Space is that big, empty place outside of the Earth's atmosphere. It starts about 500km above the Earth's surface. We're used to seeing spaceships in the movies flipping around in space, doing tight turns and 'jumping to light speed'. The reality of space travel is quite different. For example, the real-life NASA 'New Horizons' mission to Pluto which took years to arrive at its destination couldn't stop when it got there to have a good look around. The problem is that the normal ways of moving around, such as pushing off something solid or catching the wind in your sails don't apply. And without air, there's nothing to slow you down. Once you get up to speed, you can keep moving at that speed with no extra effort, but you can't easily slow down again.
This Entry looks at real ways of speeding up and slowing down in space and how spacecraft go about manoeuvring.
Ways of Speeding Up
There are four theoretical ways of speeding up in space, only two of which have ever been used by real spacecraft:
Rockets - these fire stuff out the back of your ship, causing it to be pushed forwards. This is an application of Newton's Third Law of Motion - for every action there is an equal and opposite reaction. So if you fire something out the back, you'll be pushed forward. Rather than firing solids, rockets expel hot gas. The hotter the better: hot gas has more energy than cold gas, and therefore will exert a greater force forwards on the spaceship. The problem with a rocket is that you have to carry the fuel with you and it is used up as you go along. It would be great to have enough fuel to keep accelerating, but the fuel would weigh a lot, and you'd need to carry extra fuel just to support that weight.
Gravity assist - once you're in space, you can use a peculiar effect called the gravity sling, also known as the slingshot effect, to speed yourself up. You need a large planet which is orbiting around the Sun. If you aim your spacecraft to pass close to the planet, it will be pulled away from its straight course by the gravity of the planet and will end up travelling in a different direction. It will also have speeded up slightly, if you've chosen the right course relative to the way the planet itself is moving. Since total momentum can't change, the momentum your spaceship has gained must be lost by the planet, so the planet's orbital motion around the Sun will be infinitesimally slower. You can use the Sun instead of a planet, since the Sun is itself moving, orbiting around the centre of the Milky Way Galaxy. Many modern space missions use this manoeuvre, circling the Sun and looping back around the Earth, perhaps two or three times, before being fast enough to head out into the outer solar system.
Light Sail - this method has been proposed and one test flight has been performed (in June 2015), but it's never been used for any real-life space mission. The sun gives off a steady stream of light. If you put up a giant sail on your spaceship, it will be pushed by the light away from the sun. This could theoretically be used as a way of moving spaceships away from the sun. You can even make the ship go at an angle to the sun's rays by tilting the sail, but it is not possible to get closer to the sun using a light sail. You will also need a vast sail for any reasonable amount of thrust, as the pressure of light is very feeble.
Bussard Ramjet - space isn't actually empty. There is hydrogen gas very thinly spread throughout the entire galaxy. If you could get up to sufficient speed, you could collect this interstellar hydrogen at the front of the ship using some sort of a magnetic field, and push it into a jet engine, heating it up and shooting it out the back. This is known as a Bussard Ramjet, after its inventor, American physicist Robert W Bussard. It is different from a rocket - everything that comes out of a rocket was initially inside the ship. The ramjet, on the other hand, takes in stuff, heats it up and shoots it out, so it collects its fuel as it goes along. If this could be got to work, there'd be no limit on how far you could go with such a ship (within the galaxy). The down side is that this is still theoretical. Nobody's exactly sure how to build the magnetic scoop that collects the hydrogen.
How to Slow Down
On Earth we can slow down by just turning off the engine, taking down the sails or whatever. Friction will bring us to a halt. In space, without any friction, we'll keep going at the same speed forever (Newton's First Law of Motion). Slowing down turns out to be just as much of an effort as speeding up.
There are three main ways of doing it:
Rockets - firing your rockets forward in the direction you are pointing will slow you down. It takes the same amount of rocket fuel to slow down as it does to speed up.
Gravity Assist - the slingshot effect can be used in reverse to slow a spaceship down. By careful positioning of the spaceship's trajectory around a planet, the planet's gravity will transfer some of the ship's momentum to the planet, resulting in a reduction in speed of the spaceship and an infinitesimal speeding up of the planet.
Atmospheric braking - if the destination is a planet or a moon with an atmosphere, you can use that atmosphere to slow you down. For this you need to carry a heat shield which is heavy and heat resistant. The atmosphere in front of the speeding craft is compressed and this pressure pushes on the craft, slowing it down. But the compressed atmosphere also heats up to enormous temperatures and your ship is in danger of burning up, hence the need for the heatshield.
No expense was spared on the Apollo space missions - for example much of the lunar landers were gold-plated because gold is good for keeping out cosmic rays. The heatshields, on the other hand, were made from chicken-wire and concrete. These were used when returning to Earth, but since the Moon has no atmosphere, the moon landers had to use rockets to slow themselves down.
