Rama is an alien starship, initially mistaken for an asteroid categorised as "31/439". It is detected by astronomers in the year 2131 while it is still outside the orbit of Jupiter. Its speed (100,000 km/h) and the angle of its trajectory clearly indicate it is not on a long orbit around the sun, but comes from interstellar space. The astronomers' interest is further piqued when they realise the asteroid has an extremely rapid rotation period of 4 minutes and is exceptionally large. An unmanned space probe dubbed Sita is launched from the Mars moon Phobos to intercept and photograph it. The resulting images reveal that Rama is a perfect cylinder, 20 kilometres (12 mi) in diameter and 54 kilometres (34 mi) long, and almost completely featureless, making this humankind's first encounter with an alien spacecraft.