BrightKidz Library
Subjects
Three things thrown sideways from the same high place The curved surface of a planet sweeps across the picture and falls away towards the bottom right. A small marker sits high above it, and three curved paths leave that marker travelling to the right. The first path bends down quickly and meets the ground close by. The second is thrown harder and meets the ground much further along. The third is thrown harder still, and it bends downwards at the same rate as the ground curves away beneath it, so it never meets the ground at all and runs off the edge of the picture.

The Space Station Is Falling All the Time

About 10 minutes

You already know the Space Station never comes down, and that its speed is somehow the reason. The usual way of putting it is that the motion balances gravity, the way a bucket of water swung round on a rope stays full.

That picture leaves something unexplained, and if you have ever wondered about it you were right to. Balance means two things pushing against each other. So what is pushing back?

Nothing is. There is nothing up there to push on. And the honest answer turns out to be stranger and much better than the balancing one.

The Space Station is falling. It has been falling since the moment it was put up there, it is falling right now, and it will fall for as long as it exists. It simply never arrives.

Throwing something off a tower

Forget space for a minute and stand on a tall tower with a stone.

Drop it and it lands at the bottom. Throw it sideways and it still lands, but further out. Throw it harder and it lands further out still. Every one of those stones falls at exactly the same rate — throwing it sideways does not slow the falling down one bit. The harder throw just covers more ground before it gets to the bottom.

The bit everyone forgets

The ground is not flat. The Earth is a ball, so the further sideways you go, the further the surface has curved away beneath you.

Now put those two facts together, because this is the whole thing.

Falling takes you down by a certain amount. Going sideways lets the ground drop away by a certain amount. Throw hard enough and those two amounts match. You are falling exactly as fast as the ground is getting out of your way, so you never get any closer to it — and you never get any further either.

That is an orbit. Not hovering. Not balancing. Falling, and missing.

The picture at the top of this page is that idea and nothing else: three stones thrown sideways from the same high place. The first two curve down and land. The third is thrown hard enough that its curve matches the curve of the ground, and it runs off the edge of the picture without ever arriving.

Find the speed

Drag the slider to throw the stone harder. The marker shows where it is one second after it left the tower.

How fast you have to go sideways for the ground to curve away as fast as you fall The curved surface of a planet starts at the top left and sweeps down towards the bottom right. A small tower stands on it at the far left. A dashed line straight across the top shows where a stone would travel if gravity did nothing at all. A second dashed line lower down shows where the stone really is one second later, and that line is flat because the drop is the same however hard the stone was thrown. A marker sits on the lower line and slides to the right as the speed is increased. At slow speeds the marker is below the ground and the stone has landed. At high speeds the ground has curved away underneath it and the marker is out in the open. if gravity did nothing the same drop, every time one second later

The marker only ever moves sideways. Watch what the ground does instead.

The marker never rises and never sinks, at any speed. It cannot: one second of falling is one second of falling, whether you threw the stone gently or enormously hard. The only thing your slider changes is how far along the curve you got in that second — and somewhere just under 8 the ground has finally curved down further than you have fallen.

That number is not made up for the picture. About 7.9 kilometres every second is the real speed, and it is the speed the Space Station is doing above your head. Once round the planet takes it about ninety minutes.

The drawing exaggerates the drop enormously, because the true shape is impossible to put on a page. In one second the Station falls about five metres — roughly the height of a house — while travelling nearly eight kilometres sideways. Over those eight kilometres, the Earth's surface curves away by very close to five metres. That is the coincidence that is not a coincidence.

What this explains

Why astronauts float. Not because gravity is gone — up there it is still about nine tenths as strong as it is in your kitchen. They float because they, the walls, the water and the cameras are all falling together at the same rate. Nothing is pressing on anything. It is the feeling you get in the first moment of a rollercoaster drop, held for six months.

Why it has to be so high. The Station is above almost all of the air. Down where you are, air would drag on it, slow it down, and a slower orbit is a lower one. Even at 400 kilometres there is the faintest wisp of atmosphere left, so the Station does gradually sink — and every so often an engine gives it a shove back up. Left alone, it really would come down.

Why "escaping gravity" is the wrong picture. Nothing up there has escaped anything. The Moon is doing the same trick round the Earth, and the Earth is doing it round the Sun. Falling and missing is not a rare stunt. It is what almost everything in the sky is doing.

A stone thrown sideways from a tower falls the same distance in the first second however hard it is thrown. So what does throwing it harder actually change?

Next time the Station goes over as a bright dot sliding across the evening sky, you are watching something drop. It has been dropping the whole time you have been standing there, and the ground has been sliding out from under it at precisely the rate that keeps it up.