Travelling at a speed of 0.7c is the “break even” point, where the combination of spacecraft velocity and Lorentz Contraction means you are travelling at “functional light speed” (the distance to Alpha Centauri in that frame would be 3.1 light-years and the travel time 4 years, 5 months). Of course time passes at different rates based upon their relative speeds as well, a phenomena called relativistic time dilation, so if Phaeton were travelling at a speed of .7c, for every year that passes for the crew, a year and five months would pass for The Edge of Never viewers back on Earth. Billie Kashmiri alludes to this in her confessional near the end.

Really crunchy, and really interesting ...