Conditions in space are different from those on Earth, in which our bodies have evolved. This can cause us problems and so the design of space craft need to offer us some protection.
Conditions aboard the ISS
Propulsion systems work by throwing something one way and the reaction for pushing the craft the other way. This is normally hot gasses in the case of a rocket. As fuel is burnt the gasses heat up, expand and leave through the exhaust at high speed. The space craft accerates more slowly (than the gas) because it has a greater mass. However the momentum is the same.
There is an action force pushing the exhaust out of the rocket and so the exhaust pushes back with a reaction force. The force exerted on the rocket is called thrust, which must be greater than the weight of an object if it is to accelerate.
Once in space, the space craft can manovere using thrusters. These are stores of compressed gass which are ejected from the craft. The gasses accelerate in one direction causes a reaction force on craft. This causes the space craft to accelerate in the oppposite dierction. Used in combinationm, thrusters can make a space crafts rotate or accerate in any direction.
There is no need for there to be anything to push against.
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