A Matter of Some Gravity

Today I’m writing about a popular misconception that seems to be out there. Orbits and weightlessness in space. I think everyone understands that when something is in orbit it is moving around the earth in space, generally at a great rate of knots. However, I’ve been asking around about why it is astronauts on the International Space Station float around as if there is no gravity. Most people answer that it is simply that: that there isn’t any gravity there. This is not correct.

The ISS is only about 400km up and you have to go a lot further than that to get outside the gravity well of the earth. There is about 90% as much gravity at that altitude as here on the surface so why do we have all those cool videos of water floating around and such?

Imagine a time when you have been weightless. I would guess a sizeable proportion of us have experienced this at some point when we’ve been on a rollercoaster. In everyday situations like this we approach the earth very rapidly and the free fall lasts only a brief time and, with any luck, without a squishy red mess on the floor.

Imagine you are doing a skydive, once again you are free falling. The presence of the atmosphere creates drag dependent upon the size and shape of the object that is falling and so you eventually reach a top speed known as terminal velocity. In space there is no atmosphere to slow you down and that’s why astronauts and their vehicles can reach speeds in the thousands of miles per hour.

If astronauts were in free fall, though, wouldn’t every mission very quickly end with the aforementioned squishy red mess? It would if it wasn’t for the fact that astronauts are falling sideways around the earth. The first time I heard that it made no sense to me whatsoever. I couldn’t picture it. How can you fall sideways around the earth? Luckily there is a great analogy that can clear this all up for us.

Let’s now imagine we have a canon atop a high tower and, to simplify matters, let’s pretend we’re doing this on the moon where there is no atmosphere. We shoot out a canon ball exactly horizontally and, as expected, after a few hundred metres or so it strikes the ground having described a beautiful parabolic arc through the sky. No surprises here. If we do the same again and again but add progressively more gunpowder each time then you can easily imagine the canon ball getting further and further each time you do it.

As a final test imagine putting so much force behind the canon ball that it flies for miles and miles. It is still free falling towards the ground, but the ground itself is moving away from the canon ball as the sphere of the surface drops away. If you use just the right amount of gunpowder then you can get these two effects to balance out and the canon ball will spend eternity free falling towards the surface at the same rate as the surface is moving away from the canon ball. Voilà, it is in orbit.

This is exactly what is happening to the astronauts. They are falling towards the earth at several thousand miles per hour, but they’re moving sideways as well as down and the earth’s spherical surface is moving away from them at almost the exact same rate. There is virtually no atmosphere up there to cause drag and so all that is needed to keep in orbit is the very occasional boost of a small rocket to keep things nicely balanced.

So there you have it, that’s why we have cool videos like this.

Astronaut Bruce McCandless free falling in space. Image courtesy of NASA

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