宇宙过河卒

Missing, extremely new, or very obvious to obtain.

A reference to the 1970 science fiction novel of the same name by Poul Anderson. The novel is about a ship and its members that were initially planned to reach a nearby star from Earth, but the decelerators were damaged and the ship's trajectory is now interminable, and crewmembers from the ship face existential questions and adapt to their new lives in the ship.