Originally Posted by
John83
The idea that the space station is in zero gravity is common and natural but mistaken. Gravity depends on the squared distance from the centre of mass (in this case the centre of the earth). You or I are around 6300 km from the centre of the earth. (Earth is very big.) The ISS (and I assume MIR had a similar orbit) is around 400 km up there, so it's 6700 km up from the centre of the earth. That means that the force of earth's gravity is around 88% as strong (100%*(6300/6700)^2) up there. Any difference will make for time dilation: we know for sure that this is true because if we didn't account for it, GPS would steadily get less and less accurate as the clocks on the satellites fall out of synch with those on the ground.
The reason the astronauts on the ISS (and previously on MIR) seem like they're in zero G is that the station is in freefall. It's just that it's moving sideways fast enough that the ground is falling away at the same speed. This is an orbit. If you could build a 200 km tall building, the ISS could whizz by the roof (though at a blazing 26,700 km/hr, it'd be a blink-and-you'd-miss-it situation). However, you'd still feel gravity (and a severe shortness of breath) standing on the roof.
To actually get to zeroish G, you need to go to the Lagrange point. That's where the James Webb telescope is sitting. It's around 1.5 million km away.
Space is weird.