The great radar pulse generator Excalibur, powered by nuclear explo­sions, had been out of service for almost half a century. It had been designed and developed in a frantic effort during the months following the transit of Rama through the solar system. When it was first declared operational in 2132, Excalibur’s announced purpose was to give Earth ample warning of any future alien visitors: one as gigantic as Rama could be detected at inter­stellar distances — years, it was hoped, before it could have any effect on human affairs.

That original commitment to build Excalibur had been made even before Rama had passed perihelion. As the first extraterrestrial visitor rounded the sun and headed out toward the stars, armies of scientists studied the data from the only mission that had been able to rendezvous with the intruder.

Rama, they announced, was an intelligent robot with absolutely no interest in our solar system or its inhabitants. The official report offered no explana­tions for the many mysteries encountered by the investigators; however, the experts did convince themselves that they understood one basic principle of Raman engineering. Since most of the major systems and subsystems en­countered inside Rama by the human explorers had two functional backups, it appeared that the aliens engineered everything in threes. Therefore, since the entire giant vehicle was assumed to be a machine, it was considered highly likely that two more Rama spacecraft would be following the first visitor.