Relativistic exploration story, with the added wrinkle that mental communications between twins is instantaneous, regardless of distance. Later adjustments to this are less scientific, and the ending is a disappointment.

The majority of the novel is a shipboard life story. The main character's twin is manipulative, to the point of manipulating which twin would go on a likely doomed voyage of exploration. The coming-of-age aspects of the book involve him growing to understand this, as additionally demonstrated by an irritating shipmate. In this respect, a good fit with other Heinlein juveniles.

I was willing to accept the ESP FTL connection, but allowing a non-related connection, along with later connections to descendants, was a bit much. The characters were pretty basic, and some of the conflict seemed artificial. I can't complain much about the ending without spoilers, but the marriage? Just no.