Question: Gravity is on earth but why not in space ?? What causes gravity ??

Derek McKay-Bukowski answered on 15 Jun 2011:

Just read the answers from the other… waiting for Derek? No pressure, right? 😉

Actually, Sarah is right. There is gravity in space. She is also right in saying that this is why the moon orbits the earth and together they orbit the sun.

Gravity is an attractive force. Anything with mass has gravity. In fact there is a very small gravitation force between you and the person sitting next to you! The bigger the mass, the bigger that force of attraction, but for small objects like people this force is very weak indeed. The only reason why the force of gravity that holds us on the earth is so strong is because the earth is so incredibly massive!

Now gravity gets weaker the further away the objects are. This is an “inverse square law”, which means that if you are twice the distance, then the force is 4 times weaker. Three times the distance, 9 times weaker and so on. Sure, gravity is weaker in space, but it is still there and satellites are still attracted to the earth.

But satellites are also moving sideways. They are moving so fast, that by the time they’ve fallen a little bit, they’ve moved off to the side and, due to the curvature of the earth, are still at the same altitude. So they simply keep on doing it and keep going round.

If you look at an astronaut or cosmonaut on the International Space Station, they seem to be weightless, but they are not. In fact they are falling towards the earth. But the space station is also falling, so compared to the space station they seem to be weightless. Both the people and the space station are falling at the same rate. (Fortunately for them, they too are also moving quickly sideways, so – like the other satellites – they don’t actually get any closer).

If you aren’t moving sideways, then you need a BIG rocket to overcome the force of gravity. That’s why rockets are needed to get people into space and onto their safe orbits in the first place!

Now, there are places where gravitational fields cancel out. For example at some point on their voyages to the moon, American astronauts had an equal gravitation force from the moon and from the earth. However, they were still falling about the sun (just like the earth and moon were).

What stops things from orbiting is friction. Even though space is pretty empty, it is not completely empty. There are a few particles up there and they slowly drag on the spacecraft causing it to lose speed. Thus, it doesn’t move sideways enough and so starts to lose altitude on each orbit. That actually makes the problem worse, because the lower it gets the more dense the upper atmosphere is becoming. Eventually the drag causes the spacecraft to descend into the atmosphere. Usually it is still going pretty fast, so that friction causes a lot of heat!

Anyway…

What causes gravity? Mass! Anything with mass will have its own gravitational field. But what causes mass to cause gravitational fields? And how exactly does it work? Well… that remains a tricky problem and physicists and still working on a good answer!