Space Oddity (Part II)

"Houston, we have a problem."

Apollo 13

Is there gravity in space?

Usual answer. No.

Correct answer. Yes.

Students tend to believe there's no gravity in space because they've heard of a phenomenon called 'weightlessness' where astronauts tend to float about in space shuttles. This makes them believe there is an absence of gravity in space. I've seen science textbooks show confusing pictures of this also.

This can be explained by answering the second question in my previous post.

When the space shuttle launches, what path does it take?

Usual answer. Straight up.

Correct answer. Down.

Down? How can it go into space by going down? Maybe that's what you're thinking right now. But, imagine that you take a ball and throw it. It may go 20-30 metres. The reason it doesn't go further is almost purely because of gravity. Gravity brings the ball down to hit the ground, and it won't go much further after that. Imagine now, that you threw the ball a bit harder. It would go that bit further before hitting the ground. Gravity always makes it fall at the same rate, so throwing it harder will get it a bit further before the inevitable happens. 

Imagine you throw a ball a third time. This time though you throw it incredibly quickly, like five miles per second. If you threw it that fast, it would go so far that by the time it came down again, the Earth wouldn't be there for it to fall onto. That is because the Earth is round, and it would curve away at the vital time. You would have thrown the ball over the horizon, which from your point of view, is DOWN.

Incredibly technically complicated schematic of a space launch

This may disagree with your belief of watching a shuttle launch, such as the image in my previous post. However, look at this...

What goes up, must come down

Thrown over the horizon

 Actually the shuttle is like a ball which has been thrown incredibly quickly and falls off the edge of the Earth over the horizon. It falls over, and then falls forever. No more rocket fuel is required. It will continue to fall toward the Earth and always miss. It's a kind of perpetual motion. All satellites in space orbiting the Earth are all falling towards us in this way, but they all miss. So is the Moon. We're doing the same with the Sun. And the Sun and the Solar system is doing the same with the centre of our galaxy.

In fact, the worst thing a shuttle could do would be to launch vertically, as it would exhaust its fuel, start to decelerate, then come to a stop. It would then fall towards the Earth pretty much straight back down the way it came (a bit like Wily Coyote when he realises he's stepped over a cliff) and burn up in the atmosphere.

So why, if all of those things are under the influence of gravity, and there definitely is gravity in space, does it appear that astronauts are weightless?

Because it is due to their point of view. They are effectively in a lift, that is falling towards the Earth. As everything that falls in gravity falls at the same rate, they too are falling at the same rate as the 'lift'.

So, as the lift is falling, there is nothing to push back on their feet where they are standing. Relatively to them though, everything is still, and they are able to 'float'. In fact, if you were stood still by the shuttle, it would rush past you at around 17,500 mph as it falls towards Earth. To them, they are falling at the same rate as the shuttle so they are able to move around. It's a like a very long skydive without any air resistance in a lift that is doing the same thing at the same time.

It is actually BECAUSE of gravity that they are weightless! 

These facts are always interesting to me.  Yet it is very rare I come across a student who realises them.  It seems a shame that something so amazing and beautiful isn't explained to students in school - or that they can get a GCSE in physics without knowing any, er, physics.