orbital speed of Earth isn't
as big a deal in that case, we don't have
to take that into as much account. But let's say
that we want to travel to Mars. Go from Earth
to Mars. Well, Mars is further out from the Sun
than Earth, right, I mean it goes my very educated mother.
(04:46):
So Mars comes after Earth. Oh, in case you've not
actually heard that mnemonic device, this is how I learned it.
It was my very educated mother just served us nine pickles.
That stands for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,
(05:08):
and Pluto the planets except well, you know, when I
was a kid, Pluto was a planet, and then Pluto
kind of got the boot as far as being a
planet goes. So I guess now you could say, my
very educated mother just served us nothing. Thanks mom. Okay. So,
Mars is further out from the Sun than Earth is,
(05:29):
and Mars's orbit is obviously a bigger circle around the
Sun than Earth's orbit is, because you know, Mars is
further out from the center of our solar system. Mars
also doesn't have the same orbital velocity as Earth. Earth
completes one orbit of the Sun every three sixty five
Earth days, with a little bit of change leftover. That's
(05:49):
why we have to have leap years, and Mars's orbit
is just a hair under six hundred eighty seven Earth
days long. But beyond that, Mars's orbital velocity is closer
to twenty kilometers per second. Remember Earth's is closer to
thirty kilometers per second. So Mars is not just moving
in a greater distance because it's orbit is larger, it's
(06:11):
moving a little slower compared to Earth, you know, and
again relative to the Sun. So that means if we
want to plot a course from Earth to Mars, we
have to take all that into account, right, because we
can't travel instantaneously. It takes time for us to get
from point A to point B. We can't just point
a rocket at where Mars appears to be to us
(06:35):
right at that moment and then launch the rocket in
that direction. You know, look at the sky, find the
red dot and say aim that away, because you know,
the positions of Earth and Mars are shifting in their orbits,
and Mars will not be at that same spot by
the time the vehicle we launch gets there. Heck, it's
not even at that spot as we look at it,
(06:56):
because it takes time for light to travel from Mars
back to us, So we're really looking at where Mars
used to be, and we would be shooting a rocket
at where Mars used to be a really long time ago. Uh.
The vehicle it would take months to travel there, so
by the time it would get to that position in space,
(07:16):
Mars wouldn't be there anymore. By the way, all this
orbital stuff is why when we talk about sending a
crew of human astronauts to Mars, we talk about missions
that typically are on the order of a couple of
years in length. The trip from Earth to Mars could
take around eight months, and that's if we time it
so that the launch vehicle UH shoots the spacecraft up
(07:40):
and has the spacecraft travel the least amount of distance
needed in order to go from Earth to Mars. That
means you're timing it right. You're aiming at where Mars
is going to be, and you want to time it
so that Earth and Mars are at one of the
closest points in their respective orbits to one another. You
out of time it just right. And because of the
(08:02):
difference in orbital velocities and the fact that mars Is
year is significantly longer than Earth's year, that means that
by the time you would arrive on Mars, Earth and
Mars would no longer be super close together anymore. Right
Like they're they're constantly in motion, so now they're moving
apart from each other. That means for you to get
back to Earth, you would need way more fuel for
(08:24):
your return trip than you did for your trip out
to Mars. Potentially, so the most fuel efficient thing to
do is to hunker down on Mars and do all
your science e stuff and you wait for the planets
to reach a point in their orbits where again you
will be traveling the least amount of distance you can
get away with. And all told, that means a mission
(08:46):
needs to last around two years to get all that done.
Plus you know, theoretically you could even spend some of
that time on Mars making rocket fuel, so you wouldn't
necessarily have to carry all of it with you on
the way there. Anyway, that's just a simple example of
how motion in space matters. But we're just getting started, right,
So I've touched on stuff like low Earth orbit and
(09:09):
interplanetary travel within a solar system. But it's not just
that the planets are moving around the Sun. Our entire
Solar system is hurtling through space. We are part of
the Milky Way Galaxy, and at the center of the
Milky Way Galaxy there is a super massive black hole
that's kind of the you know, it's like the Sun
(09:32):
is in our Solar system, except there's a supermassive black
hole in the middle of a galaxy billions of solar systems.
Scientists estimate that our galaxy has somewhere between a hundred
to four hundred billion stars in it. So think about that.
I can't. I tried, but that's just a number that's
just way too big for me to even get a
(09:53):
rudimentary grasp on it. Anyway, our solar system is traveling
in its orbit in the galaxy at a speed of
around four thousand miles per hour or two kilometers per second.
Then we have to consider that our galaxy is also
in motion. We've got other neighboring galaxies in our neighborhood
is just part of a super cluster of galaxies, and
(10:16):
that in itself is part of an even bigger super cluster.
And we're all hurtling through space at around a thousand
kilometers per second. Where are we going? Well, I mean,
I want to say, get in loser, we're going shopping,
but we're not. We're headed to the Great Attractor. And
to my surprise, that isn't Oscar Isaac. It's a gravitational
(10:38):
point in the Lania Kia super cluster. Uh. And um,
you know the Great Attractor is also moving towards another mass,
the Shapley super Cluster. I guess there's always a bigger fish.
And you know, we could keep going down this rabbit hole.
But what I really wanted to point out is that
we're talking about a lot of body ease in motion here,
(11:00):
and that makes navigation more tricky, right, I mean, this
is kind of like saying that you stop to ask
for directions, but the town you stop in is actually
moving on the map, and the place you're going to
is also moving on the map. Maybe it's moving away
from the town you are in, which is still in motion.
So the route you are going to take to get
(11:21):
there is changing by the minute. It gets complicated, so
you have to take, uh, you know, into account a
lot of things. You have to take reference points in
order to make it all makes sense. And of course
that's like talking about a map that's two dimensional. Obviously
in space you're talking about three dimensional. You're not limited
by a two dimensional plane. You're moving in three dimensional space. This,
(11:44):
by the way, is all before we even consider stuff
like relativity, which makes things even more weird. Einstein's theories
of relativity really show how our universe behaves in ways
that we don't often get to observe of directly. So
we don't have a lot of direct perspective on these things.
(12:05):
But let's use an example to explain some of relativity,
and we'll start with special relativity. That's the I would
argue the easier of the two to get a grasp on. Alright,
so we've got two people and we're gonna name them
Alice and Bob. Alice has superpowers, and Alice can travel
through space without a space suit and can move at
(12:28):
near the speed of light. So she flies through space
close to the speed of light, and she goes and
flies off on an adventure at top speed. When she
comes back and she meets up with her best friend Bob,
the two notice something unusual. So to Alice, it'll seem
like Bob aged faster than normal, as if more time
(12:49):
had passed for Bob than it did for Alice. To Bob,
it will seem like Alice has aged less than normal,
like not as much time passed for her, like like
like less time pass less than it should have. And
the reason for this is that the faster your speed
is relative to some other reference point. This is why
(13:11):
we talk about relativity. It is relative to some other
reference point, the slower time will pass for you relative
to that reference point. So again, like Bob is our
our reference point for Alice. So it appears like, you know,
Alice hasn't aged as much because time appeared to pass
(13:32):
slower for Alice than it did to Bob. Now to Alice,
time will have seemed to pass as normal for our
own frame of reference. So in other words, it wouldn't
feel to her as if time had slowed down. She
would feel like time was passing, just as it would
if she were standing perfectly still on Earth. A second
would feel like a second to her. And in fact,
(13:54):
if Alice were wearing a watch with a second hand,
it would seem to be clicking one second at a time,
just perfectly. Now, if Bob could somehow observe that watch
while Alice is traveling at near the speed of light,
Bob would see that the second hand is moving really slowly.
It would be taking way longer than a second for
(14:15):
it to take each tick. In fact, the faster you go,
the slower it gets. And if you got to the
point where you could travel at the speed of light,
it would stop like the second hand wouldn't move anymore.
For Bob, he wouldn't see the second If somehow Alice
could move faster than the speed of light, which is
as Einstein would put it impossible, it would look as
(14:37):
though the second hand was going backwards. She would be
traveling back in time. Now that's physically impossible. So I
just thought I would throw head in there as an
interesting you know, side note. But again to Alice, it
would seem like time was passing as normal. And likewise,
let's say Bob's wearing a watch. He has a second hand.
To Bob, time is passing just as normal. A second
takes a second, he can watch the little second hand
(14:59):
click on buy on this on his watch. Now, let's
say that Alice is able to see Bob's watch. While
Alice is traveling at near light speed, it would look
to her as if the second hand was going way
too fast, like it was just spinning around the watch face.
And again, Bob and Alice would each feel the passage
of time as if it were just normal. It's only
(15:21):
when they compare it to a point of reference, when
they are relative to something else, that they see that
there is any type of difference. Now, for most of
our experiences on Earth, we don't notice this effect, and
that's partly because we're usually traveling at a similar velocity
relative to one another. We're all on this planet. Most
(15:42):
of us aren't going super duper fast. However, there are
cases where we can measure a difference. It wouldn't be
observable like to our normal senses, but with very sensitive
you know, metrics, we could see that there was a
difference there. It's settled because we are not able to
go anywhere close to the speed of light, at least
(16:03):
not yet. So for example, we have the case of
Mark and Scott Kelly. These are twin brothers who are
both astronauts. Now, both of the Kelly's have spent time
in orbit. Mark Kelly was born first, he's the older
of the two twins, and he's logged fifty four days
in space. Pretty respectable, right, I mean extremely respectable, Mr Kelly.
(16:27):
I don't mean to, you know, dismiss that incredible achievement.
But Scott Kelly has spent five hundred twenty days in orbit,
almost ten times as much time in orbit, and a
lot of that was aboard the International Space Station, which
orbits the Earth at the speed of around twenty eight
thousand kilometers per hour or seventeen thousand, five hundred miles
(16:50):
per hour. So for a significant amount of time, Scott
was traveling much faster relative to his brother Mark, who
was back here on Earth. And since traveling faster means
that time passes more slowly relative to an outside observer,
it means that the gap between Mark and Scott actually
got bigger. Mark aged faster here on Earth than Scott
(17:14):
did out in space because of that speed of travel.
Mark once said that he used to be six minutes
older than his brother, but now, thanks to all that
space travel that Scott did at high speeds, Mark is
six minutes five milliseconds older. And that's a funny thing
to say. Uh And sure you could argue, well, that's
(17:35):
not really significant to our normal frame of reference. I mean,
what's five milliseconds, But it does show that we actually
have to keep this in mind when it comes to
space travel. It does matter. Now when we come back,
we're gonna tackle a little bit more of a relativity.
But first let's take a relatively quick break. Okay, we're back,
(18:05):
and we are not done with relativity yet. The speed
and time thing is all part of special relativity. But
Einstein was a real you know Einstein, and he also
published his theory on general relativity. This includes an explanation
that if you were to have two clocks and one
of them is closer to a gravitational mass than the
(18:27):
other one, the other one is much further out from
that gravitational mass. The one that's closer to the gravitational
mass will take more slowly than the one that's further out.
This is separate from the whole you know, speed thing.
