Orbits & satellites

Your sat nav knows where you are, the weather app shows cloud pictures from space, and in the evening you sometimes see a bright dot glide silently across the sky. Behind all of that are satellites, and behind satellites is a surprisingly simple idea: a fall that never reaches the ground.

Newton's thought experiment

Imagine a cannon on a very high mountain. Fire the ball slowly and it lands nearby. Fire it faster and it travels further before it lands. And if it is fast enough, something astonishing happens: the Earth is a sphere, so the ground curves away beneath the ball exactly as fast as it falls. The ball falls and falls but never arrives at the bottom. That is precisely what an orbit is. A satellite is nothing but a throw that misses the Earth, lap after lap.

Gravity as a central force

For something to fly in a circle, it needs a force that constantly pulls it toward the centre. In hammer throwing, that is the wire in the thrower's hand. For a satellite, gravity takes on this role: it pulls it toward the centre of the Earth at every moment and bends its path into a circle. If the thrower lets go, the hammer flies off in a straight line. If gravity suddenly vanished, the satellite would do the same. Our Moon has been doing nothing else for over four billion years: it is a natural satellite, endlessly falling around the Earth.

Why do astronauts seem weightless?

Many people think there is no gravity on the space station. Wrong: at an altitude of 400 km, gravity is still almost 90 percent as strong as on the ground. The astronauts float for a different reason: they are falling, together with the whole station, continuously around the Earth. When you fall, you don't feel your own weight, you know that tingle in your stomach when a lift starts moving downwards or a roller coaster dives. This fall is not completely undisturbed, though: up there the last traces of the atmosphere slow the station down, so a few times a year its engines give it a short push forwards and lift the orbit again. So weightlessness is permanent falling, not missing gravity.

Satellites in everyday life

GPS satellites orbit at an altitude of about 20,000 km and constantly broadcast their position and the exact time. Your phone compares the signals from at least four satellites and works out where you are. Weather satellites photograph clouds and storms from above, and some appear to hang still over one spot on Earth because at around 36,000 km above the equator they take exactly one day per lap and so turn along with the Earth.