[motoboi]

When you achieve speed in space, after acceleration, the speed won’t change forever unless you encounter some other force, like a celestial body gravity to change it. So if you achieve interestelar comet’s speed, you can shutdown the rocket and just travel at that speed for eternity like the comet does.

Even better: you can forget the comet, accelerate, keep accelerating until there is no more power or even a working motor while also extending a big sail to let solar wind accelerate you a little more.