So in other words, gravity also affects the rate at
which time passes. It passes more slowly when you're closer
(18:48):
to a big center of gravity. And this becomes really
important for navigation just here on Earth, not even just
space navigation, but navigation here. And you might wonder why
that is. Well, a lot of us depend upon GPS
apps or devices, right like we pull that up whenever
we're going someplace new, and these devices they work by
(19:11):
relying on signals that are coming from GPS satellites. It
gets a few of these different signals and then it's
able to pinpoint the location where on the surface of
the Earth we happen to be at that given time. Well,
those satellites are really far away from us. They are
beyond low Earth orbit, so they're beyond where the International
(19:32):
Space Station is, for example. They're out in medium Earth
orbit somewhere around twenty two KOs out from the Earth,
or around twelve fifty miles. So these satellites are much
further out from the Earth's gravitational mass than say your
watch or come on, we'll be real here your smartphone.
(19:54):
So because of that, the clocks on the GPS satellites
tick slightly fat stir. Then the clocks here on Earth tick. Remember,
the closer you are to a gravitational mass, the slower
time will pass for that particular frame of reference compared
to a different frame of reference. I always have to
(20:15):
throw that part in because again, in the moment it passes,
the way time passes, like the way our experience of
time is remains the same, not to our own frame
of reference. Now, the GPS satellites are also traveling really
fast relative to us, so that actually means that we
have to take a bit off the top right, because
(20:36):
we know that the faster you travel, the slower time
uh affects you relative to someone who's not traveling at
that speed. So the effects of general relativity mean that
a clock on a GPS satellite has on average around
forty five micro seconds more than an Earth clock at
the end of a day, a full day. But again,
(20:58):
because these satellites are link faster relative to us, the
clocks also have a negative seven micro seconds two factor,
and compared to our clocks, due to special relativity, so
we have to combine those two together forty five and
negative seven. That gives us thirty eight micro seconds that
are extra on the GPS clocks. So if if we
(21:22):
were to stop it right at midnight for both clocks,
you know here on Earth and at the GPS satellite,
we would see midnight on our clock and midnight plus
thirty eight million micro seconds, not milliseconds micro seconds on
the GPS side. Now, because our navigation depends upon taking
signals from satellites and essentially measuring how long it took
(21:43):
for that signal to go from the satellite to us,
in order for us to calculate where we are on Earth,
we actually have to account for that difference between our
clocks and the satellite satellites clocks, or else we start
to get some drift, and that means that over time,
and and we're actually talking about years here, but it
(22:04):
does happen, our navigation systems would become less accurate, which
would eventually get to a point where our GPS device
wouldn't even really show us where we are because it
would be miscalculating based upon the the differences in the
clocks on the GPS satellites versus the clock on our
phone or that our phone is connected to, and instead
(22:27):
it would show you where it thinks you are, but
there would be a growing gap between where it thinks
you are and where you really are. Based upon this
gap in time. It would take like seven years for
that to get to a point where we might even
notice it, and we do correct for it. So essentially
what happens is we shave thirty eight micro seconds off
(22:48):
the clocks every midnight that the that the clock's hit.
So when the clocks hit midnight on the GPS satellites,
they kind of hold for thirty eight microseconds, which puts
them I can sync with the clocks here on Earth.
And then we have to do it every single day
because every single day we get the effects of relativity.
(23:08):
All right, well, that's something we have to take into
account with navigation, Like these are things that we don't again,
we don't necessarily have to think about here on Earth. Typically,
most of us don't come into situations where special and
general relativity have a noticeable impact on our day to
(23:28):
day experience. UM. One really important element in space navigation
is something called the deep Space Network, which is not
a really cool science fiction channel, uh, no, it's it's
actually a bunch of antennas, and it's it's more than that.
But that's a big part of it. If we go
back to the nineteen fifties, we had the space race
(23:51):
ramping up. In nineteen fifty seven, the then Soviet Union
launched spot Nick into orbit. The US was already working
on its own satellite, and of course there were reasons
for this beyond the scientific push. Scientific push was a
big part of it, but there were other political reasons.
For one thing, Demonstrating that you could put a payload
into space also sent the message of hey, comrade, we
(24:15):
can build rockets big enough to reach you, even though
we're on the other side of the world, and you know,
nuclear weapons are a thing, so you know, this wasn't
just you know, a science thing. But that's another tangent
that I won't go down any further. But on the
U s side, one of the things that the Jet
Propulsion Laboratory or JPL undertook was a job from the
(24:39):
United States Army, which effectively you know, ran the jp
L to establish radio tracking stations in certain parts of
the world, including places like Singapore, Nigeria, and California. So upon,
launching the first successful US satellite, which was called Explorer one.
These ground stations would receive data from the satellite to
(25:01):
be able to track it as it passed over overhead.
This was essentially telemetry data, and you might wonder what
does that mean. Well, telemetry is essentially the process of
using some sort of device to measure something. It could
be temperature, it could be pressure, it could be speed
or velocity, and then it transmits that information to a
(25:23):
distant receiver. Now, in this case, the telemetry was mostly
about the Explorer one's orientation and velocity as it went
through its orbit. Engineers at mission control could take that
data and plot out the Explorer one's orbital path. In October,
the US government established NASA, and this was really to
(25:45):
consolidate the space efforts from various independent groups, mostly in
the military, like the Army, Navy, and Air Force all
had independent space exploration UH initiatives, so this was to
kind of bring them all under one civilian umbrella and
combine all those resources to be more effective. So this
(26:07):
included the Jet Propulsion Laboratory that had previously been run
by the U. S Army that became part of NASA.
In December of nineteen and NASA would assign to the
jp L the responsibility of planning out planetary and lunar
exploration missions that would use unscrewed spacecraft, that is, spacecraft
(26:27):
that did not have human beings abort robotic spacecraft if
you prefer. Now. That would necessitate a network system of
receivers here on Earth to be able to receive communications
with and then to send communications too, as well as
to just keep track of these robotic spacecraft as they
traveled away from the Earth. You can't just have one
(26:48):
really big antenna in Houston, because you know, the Earth
rotates and sometimes Houston would be pointing the wrong way.
Get together, Houston. So yeah, to establish that these antenna
around the Earth need to be pointing outward in such
a way that you have maintained contact with distant spacecraft. Moreover,
(27:10):
as the spacecraft move further from the Earth, the signal
strength will decrease. In fact, you know, here on Earth
we describe radio frequencies as obeying the inverse square law,
which means that the power of a signal is inversely
proportional to the distance from a source. Hey, it's Jonathan
(27:30):
from one day further out from when this episode originally published. Okay,
so the first time I tried to explain this, I
just playing god it all wrong. Uh, we published the episode,
they had an error in it, and this was because
of a fundamental misunderstanding on my part. But fortunately Charlie
Tango Bravo on Twitter set me straight and I'm gonna
try harder to get it right this time. So the
(27:54):
inverse square law will first imagine that you have a
source of electro magnetic radiation. And this can be anything
from you know, a radio antenna to a microwave source
to light. In fact, let's talk about light because that's
pretty easy for us to wrap our heads around it
because we can directly observe it. Right, We've all seen
(28:14):
that as you move away from a source of light,
then you get less light to work with, Right, that's intuitive.
So if you're walking around I don't know, a spooky
attic and it's lit by a single light bulb hanging
down from a chain, well you know that as you
get towards the corners, a ghost is gonna get you.
I'm kidding. Ghosts don't exist, but you do know that
(28:37):
as you move further away from the light, bulb, it
gets darker, Right, That just something we've experienced. Well, we
can actually describe this phenomena with the inverse square law
of propagation. We can think of radiation moving out from
a source as similar to that of an expanding sphere,
(28:58):
like it's going out in all directions and growing as
it moves outward. Right, the sphere gets bigger and bigger.
The center is still stationary in this frame of reference.
So the further we get out from the center of
the sphere, the more surface area that electromagnetic radiation is covering. Right,
the the outside of the sphere is bigger. That means
(29:21):
that the signal strength is growing weaker. You have the
same amount of signal to go around, but you're covering
a larger area, so you can think of it as
spreading across more space. And we can describe the relationship
between signal intensity and distance as intensity equals one divided
by r, that being the distance squared. So if you
(29:42):
double the distance between you and a source, if you
make are twice as big, the intensity you observe will
drop by a factor of four. So if you went
from one to two, then the intensity would go down
to a quarter of what it used to be, you've
reduced it by a factor of four. If you were
to triple the distance between you and the source, then
(30:04):
the intensity would reduce by a factor of nine. Three
squared is nine, and so on and so forth. So
I described in the previous version that signal does decrease
as distance increases. That part was right, but the relationship
I got totally wrong. The important thing to remember is
that the signal strength drops off as we get further
(30:25):
away from the source. So for spacecraft that are traveling
further from Earth, that's a big factor we have to
take into consideration. All right, let's get back to the
original episode, and again, thank you to Charlie Tango Bravo
for setting me on the right path. I appreciate it.
(30:45):
And we also have to remember that there's a lot
of stuff that generates radio signals. I mean, there's a
ton of stuff here on Earth that we create, like
TV and radio and cell phones and that kind of stuff.
Those generate radio signals. But they're also a lot of
things in space that generate radio signals, like pulsars and
nebula and quasars. So in other words, there's a lot
(31:08):
of potential noise to deal with when we're looking for
a radio signal. So again, finding that that signal amid
all the noise is a big challenge. It doesn't it's
not just important that our antenna is sensitive. We have
to have it really directional so that we can make
certain that we're pointing at the thing we want to
(31:29):
to listen to. Otherwise we might mistake some errant Earth
generated signal as being our spacecraft and then we're on
the wrong track. So to meet these challenges, NASA, through
the JPL, established the Deep Space Network or d s N.
The DSN has three facilities that are approximately a hundred
(31:51):
twenty degrees apart in longitude, so that means they're roughly
equidistant from each other Longitudinally, you multiply one twenty by three,
you get three sixty. That's a circle, right. So one
of the three is in Goldstone, California. That's actually in
the desert. It's northeast of Los Angeles, it's south of
(32:13):
Death Valley, and it's pretty far away from stuff that
generates radio waves. The second is in Madrid, Spain, so
that's almost smack dab in the middle of Spain. The
third is in Canberra, Australia, which is on the coast
in the southeast of Australia, and it's probably covered with
(32:33):
venomous animals. I mean it's Australia, so it's a safe bet.
So again, because these sites are a hundred twenty degrees
apart from each other, we get that three hundred sixty
degree view if you will, of the space around Earth.
And what this means is that at any given time,
at least one of the three DS insights has the
(32:56):
ability to establish a line of sight communication channel with
a distant spacecraft. And as the world rotates and one
of these sites begins to lose contact, the next one
will pick it right up, so communications can remain online.
You don't have an interruption. This is a big thing.
(33:17):
Like you might remember when I was talking about space stations.
