At 0.9c the relativistic time dilation is such that 2.3 days on Earth pass for every day shipboard time, meaning the crew would experience less than 4 years of

At 0.9c the relativistic time dilation is such that 2.3 days on Earth pass for every day shipboard time, meaning the crew would experience less than 4 years of travel in total: https://www.fourmilab.ch/cship/timedial.html https://www.fourmilab.ch/cship/timedial.html