Until we learned to properly navigate our
way across the oceans, early explorers were
fearful to lose sight of land in case they’d
be lost at sea.
They learned to use the water currents, winds,
movements of birds and of course, the positions
of the Sun, the Moon and the stars to find
their way across the seas to distant lands.
As we learned to launch spacecraft into orbit
and out into the Solar System, mission planners
needed to develop entirely new methods of
navigation.
In today’s episode, we’re going to talk
about how navigation in space works.
How did the missions to the Moon find their
way so accurately?
How is it done today?
And what are some clever ideas for the future
of space navigation?
Before I go into space navigation, I’d like
to talk about regular navigation here on Earth.
Your phone is equipped with a navigation system,
of course: the Global Positioning System.
This is a system of satellites surrounding
the Earth at an altitude of 20,000 km.
There are always a handful of satellites in
view of your phone.
These satellites are constantly broadcasting
their position and the current time.
From these signals, your phone calculates
its current location on Earth.
Before GPS, navigation was much more difficult.
But the main method, used for centuries, was
to use the stars as a guide: celestial navigation.
Imagine the Earth in space, with the stars
in a vast sphere around it.
At any point, there’s a star directly overhead
any part of the Earth.
And if the Earth wasn’t turning and you
wanted to go to a spot on Earth, all you’d
have to do is sail until that star was directly
above you.
Of course, the Earth is turning, so it gets
a little tougher.
As long as you know exactly what time it is,
and can measure the angle to various stars,
you can calculate your position on the Earth.
Sailors used a tool called a sextant which
allows you to measure the angle between a
star and the horizon.
For your north/south navigation, or latitude,
all you need to do is measure the angle to
the North Star, Polaris.
When Polaris is directly overhead, you’re
on the north pole, and when it’s right on
the horizon, you’re at the equator.
There’s a similar situation when you’re
headed to the south pole.
To measure east/west is a little tougher.
As the Earth turns, “noon” is overhead
in a different spot on Earth every moment.
When it’s Noon for you, you just have to
compare your time with the Prime Meridian
- an imaginary line running through Greenwich,
England - and that’ll tell you how far east
or west you are.
Learning celestial navigation isn’t that
difficult and it’s actually pretty fun once
you learn to do it.
And if you’re ever stuck without your GPS,
you’ll have a way to sail your ship at sea.
Now, let’s talk about how these systems
have been adapted for the purposes of spaceflight.
Spacecraft navigation is largely handled here
on the ground.
Radio dishes on Earth monitor the position
and velocity of spacecraft, and then upload
commands if they need to maneuver themselves
into a new orbit.
But once you want to leave Earth things get
more complicated.
For the Apollo missions to the Moon, NASA
developed the Apollo Guidance Computer, which
was a computer a little bigger than a modern
desktop PC.
There was one in the Command Module, and then
another in the Lunar Module.
The vast majority of the navigation for the
mission was done from Mission Control on Earth,
which was constantly tracking the position
and orbits of the Apollo spacecraft.
But as a backup, the astronauts used a space
sextant to compare the positions of stars
to the horizons of the Earth and Moon to make
sure the navigational calculations done from
Earth were correct.
It took the astronauts about 10,500 keystrokes
over the course of a lunar mission to input
their navigation data, and it was often referred
to as the “fourth crew member”.
The astronauts could input navigational information,
like the positions of stars, and then calculate
trajectories and maneuver angles.
It’s amazing to think that the astronauts
who flew to the Moon used a similar navigation
tool to what sailors here on Earth have been
using for centuries.
Even in space, it’s all just geometry.
Of course, your wristwatch is probably much
more powerful than the Apollo Guidance Computer.
But what if you want to go deeper into the
Solar System?
In order to properly navigate across the Solar
System, you need to know where the spacecraft
is, where you are, have an accurate map of
the Solar System, and know where your spacecraft
is going.
NASA spacecraft use the Deep Space Network,
which is an array of giant radio antennas
positioned around the world.
There are three main facilities in the DSN:
Goldstone in California, Madrid, Spain and
Canberra, Australia.
With these three main instruments, NASA can
communicate with any probe across the Solar
System.
As the Earth rotates, one spacecraft will
fall below the field of view from one antenna,
but another can pick up the signal.