When the most recent as of this recording, anyway, when
the most recent module joined the International Space Station, it
was the Naka from the from Russia. When it docked,
its thrusters misfired. They weren't supposed to fire, but they did,
(33:40):
and it caused the space station to rotate and move
into the wrong orientation relative to the Earth. Well, when
that happened. The space station was not in range of
Russia's mission control because that was on the wrong side
of the Earth at the time, and that was a
problem because Russia was the only entity that had the
(34:01):
ability to control the thrusters on the Knocka. So that's
an example of why it's important to have established these
points where we can have uninterrupted contact. All right, we've
got more to say about navigation, but before I get
totally lost, let's take another quick break. So at these
(34:29):
different sites at Goldstone, California, and Madrid, Spain, Canberra, Australia,
they have a series of radio telescopes, including one really
big one at each of these sites, and the deep
space radio antenna can be extremely impressive. So the Goldstone
Mars dish, that's the biggest one at gold Stone, California.
(34:52):
That one was built in nineteen but it was later
upgraded in night so today it measures seventy eaters across,
that's about two thirty feet. Now, this dish has a
surface area of around an acre or square feet or
three thousand, eight hundred fifty square meters. It weighs nearly
(35:14):
three thousand tons. It's mounted on massive machinery that can
tilt and turn the dish so that it can be
aimed precisely where a spacecraft is overhead and get you know,
that laser like focus with a communications channel with a
spacecraft that could be millions or billions of miles from
(35:36):
the Earth. And the purpose of those big big dishes,
I mean, it's all about collecting the very weak radio
energy that's being sent back from the spacecraft. You know,
because again that distances is intense, like the signals are
very very weak by the time they get back to Earth,
especially for super distant spacecraft. So this big dish is
(35:59):
able to like that energy and then direct it toward
the antenna itself, so you can think of it this parabola.
It's all focusing that radio energy to a specific point,
that point being the end of the actual radio antenna.
Otherwise the signal would be so weak that it would
be difficult, if not impossible, to detect that signal through
(36:21):
all the noise, and engineers have to position and antenna
precisely to beam radio instructions back to the distant spacecraft.
If you're off even by a little bit, the message
is not going to end up going to where you
need it to. Go, and the further out the spacecraft
is from Earth, the more critical it becomes to get
that just right. So all of that is to set
(36:44):
up the actual talk about navigation itself. We had to
set all those parameters to talk about the process. Without
the system like the Deep Space Network in place to
communicate with spacecraft navigation would be impossible. We would not
be able to build a spacecraft capable of detecting its
own orientation and velocity and to make changes on the fly.
(37:07):
It's not like Star Trek or Star Wars, where you
just tell an on ship computer to plot a course
for Javin or whatever. NASA describes space navigation as being
the domain of three large departments within NASA. There's mission design,
there's orbit determination, and there's flight path control. These three
(37:29):
things all inter relate to one another. So first we
have mission design. Now, this is the part of navigation
in which mission control has to determine what the intended
trajectory is for the spacecraft, where is it supposed to go. Now,
this alone is tricky for all the reasons that I've
talked about earlier in this episode. You've got to take
(37:50):
all those different elements into consideration, like relativity. You know,
what is it that you're hoping to do? What what
is the purpose of the spacecraft? What are the things
that are going to affect the spacecraft as it travels
from Earth to get to where it's supposed to go.
How do you account for those things? And how do
you either incorporate stuff so that it, you know, it
(38:14):
becomes part of your mission, or how do you find
a way around an obstacle or challenge. Now, keep in mind,
we learn new stuff the more we send spacecraft up,
Like we learn more every single time, and some of
that stuff is important and it affects calculations, like important
for the sense of navigation, that is, it's always important.
(38:36):
Someone has to take the things we've learned and then
build that into software that we use to calculate complex
equations in order to plot out navigation. So space navigation
is something that has evolved over time, and as we
learn more and incorporate what we've learned into the next
generation of software, it's constantly in a state of change.
(38:59):
So one thing that engineers have to factor in is
the fact that a spacecraft is always orbiting. Something I
kind of indicated this at the top of the show.
You know, it could orbit the Earth. It could be
in low Earth orbit and just stay there, but it's
in orbit. Or you might want it to leave Earth
orbit and enter into a solar orbit, so now it's
(39:22):
orbiting the Sun just like Earth or Mars, or you know,
any of the other planets are. Maybe you want to
enter into a different planets orbit. Maybe you want it
to leave the Solar System entirely, in which case it's
in a galactic orbit. It's orbiting the center of the
Milky Way, just like our Solar system is. It does
(39:42):
require an awful lot of velocity in order to escape
the Solar System. By the way, but we have done it,
not with you know, humans obviously, but with spacecraft we've
sent out anyway. Orbits are a huge part of navigation calculations.
And again you're talking about an origin and a destination,
and that are from an outside reference both in motion,
(40:04):
and we have to take into account the effects of
stuff like relativity with spacecraft. That's going to affect things
like the clocks on board the spacecraft. That means we
have to account for those changes due to relativity or
else we risk losing track of the spacecraft. We might
point an antenna at where a spacecraft either used to
(40:24):
be or might be in the future, but isn't right now,
just because we have these deviations from between our earthbound
clocks and the ones that are aboard the spacecraft. So
the software has to plot a trajectory that has to
take all these different things into consideration, and as you
can imagine, this means those calculations get pretty darn complicated,
(40:47):
particularly when you're looking at something you know, really ambitious,
which sounds a little weird to say, because I still
think just getting something into low Earth orbit is being
really ambitious. But you know, there's there is a scale,
I guess, and as I'm sure we're all aware, software
does not always come out perfect, right. You've probably used
software where you've encountered a bug or a glitch, Like
(41:10):
maybe you're playing a video game and the textures failed
to load and everything looks weird. Well, that's irritating when
it's a video game, but when you're talking about like
interplanetary navigation, a bug or glitch can become an enormous challenge. Now,
it might not be a show stopper. You might be
able to work around it, but it likely will require
(41:30):
a lot of people to work out a solution on
the fly in order to make a sure that a
spacecraft's route is in fact the right one to do
whatever it is you want that spacecraft to do, all right. So,
once all of that has been taken into account, engineers
calculate the navigational route for a spacecraft. This planned route
is the reference trajectory, so this is the route the
(41:53):
spacecraft should be on. Also, there's a cesall relationship between
the navigator for a mission and the software developers who
are making the navigation software. So as navigators find bugs
or they encounter new situations that necessitate new features in
the software, they can relay that to the developers, and
then they take the feedback and they produce new versions
(42:15):
of the software. I imagine that gets increasingly challenging to do,
especially to incorporate new features. Any developer can tell you
that it can be a nightmare to put something new
into code that you've just gotten to work, because the
chances are you're going to break something that previously had
been working. Okay, just imagine that for things that are
traveling through space and you have to deal with relativity
(42:38):
and stuff. Now, even when you do that correctly, which
you know obviously requires a lot of work. It does
not mean that a spacecraft is just going to magically
stick to that reference trajectory. All sorts of things can
cause the spacecraft to deviate from the planned route. In
some cases, it might be on purpose us such as
(43:01):
you know, you might have to do a maneuver to
avoid a potential collision. Or it might be that your
pathway is taking you close to a planet and you're
planning on using a gravity assist two make the spacecraft
continuance journey. But in other cases, something might pop up
that wasn't anticipated, Like it happens very quickly. Maybe the
(43:25):
spacecraft passes some large asteroids and the gravitational attraction between
the spacecraft and the asteroids pulls the craft out of
its trajectory a bit. Or maybe it turns out that
the software had a bug in it or a blind
spot that failed to account for something, and the spacecraft
is veering off course a bit as a result. I mean,
even solar pressure, that is pressure from light itself hitting
(43:50):
the spacecraft can be enough to push the spacecraft off
its reference trajectory. This is where orbit determination comes in. Now,
as the name implies, this part of space navigation is
just keeping track of a spacecraft's actual position. We know
where we want the spacecraft to go, but this is
(44:11):
about us figuring out where the spacecraft actually is. And
that is another kittle of fish. NASA breaks down orbit
determination into three sub processes or subgroups, So there's orbit reconstruction.
This is asking the question where has this spacecraft been?
(44:31):
This is all about determining the past route, the past
locations for the spacecraft to understand its actual trajectory versus
the reference trajectory. Then you've got orbit determination. This is
like asking where the heck is the ding dang thing
right now? Then you've got orbit prediction. This is like
asking where the heck is this thing going to next?
(44:55):
And and at this point I kind of wish I
hadn't burned the mean Girls reference about gettin loser we're
going shopping, But I did that one already. Anyway, A
bit of consideration reveals that all three of these things
are important. See, we're not controlling these spacecraft in real time.
You know, you don't have someone sitting looking at a monitor.
(45:17):
They get a first person view of a spacecraft's viewscreen.
They've got a joystick and they're just making it fly
all over the place. That's not how this works. And
it does take time for a signal to pass from
one point in space to another. The fastest that this
can happen at is the speed of light, and you know,
light is wicked fast. It's in fact the fastest stuff
(45:38):
there is. But light can't traverse great distances in an instant.
I mean it takes about eight minutes for light to
go from the Sun to hit us here on Earth.
So as as spacecraft gets further from the Earth, the
information that we get back from those spacecraft becomes more dated. Right,
it's more about where the space was at the time
(46:01):
it transmitted, but minutes have passed between then and when
we're able to actually look at the data. So as
engineers get the latest telemetric data, they're actually looking at
stuff that's several minutes old. So we're not observing the
spacecraft directly. We're getting information back from the spacecraft, and
then we have to draw conclusions about what's going on
(46:23):
based upon the information we have. So if the data
indicates that perhaps a ship is drifting away from its
reference trajectory, we need to be able to look back
and see what the data has said about that. When
did that deviation begin? How long has it been going on?
Where is the spacecraft now? Based on the information we have,
(46:43):
keeping in mind we're projecting forward a few minutes because
the information we have is older. And then, knowing that
our ability to pinpoint a spacecraft position with precision decreases
the further out the spacecraft gets from us, we have
to start building in margins of error, and then, based
upon all we know, where is it heading to now
(47:06):
and how can we get it back on track? If
we were not able to determine this, we wouldn't know
where to point the antenna with the d s N
in order to track the spacecraft. We would lose the spacecraft,
let alone being able to figure out how to correct
its its course. And you know space is big, so
(47:27):
if you lose something like a spacecraft, good luck finding
it again, because you'd you'd just be scanning regions of
space looking for the faintest of radio signals to try
and get back on track. It's not something you want
to have happen. Well, we've got some more to talk
about with navigation before we wrap all this up, So
(47:48):
let's take one last break. Okay, I was talking about
orbit re construction before the break, and we also need
to reconstruct the path of the spacecraft in order to
make sure that the scientific data that we're collecting with
(48:08):
this satellite. I mean, presumably we've sent it up there
to do something, well, some of it is determined by
the trajectory path, Like we have to know the trajectory
path in order for the data to make sense. There's
imaging data. There's like a type of imaging called synthetic
aperture radar imaging, and that requires that we have a
(48:29):
precise knowledge of the spacecraft's trajectory so that the software
we use to process that information can create a meaningful
image from that data. Like it's not like it's sending
a JPEG to us. It's sending us data that we
then use to create an image with sophisticated software we
(48:50):
have running on computers here on Earth. If we don't
know the trajectory precisely, then it's almost like we're building
a picture, but we're doing it from the wrong perspective,
and it would leave us with an image that's nothing
like what we were actually trying to capture with the satellite.
