how do spacecraft navigate in space

How do spacecraft navigate in space ?

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How does a space probe like Voyager 2, which was launched in 1977, visit the 4 outer planets and travel over 17 billion kilometres over 40 years with almost next to nothing in the way of fuel.

By the time Voyager 2 had reached Neptune, it had swung by Jupiter, Saturn and Uranus, travelled 7 billion kilometres and was still within 100km of it’s target and all with mid 1970’s technology.

In the movies space craft just seem to fly to where ever they want and get there in no time at all but in our version of reality its somewhat more complicated and takes much, much longer to get around. Can you image Spock saying to Kirk, “We just passed Pluto, almost home, only 9 years to go”, just incase you missed the relevance of that, it took 9 years for the New Horizons probe to get from Earth to Pluto, a distance of about 5 billion kilometres and that was one of our fastest space craft.

It might seem like an impossible task but when you know how space and physics work, it becomes a set of procedures, science fact instead of science fiction. And key to all of this is knowing how gravity works and how it affects not only you and me but everything in the universe.

The German mathematician Johannes Kepler first worked out the laws of planetary motion 400 years ago. Isaac Newton then used these as the basis for Newtons laws of motion and the creation of Classical Mechanics, the means by which we can predict the movement of everything in the solar system and beyond including planets, comets, asteroids and space craft with incredible accuracy.

Newtons 1st law states that an object at rest or travelling in a straight line will stay that way unless an external force is acts upon it.

A rock for example on the ground won’t move by its self unless something picks it up or pushes it along. If that same rock was in space and moving in a straight line, it will not change it’s speed or direction of travel unless an external force acts upon it.

In space there is always a force acting on moving body and that force is gravity, be that from the sun or a planet even another rock. Anything with mass exerts a gravitational force, the larger the mass the larger the force. The other component to a moving object is its speed.

Newtons 2nd law states that an objects speed will change when a force is applied to it, this is also reversible, so a force is generated when it speed changes. This also the reason why an asteroid travelling at 17km per second and hitting the earth can be so devastating, it can release a huge amount of kinetic energy with the sudden change in velocity.

If you fire a projectile on the earth parallel to the ground it will eventually fall under the influence of gravity back to the ground.

If you fire your projectile fast enough and it maintains that speed, its still travelling in a straight line, however earth’s gravity continuously pulls on it and when the curvature of this trajectory matches that of the earth and it’s is now said to be in orbit around the earth, in other words the force of the projectile trying go in a straight line is matched by of the gravity pulling it back to earth.

This is how Satellites and space stations stay in orbit. But they are also affected by the tiny amount of drag of the very thin atmosphere high above the earth, this slows them down and the force keeping them in their orbit becomes smaller. The balance between this and gravity gradually tips towards gravity as it pulls them further down. As they get lower, the atmospheric drag will become greater reducing the speed even more. Without a periodic boost in speed to increase their orbit they eventually come back to earth.

If, however a space craft increases its speed, the orbit will become larger and more elliptical, but it will all ways return to pass thru the point where the speed was boosted. If the speed is increased enough then it will escape the pull of earth’s gravity and enter an orbit around the sun.

Increase the speed more and it will increase the size of its orbit. If we get this speed boost correctly timed with an approaching planet in what is called an “Opportunity”, we can get the orbit of our space craft to intersect the orbit of the Planet in a method known as the Hohmann transfer approach, which is one of the most common ways to get from one moving body to another, although there are now more efficient but much longer ways like the Low thrust transfer and Interplanetary Transport Network methods.

Once there our space craft can either enter in to an orbit around the planet or we can use the planets gravity to sling shot around it or gravity assist as it’s known and increases the craft’s speed relative to the sun.

Gravity assist works by using a planets gravity to pull on our space craft as it flies close by and can be used to increase or decrease a space crafts speed and as such make its orbit larger or smaller and change to its direction.

If our craft is flying in the direction of motion to of that the planet then it will speed up. If it flies in an opposing direction, then it will decrease its speed. Depending on how it approaches the planet its course can be changed dramatically and can even leave travelling in the opposite direction.

But the is no such thing a free lunch and in order to obey the law of conservation of energy, what energy our craft gains the planet must loose. When the Voyagers used Jupiter to increase their speed to get to Saturn, Jupiter’s orbit around the sun slowed but only by 1 foot per trillion years.

We can use this gravity assist method to move from planet to planet, further and further away, increasing our crafts speed as we go until it reaches escape velocity, the point where it will be travelling fast enough to escape the pull of the sun and leave the solar system just like Voyager 1 has already done.

But the suns gravity will still pull on the craft and slow it down, if fact the suns gravitational effect extends out to about 2 and a half light years and will take voyager 40,000 years to reach the point where the suns gravity no longer dominates.

Newtons 3rd law states that every action has an equal and opposite reaction, basically the trust from an engine pushing backwards moves the craft forwards. Some people think that the thrust of a rocket pushes against the ground or the atmosphere and thus it’s impossible for them to work in space. This is clearly not the case as our rockets and thrusters don’t stop working once they are in space where there is nothing to push against. We use this thrust to increase or decrease speed and as such change our space craft’s orbit as well as to it move it in its X & Y planes to orientate its antenna with earth or point its cameras at a target.

Once we know how gravity affects our space craft and that we can use it to move from planet to planet, the next thing we need is an accurate model of the solar system. This will show us where the planets will be in relation not only to the sun also to each other and other objects like comets and asteroids.

This model is created from the planetary ephemeris which is like a timetable for all the major bodies in the solar system and gives their position relative to the sun for any given time both in the past and for the future. This data has been built up over centuries with the first one created by the Babylonians as far back as 1200BC. By using Celestial mechanics, it is possible to calculate ephemeris for several century’s into the future.

