Relativity - this is why a "rocket', with only a few pounds of thrust, with an exhaust velocity of only say, 100mph, could still get you (over TIME) NEAR the speed of light, in space.

Obviously here where airplanes fly you need more than 100mph of thrust to get to 100mph.

But in space (external gravitational influences disregarded) it doesn't matter if you're going 20,000mph relative to earth, or 2500mph relative to the ISS, relative to YOURSELF you are AT REST...

A 100mph exhaust velocity would still accelerate you even if you were going 1,000,000mph relative to anything ELSE (again excluding external gravitational influences)...