So it is imperative that we know the actual trajectory
(49:11):
of a spacecraft. Then we have flight path control, the
third of the three departments I was talking about. Again,
you could probably guess what this is about based on
the name. We can summarize this by saying that this
is the part of space navigation where we figure out
how to get the spacecraft from where it actually is
(49:32):
to where it is supposed to be. How can you
get the spacecraft to return to the reference trajectory that
you had set in the beginning. And here's another good
reason to put so much emphasis on orbit determination, because
spacecraft have a very limited set of options that they
can use in order to return to the correct course.
(49:52):
An interplanetary spacecraft, like a satellite that's designed to fly by, say,
one of Saturn's moons, it needs to on course once
it separates from the launch vehicle. You know, once it
separates from the rocket that pushes it up into space.
So this is kind of like a bowler releasing a
bowling ball. You know, once that ball leaves your hand,
(50:14):
there ain't no amount of you waving or leaning that's
really going to affect the ball's trajectory. You've set it
on a path and you can't really influence it anymore. Well,
we can still influence spacecraft a little bit, but that
initial release from the launch vehicle is the primary thing
(50:35):
of putting it on its proper trajectory. So spacecraft can
have like thrusters or rocket engines that can fire to
adjust their course a little bit. But obviously there's actually
a limit to how much fuel any spacecraft is able
to carry. And there ain't no gas stations in space,
at least not in our neighborhood. So fuel is a
limited resource. And that's you know, an understatement, but it's
(51:00):
important to remember. So let's say a spacecraft has gone
a little bit off course. The flight path control group
then has to figure out how far off course is it.
Then they have to figure out the commands needed to
have the spacecraft returned to its reference trajectory. So they
take the data from the orbit determination group that tells them, okay,
(51:20):
how far off course are we. Then they start running
the calculations how much is it going to take for
us to get back to where we need to be? Uh?
This means figuring out the proper change in velocity for
the spacecraft and if it's been a while since you've
had physics, I want to remind you that velocity isn't
just speed. A lot of folks use the word velocity
to stand in for speed, but that's just part of it.
(51:42):
Velocity is a vector. So in addition to speed, we
have to have a direction. So a change in velocity
is an acceleration. It can be a change in speed
or direction or both. The flight path control team figures
out what change in velocity is necessary, so they indicate
the magnitude and the direction that is required in order
(52:04):
for the spacecraft to return to its reference trajectory. Now,
the flight path control team doesn't initiate the actual maneuver.
They just designed the maneuver, or at least they design
the parameters of the maneuver. It needs to have this
change in direction and this magnitude. They send that information
to another team, a spacecraft engineering team, and it's this
(52:25):
group that then use stuff like attitude control systems and
thrusters or rocket engines to produce the change in velocity
that was indicated by the flat flight path control teams. So,
in other words, they're the ones to take this data
that says, here's what we need to have happen, and
they're the ones to actually activate the systems to make
it happen. NASA refers to small flight path control maneuvers
(52:48):
as trajectory correction maneuvers, which makes sense. You're trying to
correct it move it back to its reference trajectory. Now,
as we conduct more space exploration, we learned things that
are really able for future missions. So, for example, if
we were to plot out a fly by satellite to
go to Saturn, we would incorporate some gravity assist fly by.
(53:08):
It's most likely this is where you leverage the gravitational
pull of celestial body like another planet, to assist the
spacecraft on its way by giving it kind of a
slight pull slash push towards its destination and a change
in velocity. It's a boost, kind of like someone giving
you a little push when you're swinging on a swing set,
(53:28):
although it can also you know, change your direction somewhat.
And as we understand these things and we're able to
build it in, we can have that planned from the beginning,
so we can build them into an actual mission. When
we've got a good handle on those things, NASA can
call any sort of velocity maneuvers that we know we're
going to have to do deterministic. That is, we have
(53:52):
already determined the velocity maneuvers that we will need to
conduct in order to maintain our reference true jectory given
the route that we're following. We know that they are
going to be maneuvers necessary to stay on course, and
we have a good idea of what they are and
when we will need to execute them. But there are
(54:13):
other types of maneuvers that will call stochastic. These are
maneuvers that we know we're gonna need to make, but
we don't necessarily know. More like, we don't know what
the magnitude of those changes might have to be. We
might not fully understand the effect of those maneuvers ahead
of time, because we're going through uncharted ground, if you
(54:35):
if you will. We're not following something that we've already done,
where we already kind of have a grasp on what
we need to do. And then, of course occasionally sometimes
we have to do these maneuvers when something we didn't
anticipate at all happens and we have to design and
conduct a maneuver kind of on demand. Also, I should
mention that attitude control I mentioned it earlier Attitude control
(54:58):
is not about whether or not you're spacecraft is sassin you.
It has nothing to do with sass. Attitude refers to
the angular orientation of a spacecraft given some other point
of reference. Again, you have to have a point of reference,
because I mean, if you think about it, outer space
doesn't really have an up or down. You have to
(55:19):
have a point of reference and compare your position to
that point of reference, or you have to, you know,
use your point of reference to in order to make
a determination about something's positions. So you've gotta have a
point of reference to start from in order for you
to say something like that durned things upside down or
backwards or you know, or whatever. Also, there are interesting
(55:41):
ways to change the angular orientation of a spacecraft, and
some of them do not involve thrusters or rocket engines. Instead,
they might involve something like momentum wheels, as in, you know,
physical wheels a rotor spun by a motor. Now I
would get into this further, but that requires a pretty
(56:02):
long discussion about things like the conservation of momentum and
equal and opposite reactions and stuff like that, and we
don't really have time for that this episode. Is already
going super long, and I don't want Tardy to hate
me more than she does already. So she doesn't hate me, folks,
She's super nice to me. I just want to make
that clear. That was more of a jest. But the
(56:24):
short version of all this is that using stuff like
momentum wheels can make it possible to change the attitude
of a spacecraft without having to use thrusters. And as
we've previously established, fuel is a precious resource. So that's
a good thing to know, right to build in systems
that allow us to make changes to a spacecraft's orientation,
(56:45):
for example, without having to burn fuel to do it. Still,
it's not uncommon that the spacecraft engineering team will actually
have to initiate a rocket engine or a thruster ignition
to provide the thrust needed to get back onto the
reference trajectory. Uh, the timing on these maneuvers has to
be incredibly precise, both because you don't want to waste
(57:07):
even a drop of fuel if you can help it,
and also that if you fire a thruster for too
long or not long enough, then you're not going to
return to your reference trajectory. So imagine, for a moment
that you're sitting in mission control and you are relying
on complex calculations from data sent back to you from
(57:27):
space just to tell you where that something is right,
and that's something the spacecraft is so far away that
there is no means for us to observe it directly.
All you have is the data coming back to go by,
and then you have to come up with the command
to send via radio back up to this object to
(57:48):
get it back onto the course it's supposed to be
following the data you have indicates that the thing you
know was in this one particular position several minutes ago,
So you have to figure out where it is now
based on calculations with the data that you have, knowing
that that is at best maybe a precise approximation, but
(58:09):
still an approximation. Then you figure out the command you
need to send to where the spacecraft is going to
be in order to get it to take the action
to get it to where it should be. This whole
thing is mind boggling to me. It kind of makes
me think of like submarine navigators who use precise charts
(58:30):
and timing like a stop watch in order to plot
underwater courses. Because they're not able to just look outside.
I mean, at the depths that submarines can travel. You
can't have windows because they would collapse in from the pressure.
So you're in a tin can underwater, and you're using
(58:51):
very precise maps and a stopwatch and knowledge of how
fast you're moving in order to make calculations to determine
whether or not you're going to bump into something. It's
hard for me to even contemplate. Well, once the spacecraft
engineering team has done their thing, that whole process has
to repeat itself. The orbit determination team has to figure
(59:12):
out if the spacecraft is in fact back on its
reference trajectory, or if they'll need to conduct another maneuver,
and so on and so forth. And because there's so
many little things that can pull a spacecraft off track,
this becomes a continuous process. It's incredible to me that
people figured this stuff out, that they figured out how
(59:34):
to not just not just how the universe works, Like
it's already incredible to me that Einstein was able to
determine these in incredible theories of special and general relativity,
but then for people to build on that and to
make technologies that take that into account, it's it's phenomenal.
It also tells you by the way science works, right,
(59:57):
because if science didn't work, the technology we build that
leverages that science, it wouldn't work. So science works or
else are tech wouldn't work, especially when it comes to
space navigation. Now, there is a lot more that we
could say about spacecraft navigation. I haven't really gone into
deep details here, uh like how do you determine what
(01:00:21):
a spacecraft's velocity is? For example? I haven't talked about
that process, but I felt this was a good if
you will high level overview of the topic. If you
would like to know more details, let me know. I'll
research and write and do a follow up episode and
it will reference this one, but it will go into
much more detail to talk about the actual processes and
(01:00:43):
technologies we use to do the things I've talked about
in this episode. If you want that episode or any
other topic on tech Stuff, reach out to me on Twitter.
That's the best way to get in touch with me.
The handle we use for the show is tech Stuff
H s W and I'll talk to you again really soon. Yeah.
(01:01:07):
Text Stuff is an I Heart Radio production. For more
podcasts from I Heart Radio, visit the I heart radio, app,
Apple podcasts, or wherever you listen to your favorite shows.
Hey there, listeners, this is Jonathan. Before we get into
the episode, I want to acknowledge something. Uh. Twitter user
Charlie Tango Bravo helpfully pointed out that I made a mistake.
When we first published this episode, I completely misrepresented the
inverse square law, and so later in this episode you
(00:23):
will hear me kind of interrupt to give a proper explanation.
My apologies for that. Uh and um, yeah, I mean,
this one's totally on me. I made a mistake, and
I feel real dumb about it. But you know, mistakes happen.
It doesn't. It just tells me I need to be
(00:45):
even more careful in the future to make sure that
I'm not misrepresenting something, which I totally was in the
original version of this episode. Anyway, let's get to the episode,
and when we get to the part where I'll correct it,
I'm pretty sure we'll be apparent. Enjoy. Welcome to Tech Stuff,
(01:08):
a production from I Heart Radio. Hey there, and welcome
to tech Stuff. I'm your host, Jonathan Strickland. I'm an
executive producer with I Heart Radio and I love all
things tech and listener Terry A. Carlson, and I apologize
for very likely Butchering your name, asked if I might
(01:32):
cover how space vehicles navigate. So we're going to talk
about navigation, and this is one of those things I
find really fascinating and also sometimes frequently actually a bit confusing.
Now I blame part of that confusion on my own
fascination with stuff like Star Wars, which some folks call
science fiction. I think of it as fantasy that happens
(01:53):
to be set in space, or even Star Trek, which
is closer to science fiction than Star Wars, but can
play a bit fast and loose with science and technology.