In addition to sending instructions to spacecraft,
as well as retrieving data and photographs,
the antennas of the Deep Space Network help
with their navigation.
As signals go to and from the probes, they
experience a frequency shift.
Computers can then calculate velocity and
distance to the spacecraft based on this frequency
shift and the time it takes for the signals
to make a return journey.
By comparing its locations to a static map
of the stars in the sky, astronomers calculate
where it is and how fast it’s going.
The more measurements they make, the more
accurate their calculations.
Then they can compare these measurements to
a known map of the orbits of all the objects
in the Solar System to know where a spacecraft
is with amazing accuracy.
This navigation technique has enabled landings
on other worlds and precise gravitational
slingshot maneuvers from millions of kilometers
away.
At the JPL Solar Systems Dynamics website,
you can download highly accurate Solar System
maps, which give the current position and
motion of 780,000 asteroids, 3,525 comets,
8 planets, 178 moons, dwarf planets, the Sun,
and a few spacecraft.
The key to accurate navigation is to have
an accurate clock.
Your GPS is most accurate when it knows exactly
what time it is, down to about 2 nanoseconds,
or the amount of time it takes for light to
travel half a meter.
NASA is working on the most accurate clock
that will ever fly to space, and it’s called
the Deep Space Atomic Clock.
Right now, spacecraft rely on a ground signal
to give them an accurate time signal.
But if the spacecraft loses contact with Earth,
or runs out of navigation commands, there
isn’t much it can do but wait.
Normal atomic clocks are the size of a refrigerator,
but the Deep Space Atomic Clock is only the
size of a toaster, and uses a mercury-ion
trap to achieve an accuracy of down to 0.3
nanoseconds lost over a day.
In fact, this clock will be about 50 times
more accurate than the most accurate GPS clocks
already in space.
Future missions could be equipped with these
atomic clocks, to make sure they always know
what time it is, even when they’re far away
from Earth.
That’s how spacecraft navigate through the
Solar System right now, but there’s a better
way: pulsars.
And I’ll get to that in a second, but first
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Earlier this year, NASA announced that they
had successfully tested out a new technique
that might completely change the way we’ll
navigate through space in the future - by
using pulsars.
Pulsars, of course, are the rapidly spinning
neutron stars left over from supernovae explosions.
These pulsars can spin hundreds of times a
second putting out powerful radio waves.
As the pulsars spin, their radio signals sweep
past the Earth in absolutely precise intervals,
which can be used for high-precision timing
signals, similar to how GPS navigation works.
Engineers at NASA developed a clever experiment
that used these pulsar signals to figure out
the precise position of the International
Space Station.
It’s called the Station Explorer for X-ray
Timing and Navigation Technology, or SEXTANT.
Okay, that’s definitely a backronym.
The technology demonstration used the 52 X-ray
detectors and silicon-drift detectors on an
experiment attached to the station called
the Neutron-star Interior Composition Explorer
(or NICER).
This experiment is being used to study the
internal structures of neutron stars and especially
pulsars, but the SEXTANT team was able to
borrow it to do an amazing proof of concept.
In 2017, they focused NICER on four well known
millisecond pulsars and gathered 78 measurements
of radio signals from the pulsars.
Then an on-board computer calculated all these
signals together to figure out precisely where
the International Space Station was with respect
to the pulsars.
Within 8 hours of observations, they were
able to target the station’s location within
16 kilometers and at times within 5 km of
its position compared to GPS signals.
All while the station was hurtling at 28,000
km/h.
In theory, attaching one of these NICER instruments,
any spacecraft would then be able to know
precisely where it is in the Solar System
at all times.
Spacecraft could navigate themselves around
the moons of Jupiter or deep in the Kuiper
Belt, without needing precise updates from
NASA, always knowing exactly where they are
in the Solar System.
In theory, they could navigate themselves
to nearby stars.
And of course, if more pulsars were mapped
out across the galaxy and a timing system
was integrated together, you could imagine
a future version of this technology allowing
spacecraft to find their way across the entire
Milky Way.
Just download the pulsar maps for the region
of the Milky Way that you want to visit.
It’s a pretty cool idea.
What do you think?
Let me know your thoughts in the comments.
I’m always looking for new topics to cover
here in the Guide to Space, so if you have
any ideas for episodes, I’d love to hear
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