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Originally Posted by gesturen
Hi
I'm pretty new to this kind of thing but I have a question about the actual speed of spacecraft.
Is all speed of spacecraft measured in space relative to the things we consider to be "still" (i.e the solarsystem) or is it measured to absolute "stillness"? If so, how do you stay completly still in space if its even possible.
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There is no such thing as absolute rest. All motion is measured relative to some convenient reference system. In the case of space craft, the motion is usually measured with respect to Earth (for near-earth orbital missions), the Sun (for interplanetary missions), or the target planet (for orbiter missions, once orbit of the target is achieved).
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Another question is, since the galaxy is moving and spinning, would it be possible to "break" a spacecraft in order for stars etc come to you? That would mean to decelerate instead of accelerate which in my mind anyways seems more efficient but I'm probably wrong 
Many thanks!
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I think you mean "brake", not "break". It is possible, and is done in some limited cases (for example the Messenger mission), but the cases where it is used generally rely on gravitational assists for the lion's share of the breaking. Adjusting speed in space by any significant amount using strictly propulsion requires rediculous amounts of fuel.