And these kind of properties gave me a really cool
but an accurate feel for how space navigation works. Terry,
you asked that I cover what references and methods do
(02:15):
space programs used to actually do space navigation? And that's
a great question, because all of this really does rely
on reference or relationships between a vehicle and something else,
Like it's all relative. You know, we're going to talk
about that a lot in this episode. And I guess
(02:35):
on some level this is intuitive, but I had not
really thought about it in concrete terms before. So, for example,
here on Earth, if you're giving directions to someone, you
would tell that person how to get to a place
relative to where they are right now, Right, I mean,
the same thing holds true for space vehicles. But we've
(02:56):
got to remember that everything in space is moved ing
all the time. So here on Earth, if you need directions,
you can start from a position in which you know
you're not moving relative to the Earth. You're standing still.
On Earth, You're still moving, but that's because the arts moving.
We're going to get to that. And you know you're
(03:18):
standing still with reference to the Earth. But in space,
everything is moving in reference to everything else. Now you
could be moving at a similar velocity relative to your surroundings.
So from your perspective, it might seem like you're not
all really moving together, but trust me, you totes are.
Now what I'm about to go into it really matters
(03:39):
when we start thinking about the possibility of traveling beyond
our solar system to another. But it's it's, you know,
something that we have to take into consideration, even when
we're talking about travel within our Solar system. So let's
start with the easiest stuff first and then work our
way up. The Earth travels in a nearly circular orbit
around the Sun, right, I mean, this is not news
(04:02):
to you. I imagine. So the Earth is moving, it's
moving in an orbital path around the Sun. The orbit
moves at a speed that's around sixty thousand miles per
hour if you prefer, that's about or almost thirty kilometers
per second. Now, if we're talking about going to space
to do stuff in low Earth orbit, we're pretty much
(04:24):
thinking to Earth, right, We're not too worried about everything
else that's going on. The orbital speed of Earth isn't
as big a deal in that case, we don't have
to take that into as much account. But let's say
that we want to travel to Mars. Go from Earth
to Mars. Well, Mars is further out from the Sun
than Earth, right, I mean it goes my very educated mother.
(04:46):
So Mars comes after Earth. Oh, in case you've not
actually heard that mnemonic device, this is how I learned it.
It was my very educated mother just served us nine pickles.
That stands for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,
(05:08):
and Pluto the planets except well, you know, when I
was a kid, Pluto was a planet, and then Pluto
kind of got the boot as far as being a
planet goes. So I guess now you could say, my
very educated mother just served us nothing. Thanks mom. Okay. So,
Mars is further out from the Sun than Earth is,
(05:29):
and Mars's orbit is obviously a bigger circle around the
Sun than Earth's orbit is, because you know, Mars is
further out from the center of our solar system. Mars
also doesn't have the same orbital velocity as Earth. Earth
completes one orbit of the Sun every three sixty five
Earth days, with a little bit of change leftover. That's
(05:49):
why we have to have leap years, and Mars's orbit
is just a hair under six hundred eighty seven Earth
days long. But beyond that, Mars's orbital velocity is closer
to twenty kilometers per second. Remember Earth's is closer to
thirty kilometers per second. So Mars is not just moving
in a greater distance because it's orbit is larger, it's
(06:11):
moving a little slower compared to Earth, you know, and
again relative to the Sun. So that means if we
want to plot a course from Earth to Mars, we
have to take all that into account, right, because we
can't travel instantaneously. It takes time for us to get
from point A to point B. We can't just point
a rocket at where Mars appears to be to us
(06:35):
right at that moment and then launch the rocket in
that direction. You know, look at the sky, find the
red dot and say aim that away, because you know,
the positions of Earth and Mars are shifting in their orbits,
and Mars will not be at that same spot by
the time the vehicle we launch gets there. Heck, it's
not even at that spot as we look at it,
(06:56):
because it takes time for light to travel from Mars
back to us, So we're really looking at where Mars
used to be, and we would be shooting a rocket
at where Mars used to be a really long time ago. Uh.
The vehicle it would take months to travel there, so
by the time it would get to that position in space,
(07:16):
Mars wouldn't be there anymore. By the way, all this
orbital stuff is why when we talk about sending a
crew of human astronauts to Mars, we talk about missions
that typically are on the order of a couple of
years in length. The trip from Earth to Mars could
take around eight months, and that's if we time it
so that the launch vehicle UH shoots the spacecraft up
(07:40):
and has the spacecraft travel the least amount of distance
needed in order to go from Earth to Mars. That
means you're timing it right. You're aiming at where Mars
is going to be, and you want to time it
so that Earth and Mars are at one of the
closest points in their respective orbits to one another. You
out of time it just right. And because of the
(08:02):
difference in orbital velocities and the fact that mars Is
year is significantly longer than Earth's year, that means that
by the time you would arrive on Mars, Earth and
Mars would no longer be super close together anymore. Right
Like they're they're constantly in motion, so now they're moving
apart from each other. That means for you to get
back to Earth, you would need way more fuel for
(08:24):
your return trip than you did for your trip out
to Mars. Potentially, so the most fuel efficient thing to
do is to hunker down on Mars and do all
your science e stuff and you wait for the planets
to reach a point in their orbits where again you
will be traveling the least amount of distance you can
get away with. And all told, that means a mission
(08:46):
needs to last around two years to get all that done.
Plus you know, theoretically you could even spend some of
that time on Mars making rocket fuel, so you wouldn't
necessarily have to carry all of it with you on
the way there. Anyway, that's just a simple example of
how motion in space matters. But we're just getting started, right,
So I've touched on stuff like low Earth orbit and
(09:09):
interplanetary travel within a solar system. But it's not just
that the planets are moving around the Sun. Our entire
Solar system is hurtling through space. We are part of
the Milky Way Galaxy, and at the center of the
Milky Way Galaxy there is a super massive black hole
that's kind of the you know, it's like the Sun
(09:32):
is in our Solar system, except there's a supermassive black
hole in the middle of a galaxy billions of solar systems.
Scientists estimate that our galaxy has somewhere between a hundred
to four hundred billion stars in it. So think about that.
I can't. I tried, but that's just a number that's
just way too big for me to even get a
(09:53):
rudimentary grasp on it. Anyway, our solar system is traveling
in its orbit in the galaxy at a speed of
around four thousand miles per hour or two kilometers per second.
Then we have to consider that our galaxy is also
in motion. We've got other neighboring galaxies in our neighborhood
is just part of a super cluster of galaxies, and
(10:16):
that in itself is part of an even bigger super cluster.
And we're all hurtling through space at around a thousand
kilometers per second. Where are we going? Well, I mean,
I want to say, get in loser, we're going shopping,
but we're not. We're headed to the Great Attractor. And
to my surprise, that isn't Oscar Isaac. It's a gravitational
(10:38):
point in the Lania Kia super cluster. Uh. And um,
you know the Great Attractor is also moving towards another mass,
the Shapley super Cluster. I guess there's always a bigger fish.
And you know, we could keep going down this rabbit hole.
But what I really wanted to point out is that
we're talking about a lot of body ease in motion here,
(11:00):
and that makes navigation more tricky, right, I mean, this
is kind of like saying that you stop to ask
for directions, but the town you stop in is actually
moving on the map, and the place you're going to
is also moving on the map. Maybe it's moving away
from the town you are in, which is still in motion.
So the route you are going to take to get
(11:21):
there is changing by the minute. It gets complicated, so
you have to take, uh, you know, into account a
lot of things. You have to take reference points in
order to make it all makes sense. And of course
that's like talking about a map that's two dimensional. Obviously
in space you're talking about three dimensional. You're not limited
by a two dimensional plane. You're moving in three dimensional space. This,
(11:44):
by the way, is all before we even consider stuff
like relativity, which makes things even more weird. Einstein's theories
of relativity really show how our universe behaves in ways
that we don't often get to observe of directly. So
we don't have a lot of direct perspective on these things.
(12:05):
But let's use an example to explain some of relativity,
and we'll start with special relativity. That's the I would
argue the easier of the two to get a grasp on. Alright,
so we've got two people and we're gonna name them
Alice and Bob. Alice has superpowers, and Alice can travel
through space without a space suit and can move at
(12:28):
near the speed of light. So she flies through space
close to the speed of light, and she goes and
flies off on an adventure at top speed. When she
comes back and she meets up with her best friend Bob,
the two notice something unusual. So to Alice, it'll seem
like Bob aged faster than normal, as if more time
(12:49):
had passed for Bob than it did for Alice. To Bob,
it will seem like Alice has aged less than normal,
like not as much time passed for her, like like
like less time pass less than it should have. And
the reason for this is that the faster your speed
is relative to some other reference point. This is why
(13:11):
we talk about relativity. It is relative to some other
reference point, the slower time will pass for you relative
to that reference point. So again, like Bob is our
our reference point for Alice. So it appears like, you know,
Alice hasn't aged as much because time appeared to pass
(13:32):
slower for Alice than it did to Bob. Now to Alice,
time will have seemed to pass as normal for our
own frame of reference. So in other words, it wouldn't
feel to her as if time had slowed down. She
would feel like time was passing, just as it would
if she were standing perfectly still on Earth. A second
would feel like a second to her. And in fact,
(13:54):
if Alice were wearing a watch with a second hand,
it would seem to be clicking one second at a time,
just perfectly. Now, if Bob could somehow observe that watch
while Alice is traveling at near the speed of light,
Bob would see that the second hand is moving really slowly.
It would be taking way longer than a second for
(14:15):
it to take each tick. In fact, the faster you go,
the slower it gets. And if you got to the
point where you could travel at the speed of light,
it would stop like the second hand wouldn't move anymore.
For Bob, he wouldn't see the second If somehow Alice
could move faster than the speed of light, which is
as Einstein would put it impossible, it would look as
(14:37):
though the second hand was going backwards. She would be
traveling back in time. Now that's physically impossible. So I
just thought I would throw head in there as an
interesting you know, side note. But again to Alice, it
would seem like time was passing as normal. And likewise,
let's say Bob's wearing a watch. He has a second hand.
To Bob, time is passing just as normal. A second
takes a second, he can watch the little second hand
(14:59):
click on buy on this on his watch. Now, let's
say that Alice is able to see Bob's watch. While
Alice is traveling at near light speed, it would look
to her as if the second hand was going way
too fast, like it was just spinning around the watch face.
And again, Bob and Alice would each feel the passage
of time as if it were just normal. It's only
(15:21):
when they compare it to a point of reference, when
they are relative to something else, that they see that
there is any type of difference. Now, for most of
our experiences on Earth, we don't notice this effect, and
that's partly because we're usually traveling at a similar velocity
relative to one another. We're all on this planet. Most
(15:42):
of us aren't going super duper fast. However, there are
cases where we can measure a difference. It wouldn't be
observable like to our normal senses, but with very sensitive
you know, metrics, we could see that there was a
difference there. It's settled because we are not able to
go anywhere close to the speed of light, at least
(16:03):
not yet. So for example, we have the case of
Mark and Scott Kelly. These are twin brothers who are
both astronauts. Now, both of the Kelly's have spent time
in orbit. Mark Kelly was born first, he's the older
of the two twins, and he's logged fifty four days
in space. Pretty respectable, right, I mean extremely respectable, Mr Kelly.
