Rendezvous with Rama
(The first book in the Rama series)

In 2130, a new celestial body is discovered heading toward the Sun. Earthlings name this object "Rama" -- a vast cylinder, about 31 miles long and 12 miles across, with a mass of at least ten trillion tons. The spaceship Endeavor, directed by Commander Bill Norton, lands on Rama and has three weeks to explore its hollow interior. Inside the vessel they discover a completely self-contained world -- a world that has been cruising through space for perhaps more than a million years.