They calculated their entire journey with Newtonian physics. They worked it all out by assuming they could just accelerate faster and faster and the speed of light wasn’t an issue. They don’t know about time dilation. Rocky doesn’t realize that Erid experienced a whole bunch more time than he did on that trip. They don’t know about length dilation. The distance to Tau Ceti will actually increase as you slow down relative to it—even if you’re still going toward it.
