The solar survey vessel "Endeavour" is sent to study Rama, as it is the only ship close enough to do so in the brief period Rama will spend in the solar system. "Endeavour" manages to rendezvous with Rama one month after it first comes to Earth's attention, when the alien ship is already inside Venus's orbit. The crew, led by Commander Bill Norton, enters Rama through a safety system consisting of triple airlocks, and explores the 16-km wide by 50-km long cylindrical world of its interior, but the nature and purpose of the starship and its creators remain enigmatic throughout the book. Rama's inner surfaces hold "cities" of geometric structures that resemble buildings and are separated by streets with shallow trenches. A band of water, dubbed the Cylindrical Sea, stretches around Rama's central circumference. Massive spires, which are theorised as part of Rama's propulsion system, stand at its "southern" end. They also find that Rama's atmosphere is breathable.