When an astronaut learns that his fianc?e is sent on a space trip at near-lightspeed, he must find her before their age difference prevents them from ever having a family together.

This is interesting, in a "I want to know more" way. Is the fiancee an astronaut too? Why is the fiancee being sent into space? What is the astronaut doing in the meantime, is he also in space, etc. Does his hunt take place on earth or in space? Is there an antagonist?

I think some of these things need to be in the log line.

On a side note, this would be a great way for a romantic rival to "get rid" of the astronaut- by going on the trip with the fiancee so they will stay the same age, while the astronaut would become old.