Ways of Changing Direction in Space
There are basically two ways of turning in space:
Rockets - fire a rocket out the side, at 90° to the direction you are travelling. This will cause the spaceship to change direction very slightly. This method is not suitable for large-scale changes in direction, as it needs a huge amount of fuel. If for example you want to turn through 90 degrees, you will need to absorb all the energy of the forward motion and then provide enough energy to get the ship moving in the new direction. That's twice as much energy as it took to get the ship going in the first place.
Use the gravity of a nearby planet or moon - swing around it and you can end up at more or less the same speed in any direction you like.
Spinning Your Spaceship
If your spaceship is moving along without any power being applied, it will keep going in the same direction forever. There's no reason, though, why the 'front' of the ship has to be pointing the way you're going. You can spin the ship without affecting its forward motion. The ship will turn around its centre of mass1, and the centre of mass of the ship will continue to travel in the same direction at the same speed.
One way to do this would be to fire small rockets known as thrusters in opposite directions. If you fire a small rocket forwards on the left side of the ship and another rocket backwards on the right side of the ship, the ship will spin towards the left. This doesn't change the direction the ship is going, just the direction it is facing. It will continue to spin to the left until you fire some more rockets to stop it.
Another way to spin the ship is to have a large flywheel inside it. Spinning the flywheel in one direction will cause the ship to spin in the opposite direction. When you want to stop the ship spinning, just stop the flywheel spinning.
The main reason for spinning a ship is to move the main rocket engines around so that they are facing forwards, which you may need to do to slow the ship down.
Another reason would be to provide artificial gravity - as the ship spins, everything in it or attached to it which is not on the axis of spin will feel an outward force. The further from the axis, the greater this force. This would feel to people on the ship like a force of gravity pulling them outwards. Simulated gravity would make life on board a lot easier for the crew - eating, drinking and other bodily functions are all much simpler if they are done with some gravity. It also would make it easier for the astronauts to do exercise.
This is all in the future, though. The Apollo trips to the moon had no artificial gravity, and in fact no way of exercising at all, but the journeys only lasted a few days. The International Space Station has a gym so that the crew can exercise, but it is not big enough to provide a decent rotating section with artificial gravity.
Realistic Space Travel in Science Fiction
Science fiction writers rarely write about the tedium of space travel - the launches and landings are exciting but the months or years of travel in between don't make for good stories. And that's just tootling around our solar system. If you want to travel to the stars, you're going to need thousands of years at the speeds we can reach today. So science fiction tends to rely on magic devices such as wormholes, warp drives and hyperspace to get quickly to where the action is.
Very occasionally a writer will describe realistic space travel. Stephen Baxter's Titan describes a future NASA mission to one of Saturn's moons, complete with all the US government politics that goes into funding any such mission. In Arthur C Clarke's novel 2001 A Space Odyssey, the trip to Saturn2 takes so long that most of the crew are in suspended animation. In his novel The Songs of Distant Earth, the whole crew is in suspended animation for the hundreds or even thousands of years needed to reach another star system.
Larry Niven and Jerry Pournelle feature a ship powered by a light sail in their novel The Mote in God's Eye. They also describe elaborate space battles in the sequel, The Moat Around Murcheson's Eye3, which involve a lot of tedious (and frankly confusing) matching of velocities and the practical details of speeding up and slowing down. Larry Niven's short story Rammer is about a man training to be the pilot of a Bussard ramjet.
To see artificial gravity based on spinning portions of the ship, we can look to Stanley Kubrick's movie of 2001 A Space Odyssey. It features a giant wheel-like space station with artificial gravity, and there is also a rotating section on the Discovery One spaceship which travels to Jupiter. The day-to-day living of the two pilots can be carried out in more Earth-like conditions than the average trip into Earth orbit. We see astronaut Dave Bowman running around the cylindrical running track to keep fit.
Even these authors, though, will resort to unexplained 'new science' which conflicts with the currently understood laws of physics in order to move the story along.
Real Space Travel
You'll find that real space missions to distant planets, on the other hand, are long-drawn-out affairs lasting years. It's not a question of just pointing towards Jupiter and pressing the thrust button. You could of course do this if you had unlimited fuel, but in real life, it's so difficult to get moving at a decent speed that the spacecraft will typically use any way it can to save fuel.
For example, the 'New Horizons' mission to Pluto used a rocket to escape from the Earth's gravity and to set it directly on its path at a speed of about 16km/s (58,000 km/hour), the fastest speed of any object to ever leave Earth. It then encountered Jupiter along the way and used gravity assist to speed up by another 4km/s, which knocked three years off the time to reach Pluto. In the next stage of the journey, small amounts (a few grams) of hydrazine rocket fuel were burnt to achieve tiny course corrections. Arriving at Pluto nine years after launch at about 20km/s, huge amounts of fuel would have been needed to slow the craft down, and all this would have to be carried with it, which would have made the craft very much bigger and more expensive. So the mission designers settled for a fly-by, in which the craft sped past Pluto, taking lots of photos and measurements.