Question: If everything is weightless in space then why are you only allowed to take up a certain mass?

Steve Price answered on 14 Jan 2016:

Hi,
So in space things are weightless. That is because the pull of the gravity is offset by the motion which is pushing away. When these two are equal then you are in orbit.
However everything still has mass. If you we’re trying to push a box, for example, you’d still have to apply a force because of the mass of the box.
All man made items in space have been taken from the Earths surface and moved into space, I.e. Launched.
They have to be launched on a rocket. The rockets of a certain size can only carry a certain amount of the mass. These items have to gain potential energy equivalent to the height of the orbit. The engines on the rocket produce a lot of energy by burning the fuel. That energy is pushed out through the nozzle and allows the rocket, along with its payload to gain height and eventually reach orbit. The greater the mass you want to take to orbit, the bigger the rocket has to be. The rockets are as big as they can be. If you want to take more mass into orbit than the rocket can achieve then you have to use 2 rockets. That is expensive. When the ISS was built it was gradually built as you could only launch one part at a time because of the size of the rockets. Mass therefore is the limiting factor and a lot of effort is spent minimising the mass of objects so as many things as possible can be launched on one rocket.