(16:27):
I don't mean to, you know, dismiss that incredible achievement.
But Scott Kelly has spent five hundred twenty days in orbit,
almost ten times as much time in orbit, and a
lot of that was aboard the International Space Station, which
orbits the Earth at the speed of around twenty eight
thousand kilometers per hour or seventeen thousand, five hundred miles
(16:50):
per hour. So for a significant amount of time, Scott
was traveling much faster relative to his brother Mark, who
was back here on Earth. And since traveling faster means
that time passes more slowly relative to an outside observer,
it means that the gap between Mark and Scott actually
got bigger. Mark aged faster here on Earth than Scott
(17:14):
did out in space because of that speed of travel.
Mark once said that he used to be six minutes
older than his brother, but now, thanks to all that
space travel that Scott did at high speeds, Mark is
six minutes five milliseconds older. And that's a funny thing
to say. Uh And sure you could argue, well, that's
(17:35):
not really significant to our normal frame of reference. I mean,
what's five milliseconds, But it does show that we actually
have to keep this in mind when it comes to
space travel. It does matter. Now when we come back,
we're gonna tackle a little bit more of a relativity.
But first let's take a relatively quick break. Okay, we're back,
(18:05):
and we are not done with relativity yet. The speed
and time thing is all part of special relativity. But
Einstein was a real you know Einstein, and he also
published his theory on general relativity. This includes an explanation
that if you were to have two clocks and one
of them is closer to a gravitational mass than the
(18:27):
other one, the other one is much further out from
that gravitational mass. The one that's closer to the gravitational
mass will take more slowly than the one that's further out.
This is separate from the whole you know, speed thing.
So in other words, gravity also affects the rate at
which time passes. It passes more slowly when you're closer
(18:48):
to a big center of gravity. And this becomes really
important for navigation just here on Earth, not even just
space navigation, but navigation here. And you might wonder why
that is. Well, a lot of us depend upon GPS
apps or devices, right like we pull that up whenever
we're going someplace new, and these devices they work by
(19:11):
relying on signals that are coming from GPS satellites. It
gets a few of these different signals and then it's
able to pinpoint the location where on the surface of
the Earth we happen to be at that given time. Well,
those satellites are really far away from us. They are
beyond low Earth orbit, so they're beyond where the International
(19:32):
Space Station is, for example. They're out in medium Earth
orbit somewhere around twenty two KOs out from the Earth,
or around twelve fifty miles. So these satellites are much
further out from the Earth's gravitational mass than say your
watch or come on, we'll be real here your smartphone.
(19:54):
So because of that, the clocks on the GPS satellites
tick slightly fat stir. Then the clocks here on Earth tick. Remember,
the closer you are to a gravitational mass, the slower
time will pass for that particular frame of reference compared
to a different frame of reference. I always have to
(20:15):
throw that part in because again, in the moment it passes,
the way time passes, like the way our experience of
time is remains the same, not to our own frame
of reference. Now, the GPS satellites are also traveling really
fast relative to us, so that actually means that we
have to take a bit off the top right, because
(20:36):
we know that the faster you travel, the slower time
uh affects you relative to someone who's not traveling at
that speed. So the effects of general relativity mean that
a clock on a GPS satellite has on average around
forty five micro seconds more than an Earth clock at
the end of a day, a full day. But again,
(20:58):
because these satellites are link faster relative to us, the
clocks also have a negative seven micro seconds two factor,
and compared to our clocks, due to special relativity, so
we have to combine those two together forty five and
negative seven. That gives us thirty eight micro seconds that
are extra on the GPS clocks. So if if we
(21:22):
were to stop it right at midnight for both clocks,
you know here on Earth and at the GPS satellite,
we would see midnight on our clock and midnight plus
thirty eight million micro seconds, not milliseconds micro seconds on
the GPS side. Now, because our navigation depends upon taking
signals from satellites and essentially measuring how long it took
(21:43):
for that signal to go from the satellite to us,
in order for us to calculate where we are on Earth,
we actually have to account for that difference between our
clocks and the satellite satellites clocks, or else we start
to get some drift, and that means that over time,
and and we're actually talking about years here, but it
(22:04):
does happen, our navigation systems would become less accurate, which
would eventually get to a point where our GPS device
wouldn't even really show us where we are because it
would be miscalculating based upon the the differences in the
clocks on the GPS satellites versus the clock on our
phone or that our phone is connected to, and instead
(22:27):
it would show you where it thinks you are, but
there would be a growing gap between where it thinks
you are and where you really are. Based upon this
gap in time. It would take like seven years for
that to get to a point where we might even
notice it, and we do correct for it. So essentially
what happens is we shave thirty eight micro seconds off
(22:48):
the clocks every midnight that the that the clock's hit.
So when the clocks hit midnight on the GPS satellites,
they kind of hold for thirty eight microseconds, which puts
them I can sync with the clocks here on Earth.
And then we have to do it every single day
because every single day we get the effects of relativity.
(23:08):
All right, well, that's something we have to take into
account with navigation, Like these are things that we don't again,
we don't necessarily have to think about here on Earth. Typically,
most of us don't come into situations where special and
general relativity have a noticeable impact on our day to
(23:28):
day experience. UM. One really important element in space navigation
is something called the deep Space Network, which is not
a really cool science fiction channel, uh, no, it's it's
actually a bunch of antennas, and it's it's more than that.
But that's a big part of it. If we go
back to the nineteen fifties, we had the space race
(23:51):
ramping up. In nineteen fifty seven, the then Soviet Union
launched spot Nick into orbit. The US was already working
on its own satellite, and of course there were reasons
for this beyond the scientific push. Scientific push was a
big part of it, but there were other political reasons.
For one thing, Demonstrating that you could put a payload
into space also sent the message of hey, comrade, we
(24:15):
can build rockets big enough to reach you, even though
we're on the other side of the world, and you know,
nuclear weapons are a thing, so you know, this wasn't
just you know, a science thing. But that's another tangent
that I won't go down any further. But on the
U s side, one of the things that the Jet
Propulsion Laboratory or JPL undertook was a job from the
(24:39):
United States Army, which effectively you know, ran the jp
L to establish radio tracking stations in certain parts of
the world, including places like Singapore, Nigeria, and California. So upon,
launching the first successful US satellite, which was called Explorer one.
These ground stations would receive data from the satellite to
(25:01):
be able to track it as it passed over overhead.
This was essentially telemetry data, and you might wonder what
does that mean. Well, telemetry is essentially the process of
using some sort of device to measure something. It could
be temperature, it could be pressure, it could be speed
or velocity, and then it transmits that information to a
(25:23):
distant receiver. Now, in this case, the telemetry was mostly
about the Explorer one's orientation and velocity as it went
through its orbit. Engineers at mission control could take that
data and plot out the Explorer one's orbital path. In October,
the US government established NASA, and this was really to
(25:45):
consolidate the space efforts from various independent groups, mostly in
the military, like the Army, Navy, and Air Force all
had independent space exploration UH initiatives, so this was to
kind of bring them all under one civilian umbrella and
combine all those resources to be more effective. So this
(26:07):
included the Jet Propulsion Laboratory that had previously been run
by the U. S Army that became part of NASA.
In December of nineteen and NASA would assign to the
jp L the responsibility of planning out planetary and lunar
exploration missions that would use unscrewed spacecraft, that is, spacecraft
(26:27):
that did not have human beings abort robotic spacecraft if
you prefer. Now. That would necessitate a network system of
receivers here on Earth to be able to receive communications
with and then to send communications too, as well as
to just keep track of these robotic spacecraft as they
traveled away from the Earth. You can't just have one
(26:48):
really big antenna in Houston, because you know, the Earth
rotates and sometimes Houston would be pointing the wrong way.
Get together, Houston. So yeah, to establish that these antenna
around the Earth need to be pointing outward in such
a way that you have maintained contact with distant spacecraft. Moreover,
(27:10):
as the spacecraft move further from the Earth, the signal
strength will decrease. In fact, you know, here on Earth
we describe radio frequencies as obeying the inverse square law,
which means that the power of a signal is inversely
proportional to the distance from a source. Hey, it's Jonathan
(27:30):
from one day further out from when this episode originally published. Okay,
so the first time I tried to explain this, I
just playing god it all wrong. Uh, we published the episode,
they had an error in it, and this was because
of a fundamental misunderstanding on my part. But fortunately Charlie
Tango Bravo on Twitter set me straight and I'm gonna
try harder to get it right this time. So the
(27:54):
inverse square law will first imagine that you have a
source of electro magnetic radiation. And this can be anything
from you know, a radio antenna to a microwave source
to light. In fact, let's talk about light because that's
pretty easy for us to wrap our heads around it
because we can directly observe it. Right, We've all seen
(28:14):
that as you move away from a source of light,
then you get less light to work with, Right, that's intuitive.
So if you're walking around I don't know, a spooky
attic and it's lit by a single light bulb hanging
down from a chain, well you know that as you
get towards the corners, a ghost is gonna get you.
I'm kidding. Ghosts don't exist, but you do know that
(28:37):
as you move further away from the light, bulb, it
gets darker, Right, That just something we've experienced. Well, we
can actually describe this phenomena with the inverse square law
of propagation. We can think of radiation moving out from
a source as similar to that of an expanding sphere,
(28:58):
like it's going out in all directions and growing as
it moves outward. Right, the sphere gets bigger and bigger.
The center is still stationary in this frame of reference.
So the further we get out from the center of
the sphere, the more surface area that electromagnetic radiation is covering. Right,
the the outside of the sphere is bigger. That means
(29:21):
that the signal strength is growing weaker. You have the
same amount of signal to go around, but you're covering
a larger area, so you can think of it as
spreading across more space. And we can describe the relationship
between signal intensity and distance as intensity equals one divided
by r, that being the distance squared. So if you
(29:42):
double the distance between you and a source, if you
make are twice as big, the intensity you observe will
drop by a factor of four. So if you went
from one to two, then the intensity would go down
to a quarter of what it used to be, you've
reduced it by a factor of four. If you were
to triple the distance between you and the source, then
(30:04):
the intensity would reduce by a factor of nine. Three
squared is nine, and so on and so forth. So
I described in the previous version that signal does decrease
as distance increases. That part was right, but the relationship
I got totally wrong. The important thing to remember is
that the signal strength drops off as we get further
(30:25):
away from the source. So for spacecraft that are traveling
further from Earth, that's a big factor we have to
take into consideration. All right, let's get back to the
original episode, and again, thank you to Charlie Tango Bravo
for setting me on the right path. I appreciate it.
(30:45):
And we also have to remember that there's a lot
of stuff that generates radio signals. I mean, there's a
ton of stuff here on Earth that we create, like
TV and radio and cell phones and that kind of stuff.
