A group of astronauts rendezvous with an empty alien spacecraft on approach to the sun, explore it, but have to leave before the exploration can be completed.

This was incredible!  It should really be called "Physics Fiction", because it stressed everything physics does, from coriolis force to melting ice.  From beginning to end, it had action, suspense, drama.  I could see the entire book proceeding in my mind.  Worth a second read.  Now, on to the sequel!