KE=1/2mv^2

Conservation of momentum is the easiest law to use in almost every casem_{1}v_{1} + m_{2}v_{2} = 0So in the space walker (m_{1}) pushing on the space station (m_{2}):m_{1} << m_{2}Therefore v_{1} >> v_{2} and the directions are opposite.In the Star Destroyer vs Shuttle energy comparison, for the same speedE_{Vader's ship} >> E_{Luke's ship} sincem_{Vader's ship} >> m_{Luke's ship} even thoughv_{Vader's ship} = v_{Luke's ship}E=mc^{2} means you can liberate E if you annihilate m.In the equations of motion, it means that m actually increases the faster that m is moving. If I recall correctly,m_{fast} = m_{0} / sqrt (1-v^{2}/c^{2})So that added mass is only noticeable when v approaches c or for long-term orbital calculations, like on probes escaping the solar system.