Leaving and arriving: how spacecraft get about

Space is not far away. It starts about 100 km above your head, closer than many towns are to each other.

The hard part is not getting up there. It is going fast enough to stay. To keep circling the Earth without falling back, a spacecraft has to travel at about 28,000 km an hour. That is why rockets are so big: nearly all of a rocket is fuel, burned to reach that speed.

Once it is going fast enough, a spacecraft can switch its engines off. There is no air to slow it down, so it just keeps coasting, for years if need be.

To go to another planet, you do not aim at where the planet is now. You aim at where it will be when you arrive, months later, like throwing a ball to someone who is running.

Two speeds. Two numbers govern leaving any world.

Orbital speed is how fast you must go sideways to keep falling round it without coming down. Just above the Earth’s air that is about 7.8 km a second, and one lap takes about 90 minutes.

Escape speed is how fast you must go to leave for good, coasting, with no further push. It is always about 1.4 times the orbital speed at the same height.

Leaving from the surface of	Escape speed
The Moon	2.4 km a second
Mars	5.0 km a second
The Earth	11.2 km a second
Jupiter	About 60 km a second
The Sun	618 km a second

The small figure for the Moon is why the Apollo crews could leave it in a craft the size of a van, when leaving the Earth had taken a rocket 110 m tall.

Why rockets are mostly fuel. A rocket has to carry its fuel with it, and then carry the fuel needed to lift that fuel. The sums are unforgiving. The Saturn V that sent people to the Moon weighed nearly 3,000 tonnes on the launch pad, and well over four fifths of that was propellant. Less than 50 tonnes went on towards the Moon.

Coasting between planets. After the engines stop, a spacecraft is simply in orbit round the Sun, like a very small planet. The cheapest route to another planet is half of a long oval orbit that touches the Earth’s orbit at one end and the target’s at the other. A short burn at the start puts the craft on the oval, and it coasts the rest of the way.

It is slow, and it only works when the planets are in the right places. A trip to Mars this way takes seven to nine months, and the chance to set off comes round only every 26 months. That is why Mars missions leave in clusters.

Borrowing speed. A spacecraft that passes close behind a moving planet is pulled along by it and leaves faster than it arrived, as measured from the Sun. The planet slows down in exchange, by an amount far too small ever to measure. This is a gravity assist, and it costs no fuel.

Voyager 2 is the famous case. Launched in 1977, it used Jupiter to reach Saturn, Saturn to reach Uranus and Uranus to reach Neptune, arriving in 1989 after twelve years. Without the planets’ help the journey would have taken about thirty. The four giants line up that conveniently only about once every 175 years.

The trick works for slowing down too. Passing in front of a planet takes speed away. The Parker Solar Probe flew past Venus seven times to shed enough speed to drop close to the Sun, which is harder to reach than the edge of the Solar System.

Arriving. Getting there is half the job. A spacecraft arriving at a planet is going too fast to stay, and must slow down to be captured: by firing its engine, or by skimming the top of the atmosphere and letting drag do the work. If it does neither, it swings past and carries on.