Red Dwarf (1988)

Chosen answer: It can, but that doesn't mean that it's supposed to. At the time of the episode, Red Dwarf's thrusters have been firing continuously for millions of years, accelerating the ship until, in this episode, it finally achieves light speed. This is not a speed that the Dwarf was ever intended to attain, hence the need to slow it down again before the stresses tear it apart. If they wanted to hit light speed again, it would take the same sort of length of time to accelerate back up to that speed. Technically possible, but not exactly practical.

Tailkinker