How does a space probe like Voyager 2 which was launched in 1977 visit the four outer planets and travel over 17 billion kilometres over space with almost next there 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 100 kilometres of its target and all with mid-1970s technology.
In the movies spacecraft just seemed to fly where they want and get there in no time at all but in our version of reality it’s somewhat more complicated and takes much, much longer to get around. It took nine years for the New Horizons probe to get from Earth to Pluto, a distance of about five billion kilometres and that was one of our fastest spacecraft. 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 the key to all of this is knowing how gravity works and how it affects not only you and me but also 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 a basis for Newton’s 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 spacecraft with incredible accuracy.
Newton’s first law states that an object at rest or travelling a straight line will stay that way unless a force acts upon it. A rock for example on the ground won’t move by itself unless something else picks it up or pushes it along. If that same rock were in space and moving in a straight line it will not change its speed or direction of travel unless an external force acts upon it. In space, there is always a force acting on a moving body and that force is gravity be it from the Sun, a planet or 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. Newton’s second law states that an object’s speed will change when a force is applied to it this is also reversible so a force is generated when its speed changes. If you fire a projectile on 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 it’s still travelling in a straight line. However, Earth’s gravity continuously pulls on it and when the curvature of its trajectory matches that of the earth it is now said to be in orbit around the earth.
In other words, the force of a projectile trying to go in a straight line is matched by that of 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 a very thin atmosphere high above the earth. This slows them down and as the force keeping them in their orbits becomes smaller, the balance between this and gravity gradually tips towards gravity. As it pulls them down further as they get lower the atmospheric drag becomes even greater reducing the speed even more without a periodic boost in speed to increase their orbit they will eventually come back to earth.
However, if a spacecraft increases its speed the orbit will become larger and more elliptical but it will always return to pass through the point where the speed was originally boosted. If the speed of our craft is increased enough then it will escape the pull Earth’s gravity and enter an orbit around the Sun.
Increase for speed more and it will increase the size of its orbit. If we get the speed boost correctly timed with an approaching planet in what is called an “opportunity” we can get the orbit of our spacecraft to intersect the orbit of a planet, 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 method and the interplanetary transport network method.
Once our spacecraft is under the gravitational pull of another planet it can either enter into an orbit around the planet or can use the planet’s gravity to slingshot around it or use gravity assist as known and increase the craft’s speed relative to the Sun.
Gravity assist works by using a planet’s gravity to pull on our spacecraft as it flies close by and can be used to increase or decrease a spacecraft speed and as such make its orbit larger or smaller and change its direction of travel. If our craft is flying in the direction of motion of that of the planets then it’s will speed up. If it flies in an opposing direction then it will decrease its speed, depending upon how it approaches the planet its course can be changed dramatically and can even leave travelling in the opposite direction.
But there is no such thing as a free lunch and in order to obey the law of conservation of energy, what energy our craft gains the planet must lose. When the voyagers used Jupiter to increase their speed to get to Saturn, Jupiter’s orbit around the Sun slow but only by about one foot per trillion years. We can use this gravity assist method to move from planet to planet further and further away increasing our craft speed as we go until it reaches escape velocity the point where the spacecraft 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 Sun’s gravity will still pull on the craft and slow it down, in fact, the Sun’s gravitational effect extends out about two and a half light-years and it will take Voyager travelling at over 60,000 km/h 40,000 years to reach the point where the sun’s gravity no longer dominates.
Newton’s third law states for every action has an equal and opposite reaction. Basically, the thrust from an engine pushing backwards moves a 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 when there is nothing for them to push against. We use this thrust to increase or decrease speed and as such, change our spacecraft’s orbit as well as move it in its X from Y planes with thrusters to orientate its antenna with earth or point its cameras towards a target.
Once we know how gravity affects our spacecraft 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 but also to each other and other objects like comets and asteroids. This model is created from the planetary ephemeris which is like a time table for all the major bodies in the solar system and gives their positions relative to the Sun for any given time both in the past and the future. This data has been built up over centuries. Where the first ones being created by the Babylonians as far back as 1200 BC.
By using celestial mechanics it is possible to calculate ephemeris for several centuries 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 be in the years ahead. However, these ephemerides are not perfect due to the gravitational effect of unknown asteroids and maybe because of yet unknown Planet X far beyond Pluto.
NASA has updated its ephemerides almost every year for the last 20 years as new data has come to light. So knowing how our spacecraft will move in space and the position of the planets well into the future this allows navigators to plot a course for our spacecraft with incredible accuracy. This can be seen with the Voyager missions, they used a planetary ephemeris to find a once in a 175-year alignment in the planets Jupiter, Saturn, Uranus and Neptune.
This alignment was discovered by Gary Flandro in 1964 while working at JPL and allowed the planers to come up with the Grand Tour. This would allow one spacecraft to visit all four planets by using gravity assist and cut their mission time from 40 years to less than 10 if they launched in 1977. The original Grand Tour was to include Pluto but due to funding limitations it was left out but Pluto was visited by the New Horizons probe in 2015. Voyager 2 was the first to set off in 1977 on other grand tour of the four outer planets and eventually travelled out in the plane of a solar system. This same technique of gravity assist has since been used on the Galileo, Cassini and the New Horizons missions.
Voyager 1 launched three weeks after Voyager 2 on a quicker route to visit Jupiter and Saturn and do a flyby of Saturn’s moon Titan. But this would then put it on an upward trajectory and out of a plane of a solar system to interstellar space.
Now we have a plan but we still need something to guide our spacecraft along its planned trajectory. For this, they use an inertial navigation system basically this is a highly accurate system of gyroscopes, accelerometers and other sensors that can detect movement of a 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 kilometres per hour so if a journey to Mars lasted eight months, which will be 5760 hours, then the error would be about 6300 km by the time it reached Mars, far too much when you have to enter an orbit with an accuracy of just 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, spacecraft use optical sensors and cameras to determine their position and reset the inertial navigation systems. On the Apollo missions, the crew used a space sextant to correct the drift on 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 a radio signal from Earth. Newer spacecraft 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.
Even with the best-planned course things will vary along the way. Other forces can also affect a 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 spacecraft and has to be corrected for and timing is everything. Our spacecraft must arrive at particular points in space along the journey within a very small window of time. Travelling at 30 km/s and approaching a planet to use its 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 course.
To communicate and work out the distance and speed of a 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 a spacecraft at all times. By sending a radio signal to the craft and having it returned the signal and 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 living 3 meters and its speed to within 180 millimetres 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 the Rosetta probe and it’s Philae lander and to take close-up pictures of Pluto within a two-hour time window, 9 years after launch and 5 billion kilometres away and when we only had one third of Pluto’s orbit mapped.
Five spacecraft have now achieved escape velocity using these methods we have spoken about and are now the farthest objects created by man. Pioneer 10, 11, Voyagers 1 & 2 & New Horizons. It’s incredible to think that all of this was done based on theories that were developed hundreds of years ago by observation only and the desire to figure out how the heavens worked long before we even thought it was possible to get into space let alone use gravity as our main engines.