Those generate radio signals. But they're also a lot of
things in space that generate radio signals, like pulsars and
nebula and quasars. So in other words, there's a lot
(31:08):
of potential noise to deal with when we're looking for
a radio signal. So again, finding that that signal amid
all the noise is a big challenge. It doesn't it's
not just important that our antenna is sensitive. We have
to have it really directional so that we can make
certain that we're pointing at the thing we want to
(31:29):
to listen to. Otherwise we might mistake some errant Earth
generated signal as being our spacecraft and then we're on
the wrong track. So to meet these challenges, NASA, through
the JPL, established the Deep Space Network or d s N.
The DSN has three facilities that are approximately a hundred
(31:51):
twenty degrees apart in longitude, so that means they're roughly
equidistant from each other Longitudinally, you multiply one twenty by three,
you get three sixty. That's a circle, right. So one
of the three is in Goldstone, California. That's actually in
the desert. It's northeast of Los Angeles, it's south of
(32:13):
Death Valley, and it's pretty far away from stuff that
generates radio waves. The second is in Madrid, Spain, so
that's almost smack dab in the middle of Spain. The
third is in Canberra, Australia, which is on the coast
in the southeast of Australia, and it's probably covered with
(32:33):
venomous animals. I mean it's Australia, so it's a safe bet.
So again, because these sites are a hundred twenty degrees
apart from each other, we get that three hundred sixty
degree view if you will, of the space around Earth.
And what this means is that at any given time,
at least one of the three DS insights has the
(32:56):
ability to establish a line of sight communication channel with
a distant spacecraft. And as the world rotates and one
of these sites begins to lose contact, the next one
will pick it right up, so communications can remain online.
You don't have an interruption. This is a big thing.
(33:17):
Like you might remember when I was talking about space stations.
When the most recent as of this recording, anyway, when
the most recent module joined the International Space Station, it
was the Naka from the from Russia. When it docked,
its thrusters misfired. They weren't supposed to fire, but they did,
(33:40):
and it caused the space station to rotate and move
into the wrong orientation relative to the Earth. Well, when
that happened. The space station was not in range of
Russia's mission control because that was on the wrong side
of the Earth at the time, and that was a
problem because Russia was the only entity that had the
(34:01):
ability to control the thrusters on the Knocka. So that's
an example of why it's important to have established these
points where we can have uninterrupted contact. All right, we've
got more to say about navigation, but before I get
totally lost, let's take another quick break. So at these
(34:29):
different sites at Goldstone, California, and Madrid, Spain, Canberra, Australia,
they have a series of radio telescopes, including one really
big one at each of these sites, and the deep
space radio antenna can be extremely impressive. So the Goldstone
Mars dish, that's the biggest one at gold Stone, California.
(34:52):
That one was built in nineteen but it was later
upgraded in night so today it measures seventy eaters across,
that's about two thirty feet. Now, this dish has a
surface area of around an acre or square feet or
three thousand, eight hundred fifty square meters. It weighs nearly
(35:14):
three thousand tons. It's mounted on massive machinery that can
tilt and turn the dish so that it can be
aimed precisely where a spacecraft is overhead and get you know,
that laser like focus with a communications channel with a
spacecraft that could be millions or billions of miles from
(35:36):
the Earth. And the purpose of those big big dishes,
I mean, it's all about collecting the very weak radio
energy that's being sent back from the spacecraft. You know,
because again that distances is intense, like the signals are
very very weak by the time they get back to Earth,
especially for super distant spacecraft. So this big dish is
(35:59):
able to like that energy and then direct it toward
the antenna itself, so you can think of it this parabola.
It's all focusing that radio energy to a specific point,
that point being the end of the actual radio antenna.
Otherwise the signal would be so weak that it would
be difficult, if not impossible, to detect that signal through
(36:21):
all the noise, and engineers have to position and antenna
precisely to beam radio instructions back to the distant spacecraft.
If you're off even by a little bit, the message
is not going to end up going to where you
need it to. Go, and the further out the spacecraft
is from Earth, the more critical it becomes to get
that just right. So all of that is to set
(36:44):
up the actual talk about navigation itself. We had to
set all those parameters to talk about the process. Without
the system like the Deep Space Network in place to
communicate with spacecraft navigation would be impossible. We would not
be able to build a spacecraft capable of detecting its
own orientation and velocity and to make changes on the fly.
(37:07):
It's not like Star Trek or Star Wars, where you
just tell an on ship computer to plot a course
for Javin or whatever. NASA describes space navigation as being
the domain of three large departments within NASA. There's mission design,
there's orbit determination, and there's flight path control. These three
(37:29):
things all inter relate to one another. So first we
have mission design. Now, this is the part of navigation
in which mission control has to determine what the intended
trajectory is for the spacecraft, where is it supposed to go. Now,
this alone is tricky for all the reasons that I've
talked about earlier in this episode. You've got to take
(37:50):
all those different elements into consideration, like relativity. You know,
what is it that you're hoping to do? What what
is the purpose of the spacecraft? What are the things
that are going to affect the spacecraft as it travels
from Earth to get to where it's supposed to go.
How do you account for those things? And how do
you either incorporate stuff so that it, you know, it
(38:14):
becomes part of your mission, or how do you find
a way around an obstacle or challenge. Now, keep in mind,
we learn new stuff the more we send spacecraft up,
Like we learn more every single time, and some of
that stuff is important and it affects calculations, like important
for the sense of navigation, that is, it's always important.
(38:36):
Someone has to take the things we've learned and then
build that into software that we use to calculate complex
equations in order to plot out navigation. So space navigation
is something that has evolved over time, and as we
learn more and incorporate what we've learned into the next
generation of software, it's constantly in a state of change.
(38:59):
So one thing that engineers have to factor in is
the fact that a spacecraft is always orbiting. Something I
kind of indicated this at the top of the show.
You know, it could orbit the Earth. It could be
in low Earth orbit and just stay there, but it's
in orbit. Or you might want it to leave Earth
orbit and enter into a solar orbit, so now it's
(39:22):
orbiting the Sun just like Earth or Mars, or you know,
any of the other planets are. Maybe you want to
enter into a different planets orbit. Maybe you want it
to leave the Solar System entirely, in which case it's
in a galactic orbit. It's orbiting the center of the
Milky Way, just like our Solar system is. It does
(39:42):
require an awful lot of velocity in order to escape
the Solar System. By the way, but we have done it,
not with you know, humans obviously, but with spacecraft we've
sent out anyway. Orbits are a huge part of navigation calculations.
And again you're talking about an origin and a destination,
and that are from an outside reference both in motion,
(40:04):
and we have to take into account the effects of
stuff like relativity with spacecraft. That's going to affect things
like the clocks on board the spacecraft. That means we
have to account for those changes due to relativity or
else we risk losing track of the spacecraft. We might
point an antenna at where a spacecraft either used to
(40:24):
be or might be in the future, but isn't right now,
just because we have these deviations from between our earthbound
clocks and the ones that are aboard the spacecraft. So
the software has to plot a trajectory that has to
take all these different things into consideration, and as you
can imagine, this means those calculations get pretty darn complicated,
(40:47):
particularly when you're looking at something you know, really ambitious,
which sounds a little weird to say, because I still
think just getting something into low Earth orbit is being
really ambitious. But you know, there's there is a scale,
I guess, and as I'm sure we're all aware, software
does not always come out perfect, right. You've probably used
software where you've encountered a bug or a glitch, Like
(41:10):
maybe you're playing a video game and the textures failed
to load and everything looks weird. Well, that's irritating when
it's a video game, but when you're talking about like
interplanetary navigation, a bug or glitch can become an enormous challenge. Now,
it might not be a show stopper. You might be
able to work around it, but it likely will require
(41:30):
a lot of people to work out a solution on
the fly in order to make a sure that a
spacecraft's route is in fact the right one to do
whatever it is you want that spacecraft to do, all right. So,
once all of that has been taken into account, engineers
calculate the navigational route for a spacecraft. This planned route
is the reference trajectory, so this is the route the
(41:53):
spacecraft should be on. Also, there's a cesall relationship between
the navigator for a mission and the software developers who
are making the navigation software. So as navigators find bugs
or they encounter new situations that necessitate new features in
the software, they can relay that to the developers, and
then they take the feedback and they produce new versions
(42:15):
of the software. I imagine that gets increasingly challenging to do,
especially to incorporate new features. Any developer can tell you
that it can be a nightmare to put something new
into code that you've just gotten to work, because the
chances are you're going to break something that previously had
been working. Okay, just imagine that for things that are
traveling through space and you have to deal with relativity
(42:38):
and stuff. Now, even when you do that correctly, which
you know obviously requires a lot of work. It does
not mean that a spacecraft is just going to magically
stick to that reference trajectory. All sorts of things can
cause the spacecraft to deviate from the planned route. In
some cases, it might be on purpose us such as
(43:01):
you know, you might have to do a maneuver to
avoid a potential collision. Or it might be that your
pathway is taking you close to a planet and you're
planning on using a gravity assist two make the spacecraft
continuance journey. But in other cases, something might pop up
that wasn't anticipated, Like it happens very quickly. Maybe the
(43:25):
spacecraft passes some large asteroids and the gravitational attraction between
the spacecraft and the asteroids pulls the craft out of
its trajectory a bit. Or maybe it turns out that
the software had a bug in it or a blind
spot that failed to account for something, and the spacecraft
is veering off course a bit as a result. I mean,
even solar pressure, that is pressure from light itself hitting
(43:50):
the spacecraft can be enough to push the spacecraft off
its reference trajectory. This is where orbit determination comes in. Now,
as the name implies, this part of space navigation is
just keeping track of a spacecraft's actual position. We know
where we want the spacecraft to go, but this is
(44:11):
about us figuring out where the spacecraft actually is. And
that is another kittle of fish. NASA breaks down orbit
determination into three sub processes or subgroups, So there's orbit reconstruction.
This is asking the question where has this spacecraft been?
(44:31):
This is all about determining the past route, the past
locations for the spacecraft to understand its actual trajectory versus
the reference trajectory. Then you've got orbit determination. This is
like asking where the heck is the ding dang thing
right now? Then you've got orbit prediction. This is like
asking where the heck is this thing going to next?
(44:55):
And and at this point I kind of wish I
hadn't burned the mean Girls reference about gettin loser we're
going shopping, But I did that one already. Anyway, A
bit of consideration reveals that all three of these things
are important. See, we're not controlling these spacecraft in real time.
You know, you don't have someone sitting looking at a monitor.
(45:17):
They get a first person view of a spacecraft's viewscreen.
They've got a joystick and they're just making it fly
all over the place. That's not how this works. And
it does take time for a signal to pass from
one point in space to another. The fastest that this
can happen at is the speed of light, and you know,
light is wicked fast. It's in fact the fastest stuff
(45:38):
there is. But light can't traverse great distances in an instant.
I mean it takes about eight minutes for light to
go from the Sun to hit us here on Earth.
So as as spacecraft gets further from the Earth, the
information that we get back from those spacecraft becomes more dated. Right,
it's more about where the space was at the time
(46:01):
it transmitted, but minutes have passed between then and when
we're able to actually look at the data. So as
engineers get the latest telemetric data, they're actually looking at
stuff that's several minutes old. So we're not observing the
spacecraft directly. We're getting information back from the spacecraft, and
then we have to draw conclusions about what's going on
(46:23):
based upon the information we have. So if the data
indicates that perhaps a ship is drifting away from its
reference trajectory, we need to be able to look back
and see what the data has said about that. When
did that deviation begin? How long has it been going on?
