A ten million megaton starship which passed through the Sol system in 2131 AD.
Since Rama was in free fall when it entered our system, it was naturally assumed to be an asteroid and routinely cataloged as "31/439" by Spaceguard. As its peculiarities surfaced, 31/439 earned the astronomical name Rama.
It was soon realized that the asteroid was in fact a cylindical artifact 50 kilometers long and 20 kilometers in diameter. The Solar Survey ship, the Endeavor, intercepted and explored Rama.
Although the exploration posed more questions than it answered, Rama is believed to be a "space ark" on a voyage of staggering proportions. Rama's last solar fly-by was probably 200,000 years before its arrival in the Sol system. Rama was pointed directly at the Magellanic Clouds when it left our system. However, Rama's actual embarking point and ultimate destination are unknown.
The exterior of Rama is a perfect, unadorned cylinder. Air lock facilities are located at one end, Rama's reactionless drive is at the other end. This drive is capable of a cruising speed of 2000K per second, roughly one percent the speed of light.
The interior of Rama is a single "room" using the inside of the outer shell as a floor. The floor space is broken by three pair of strip lights bordering each shore of a 10K "sea." The sea forms a "ribbon" around the cylinder's epicenter. (Imagine a stripe painted around the inside of a basketball.)
The sea is actually an "organometallic soup" densely packed with spherical single-shelled organisms similar to Terran plankton of prehistoric times. The sea's apparent purpose is to supply raw materials for BIOTS and other artifacts. Biots are fabricated in a number of automated factories located on an oval island (10K x 30K) in the cylindical sea. The biots perform maintenance tasks during solar fly-byes.
Rama and its contents are built on a principle of triple symmetry and redundancy. The Ramans themselves are probably tri-pedal. Supporting evidence is the design of the various biots and devices viewed in the Illustrated Catalog of Rama Artifacts.From Rendezvous With Rama by Arthur C. Clarke