Yeah, there's a huge difference! At relativistic speeds (AKA speeds you can express as a significant percentage of the speed of light) energy increases exponen

Yeah, there's a huge difference! At relativistic speeds (AKA speeds you can express as a significant percentage of the speed of light) energy increases exponentially as you get closer to c. Put another way - since the energy of an object is infinite at the speed of light, then the energy needs to keep getting bigger and bigger as you keep putting 9s on the end, since if you're going .9c your energy is not infinite, while if you are going .(9)c your energy is infinite. To actually answer your question, the energy of an object going at .9999c would be about 50x the energy of an object going at .9c.