The smallest individual spacecraft concept proposed for the Strategic Defense Initiative was the “Brilliant Pebbles” system. Instead of vast, multi-hundred-ton battle platforms like the lasers and neutral particle beams and railguns, Brilliant Pebbles were *relatively* small rocket vehicle designed to intercept enemy missiles and warheads while in space. The system was composed of a rocket-powered kill vehicle (usually fueled by dense, easily storable propellants such as nitric acid and hydrazine), and a “cocoon.” The latter was a shroud that protected the kill vehicle while it waited out the years floating in space.
The kill vehicle, in order to do its job, had to be *extremely* high performance. It was composed of a series of thrusters, lightweight composite propellant tanks, optics to spot and track the target, a computer to run it, communication systems, batteries… and not much else. The Brilliant Pebbles (so named because they were derived from the concept of “smart rocks,” which was a jovial way to describe a hit-to-kill system: instead of taking out the target with a warhead, you actually ram the target with your vehicle) vehicles were said to have the propulsive capability of boosting themselves out of Earth orbit and doing a fast flyby of Mars. This performance was needed in order to be able to launch from wherever they happened to be and race to intercept enemy missiles.
Data, such as mass and dimensions, is sadly lacking. Guesstimate that the cocoon is about the size of a Volkswagen. Where the bigger systems such as lasers would require heavy lift launchers, Brilliant Pebbles could be launched by much smaller rockets… and a whole lot of them. *Thousands* of Brilliant Pebbles would be needed in low Earth orbit to provide basic coverage. For every Brilliant Pebble that would be in place to take on a Soviet missile, many more would not be. It was the need to launch vast flocks of these that the SDI program began studying reusable, low-cost launchers, leading to the Delta Clipper program.