OK, this is a question probably asked and answered before, but I've been wondering about this for years..... Alkatran made a comment in the space-buoy thread about how mass changes with speed in SR. As you approach the speed of light, your mass increases towards infinity, and it would take more and more fuel to keep you accelerating at a sensible pace, so you could never get to light speed. It seems like a fair enough point, if you're using chemical thrusters of some sort, but what about this... In a fission reactor, heavy elements are broken down into lighter ones, but the sum of the mass of the lighter elements/particles is less than the mass of the original. The absent mass has been converted into energy, and that energy is eventually harvested as a power source. Is this OK so far? It's been a while... So when something goes fast and hence increases its mass, where does that extra mass reside? Would the energy released from fission be increased by the same fraction that the mass increased? If so, then doesn't it stand to reason that if you power a space shuttle using a fission engine, then the amount of energy liberated by the fission reaction will increase at the same rate as the mass of the shuttle, making the relativity of mass a rather irrelevant point? So... could a fission powered rocket reach the speed of light? Hope that made sence - what do you think?