SDI provides a +75% chance of intercepting ICBMs.
SDI stands for "Strategic Defense Initiative," a research program implemented by the United States in the mid-1980s to provide a ground and space-based shield against incoming nuclear missiles. As proposed, the SDI is a defensive system that will spot incoming intercontinental ballistic missiles, track their courses, and then destroy them before they detonate over US cities.
The concept of putting weapons platforms in space has been around for years; in fact the major space powers may have already done so in secret. Some nations have certainly built "satellite killers," satellites designed to destroy enemy satellites, disrupting the opposition's communications and intelligence capabilities.
The specific method of incoming missile destruction hasn't been determined. Some have proposed creating powerful x-ray lasers powered by nuclear explosions that would melt the enemy weapons in space; others suggest ground-based anti-missile rockets on the Patriot missile model.
To date the project has made little in the way of visible progress, and its future is uncertain. Some people believe that the threat of an incoming nuclear missile is extremely remote, especially when compared to the threat of a nuclear bomb being smuggled into the country by terrorists. Others believe that the entire concept is flawed and that the money would be better spent bribing potential enemies into destroying their ICBMs rather than trying to create a system to shoot them down.