Because space missions last for years or even decades like the voyager ones, it would be impossible to plan missions without knowing where the planets would in the years ahead.
However, these ephemerides are not perfect due to the gravitational effect of unknown asteroids and maybe, an as yet unknown Planet X far beyond Pluto. NASA has updated its ephemerides almost every year for the last 20 years and new data has come to light.

So with the knowledge of how our space craft will move in space and the position of the planets known well in to the future, this allows the navigators to plot the course of our space craft with incredible accuracy.

This can be seen with the Voyager missions. They used planetary ephemeris to find a once in a 175 year alignment in the planets Jupiter, Saturn, Uranus and Neptune. This was discovered by Gary Flandro in 1964 whilst working at JPL and allowed the planners to come up with the “Grand Tour”. This would allow one space craft to visit all four planets by using gravity assist and cut the mission time from 40 years to less than 10, if they launched in 1977. The original Grand Tour was to include Pluto but due funding limitations it was left out, but Pluto was
visited by the new horizons probe in 2015.

Voyager 2 set off first in 1977 on the “grand tour” of the four outer planets and eventually to travel out in the plane of the solar system. This same technique of gravity assist has since been used on Galileo, Cassini and New Horizons missions.

Voyager 1 launched three weeks after Voyager 2 on a quicker route to visit Jupiter, Saturn and do a flyby of Saturn’s moon titan. But this would then put it on an upward trajectory and out of the plane of the solar system to interstellar space. On its way out, it was turned around for so it’s camera could face back to earth and take one last set of photos.

These are the farthest images of the solar system ever taken and one of them captured earths place in it. Covering just 0.12 pixels in size and in the middle of a lens flare, the famous “Pale Blue Dot” as Carl Sagen called it was taken 6.4 billion km away looking down at a 32 degree angle on to the solar system.

4) Now we have a plan, but we still need something to guide our space craft along its planned trajectory. For this they use an inertial navigation system. Basically, this highly accurate system using gyroscopes, accelerometers and other sensors that can detect the movement of the craft in any direction in space. Using this information, the navigators can work out if the craft is on course.

However inertial navigation systems are mechanical devices and as such suffer from what is known as integration drift, tiny errors in the gyroscopes and sensors. This is compounded over time because they calculate their position as they move along from the last previously calculated position, so the longer they go, the more the errors build up.

The error in a good system is less than 1.1 kilometers per hour. So if a journey to Mars lasted 8 months which would be 5760 hours, then the error would be about 6,300 kilometers by the time it reached Mars, far too much when you have to enter orbit with an accuracy of a few kilometres.

To compensate for the integration drift, another fixed reference system is needed, and this is the stars. Just as marine navigators used a sextant to work out their position, space craft use optical sensors and cameras to determine their position and reset the inertial navigation system.

On the Apollo missions, the crew used a space sextant to correct for the drift in the onboard navigation system. On the Voyager probes, they used a star tracker that could look for a very bright guide star, which in Voyagers case was Canopus. It also had a sun sensor that could be used in conjunction with the radio signal from earth to triangulate it’s position. Newer space craft have more sophisticated systems which use cameras to look for known objects like planets and even comets and asteroids as well as the target itself.

5) Even with the best planned course, things will vary along the way. Other forces can also affect a space craft deep in space. The solar wind for example, the flow of charged particles from the sun, can over time gradually change the course of a space craft and has to be corrected for too and timing is everything.

Our space craft must arrive at a particular points in space along the journey within a very small window of time.

Travelling at 30km per second and approaching a planet to use it’s gravity to swing by and change course, if you are out by more than a few minutes or so, it could mean the difference between being sucked into the planet by its gravity or undershooting the planned coarse.

To communicate and workout the distance and speed of the craft, NASA uses the Deep Space Network. This is a network of radio telescopes spread around the world so that at least one will be in contact with the space craft at all times.

By sending a radio signal to the craft and then having it return the signal, using the Doppler effect and a highly accurate atomic clock, the slight difference between the two signals can be used to calculate its distance from earth to within 3 metres and it’s speed to within 180mm per hour.

Combining all this information we are now able to send space probes with incredible accuracy, so much so that we can now land on a comet as we did with Rosetta and its Philae lander and take close up pictures of Pluto within a 2 hour time window, 9 years after launch and 5 billion kilometres away and when we only had 1/3 of Pluto’s orbit mapped.

Five space craft have now achieved escape velocity by using the methods we’ve spoken about and are now the farthest objects created by man, Pioneer 10, 11, Voyager 1 and 2 and New Horizons.
It’s incredible to think that all this was done based on theory’s that were developed hundreds of years ago by observation and the desire to figure out how the heavens worked long before we even thought it was possible to get in to space, let alone use gravity as our main engines.

The ability to work out the Orbits of the planets is a key fundamental in planning any space missions that we will be doing for the foreseeable future and it something that you can do yourself with the help of the guys over at Brilliant is a problem solving website that allows you to learn by solving real world or in this case outer space problems.

They have a complete section dedicated to Gravitational physics covering not only Newtonian gravity but also Keplerian orbits where you can work out how to plot a course to mars using the Hohmann transfer method just as if you were sending your own space craft to explore the solar system.

You can also test your abilities with the classical mechanics section, the discipline that Newton helped create that underpins our ability to predict the actions of any object in space, something any budding space navigator has to know.

To help support us at curious droid and learn more about Brilliant, just click on the link to and sign up for free. As a special bonus for curious droid viewers, the first 200 people to sign up will get a 20% discount off their annual subscription.

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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