Hypothetically, suppose we have the ability to accelerate a spaceship to near speed of light (lets say 0.9c). From what I remember of relativistic kinematics, the energy required to accelerate the ship to that speed be given by:
$$ KE = mc^2 (\gamma -1) $$
If so, let us suppose the spaceship is to travel to the nearest neighbouring star. What would be the energy for the spaceship to maintain that velocity of 0.9c.