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 I’d like to thank:
Steve Wolfhope Katrina Ince
Edi Kuhn Scott Bragdon
And the rest of our 836 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
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.
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