Where is the spacecraft now? Based on the information we have,
(46:43):
keeping in mind we're projecting forward a few minutes because
the information we have is older. And then, knowing that
our ability to pinpoint a spacecraft position with precision decreases
the further out the spacecraft gets from us, we have
to start building in margins of error, and then, based
upon all we know, where is it heading to now
(47:06):
and how can we get it back on track? If
we were not able to determine this, we wouldn't know
where to point the antenna with the d s N
in order to track the spacecraft. We would lose the spacecraft,
let alone being able to figure out how to correct
its its course. And you know space is big, so
(47:27):
if you lose something like a spacecraft, good luck finding
it again, because you'd you'd just be scanning regions of
space looking for the faintest of radio signals to try
and get back on track. It's not something you want
to have happen. Well, we've got some more to talk
about with navigation before we wrap all this up, So
(47:48):
let's take one last break. Okay, I was talking about
orbit re construction before the break, and we also need
to reconstruct the path of the spacecraft in order to
make sure that the scientific data that we're collecting with
(48:08):
this satellite. I mean, presumably we've sent it up there
to do something, well, some of it is determined by
the trajectory path, Like we have to know the trajectory
path in order for the data to make sense. There's
imaging data. There's like a type of imaging called synthetic
aperture radar imaging, and that requires that we have a
(48:29):
precise knowledge of the spacecraft's trajectory so that the software
we use to process that information can create a meaningful
image from that data. Like it's not like it's sending
a JPEG to us. It's sending us data that we
then use to create an image with sophisticated software we
(48:50):
have running on computers here on Earth. If we don't
know the trajectory precisely, then it's almost like we're building
a picture, but we're doing it from the wrong perspective,
and it would leave us with an image that's nothing
like what we were actually trying to capture with the satellite.
So it is imperative that we know the actual trajectory
(49:11):
of a spacecraft. Then we have flight path control, the
third of the three departments I was talking about. Again,
you could probably guess what this is about based on
the name. We can summarize this by saying that this
is the part of space navigation where we figure out
how to get the spacecraft from where it actually is
(49:32):
to where it is supposed to be. How can you
get the spacecraft to return to the reference trajectory that
you had set in the beginning. And here's another good
reason to put so much emphasis on orbit determination, because
spacecraft have a very limited set of options that they
can use in order to return to the correct course.
(49:52):
An interplanetary spacecraft, like a satellite that's designed to fly by, say,
one of Saturn's moons, it needs to on course once
it separates from the launch vehicle. You know, once it
separates from the rocket that pushes it up into space.
So this is kind of like a bowler releasing a
bowling ball. You know, once that ball leaves your hand,
(50:14):
there ain't no amount of you waving or leaning that's
really going to affect the ball's trajectory. You've set it
on a path and you can't really influence it anymore. Well,
we can still influence spacecraft a little bit, but that
initial release from the launch vehicle is the primary thing
(50:35):
of putting it on its proper trajectory. So spacecraft can
have like thrusters or rocket engines that can fire to
adjust their course a little bit. But obviously there's actually
a limit to how much fuel any spacecraft is able
to carry. And there ain't no gas stations in space,
at least not in our neighborhood. So fuel is a
limited resource. And that's you know, an understatement, but it's
(51:00):
important to remember. So let's say a spacecraft has gone
a little bit off course. The flight path control group
then has to figure out how far off course is it.
Then they have to figure out the commands needed to
have the spacecraft returned to its reference trajectory. So they
take the data from the orbit determination group that tells them, okay,
(51:20):
how far off course are we. Then they start running
the calculations how much is it going to take for
us to get back to where we need to be? Uh?
This means figuring out the proper change in velocity for
the spacecraft and if it's been a while since you've
had physics, I want to remind you that velocity isn't
just speed. A lot of folks use the word velocity
to stand in for speed, but that's just part of it.
(51:42):
Velocity is a vector. So in addition to speed, we
have to have a direction. So a change in velocity
is an acceleration. It can be a change in speed
or direction or both. The flight path control team figures
out what change in velocity is necessary, so they indicate
the magnitude and the direction that is required in order
(52:04):
for the spacecraft to return to its reference trajectory. Now,
the flight path control team doesn't initiate the actual maneuver.
They just designed the maneuver, or at least they design
the parameters of the maneuver. It needs to have this
change in direction and this magnitude. They send that information
to another team, a spacecraft engineering team, and it's this
(52:25):
group that then use stuff like attitude control systems and
thrusters or rocket engines to produce the change in velocity
that was indicated by the flat flight path control teams. So,
in other words, they're the ones to take this data
that says, here's what we need to have happen, and
they're the ones to actually activate the systems to make
it happen. NASA refers to small flight path control maneuvers
(52:48):
as trajectory correction maneuvers, which makes sense. You're trying to
correct it move it back to its reference trajectory. Now,
as we conduct more space exploration, we learned things that
are really able for future missions. So, for example, if
we were to plot out a fly by satellite to
go to Saturn, we would incorporate some gravity assist fly by.
(53:08):
It's most likely this is where you leverage the gravitational
pull of celestial body like another planet, to assist the
spacecraft on its way by giving it kind of a
slight pull slash push towards its destination and a change
in velocity. It's a boost, kind of like someone giving
you a little push when you're swinging on a swing set,
(53:28):
although it can also you know, change your direction somewhat.
And as we understand these things and we're able to
build it in, we can have that planned from the beginning,
so we can build them into an actual mission. When
we've got a good handle on those things, NASA can
call any sort of velocity maneuvers that we know we're
going to have to do deterministic. That is, we have
(53:52):
already determined the velocity maneuvers that we will need to
conduct in order to maintain our reference true jectory given
the route that we're following. We know that they are
going to be maneuvers necessary to stay on course, and
we have a good idea of what they are and
when we will need to execute them. But there are
(54:13):
other types of maneuvers that will call stochastic. These are
maneuvers that we know we're gonna need to make, but
we don't necessarily know. More like, we don't know what
the magnitude of those changes might have to be. We
might not fully understand the effect of those maneuvers ahead
of time, because we're going through uncharted ground, if you
(54:35):
if you will. We're not following something that we've already done,
where we already kind of have a grasp on what
we need to do. And then, of course occasionally sometimes
we have to do these maneuvers when something we didn't
anticipate at all happens and we have to design and
conduct a maneuver kind of on demand. Also, I should
mention that attitude control I mentioned it earlier Attitude control
(54:58):
is not about whether or not you're spacecraft is sassin you.
It has nothing to do with sass. Attitude refers to
the angular orientation of a spacecraft given some other point
of reference. Again, you have to have a point of reference,
because I mean, if you think about it, outer space
doesn't really have an up or down. You have to
(55:19):
have a point of reference and compare your position to
that point of reference, or you have to, you know,
use your point of reference to in order to make
a determination about something's positions. So you've gotta have a
point of reference to start from in order for you
to say something like that durned things upside down or
backwards or you know, or whatever. Also, there are interesting
(55:41):
ways to change the angular orientation of a spacecraft, and
some of them do not involve thrusters or rocket engines. Instead,
they might involve something like momentum wheels, as in, you know,
physical wheels a rotor spun by a motor. Now I
would get into this further, but that requires a pretty
(56:02):
long discussion about things like the conservation of momentum and
equal and opposite reactions and stuff like that, and we
don't really have time for that this episode. Is already
going super long, and I don't want Tardy to hate
me more than she does already. So she doesn't hate me, folks,
She's super nice to me. I just want to make
that clear. That was more of a jest. But the
(56:24):
short version of all this is that using stuff like
momentum wheels can make it possible to change the attitude
of a spacecraft without having to use thrusters. And as
we've previously established, fuel is a precious resource. So that's
a good thing to know, right to build in systems
that allow us to make changes to a spacecraft's orientation,
(56:45):
for example, without having to burn fuel to do it. Still,
it's not uncommon that the spacecraft engineering team will actually
have to initiate a rocket engine or a thruster ignition
to provide the thrust needed to get back onto the
reference trajectory. Uh, the timing on these maneuvers has to
be incredibly precise, both because you don't want to waste
(57:07):
even a drop of fuel if you can help it,
and also that if you fire a thruster for too
long or not long enough, then you're not going to
return to your reference trajectory. So imagine, for a moment
that you're sitting in mission control and you are relying
on complex calculations from data sent back to you from
(57:27):
space just to tell you where that something is right,
and that's something the spacecraft is so far away that
there is no means for us to observe it directly.
All you have is the data coming back to go by,
and then you have to come up with the command
to send via radio back up to this object to
(57:48):
get it back onto the course it's supposed to be
following the data you have indicates that the thing you
know was in this one particular position several minutes ago,
So you have to figure out where it is now
based on calculations with the data that you have, knowing
that that is at best maybe a precise approximation, but
(58:09):
still an approximation. Then you figure out the command you
need to send to where the spacecraft is going to
be in order to get it to take the action
to get it to where it should be. This whole
thing is mind boggling to me. It kind of makes
me think of like submarine navigators who use precise charts
(58:30):
and timing like a stop watch in order to plot
underwater courses. Because they're not able to just look outside.
I mean, at the depths that submarines can travel. You
can't have windows because they would collapse in from the pressure.
So you're in a tin can underwater, and you're using
(58:51):
very precise maps and a stopwatch and knowledge of how
fast you're moving in order to make calculations to determine
whether or not you're going to bump into something. It's
hard for me to even contemplate. Well, once the spacecraft
engineering team has done their thing, that whole process has
to repeat itself. The orbit determination team has to figure
(59:12):
out if the spacecraft is in fact back on its
reference trajectory, or if they'll need to conduct another maneuver,
and so on and so forth. And because there's so
many little things that can pull a spacecraft off track,
this becomes a continuous process. It's incredible to me that
people figured this stuff out, that they figured out how
(59:34):
to not just not just how the universe works, Like
it's already incredible to me that Einstein was able to
determine these in incredible theories of special and general relativity,
but then for people to build on that and to
make technologies that take that into account, it's it's phenomenal.
It also tells you by the way science works, right,
(59:57):
because if science didn't work, the technology we build that
leverages that science, it wouldn't work. So science works or
else are tech wouldn't work, especially when it comes to
space navigation. Now, there is a lot more that we
could say about spacecraft navigation. I haven't really gone into
deep details here, uh like how do you determine what
(01:00:21):
a spacecraft's velocity is? For example? I haven't talked about
that process, but I felt this was a good if
you will high level overview of the topic. If you
would like to know more details, let me know. I'll
research and write and do a follow up episode and
it will reference this one, but it will go into
much more detail to talk about the actual processes and
(01:00:43):
technologies we use to do the things I've talked about
in this episode. If you want that episode or any
other topic on tech Stuff, reach out to me on Twitter.
That's the best way to get in touch with me.
The handle we use for the show is tech Stuff
H s W and I'll talk to you again really soon. Yeah.
(01:01:07):
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