since only the projectile leaves the gun, a railgun system can potentially fire many times before needing to be resupplied. A hypervelocity railgun works very much like a particle accelerator in so far as it converts electrical potential energy into kinetic energy imparted to the projectile. A conductive pellet (the projectile) is attracted down the rails by electric current flowing through a rail. Through the magnetic forces that this system achieves, a force is exerted on the projectile moving it down the rail. Railguns can generate muzzle-velocities in excess of 2.4 kilometers per second. Railguns face a host of technical challenges before they will be ready for battlefield deployment. First, the rails guiding the projectile must carry very high power. Each firing of the railgun produces tremendous current flow (almost half a million amperes) through the rails, causing rapid erosion of the rail's surfaces (through ohmic heating), and even vaporization of the rail surface. Early prototypes were essentially single-use weapons, requiring complete replacement of the rails after each firing. Another challenge with the railgun system is projectile survivability. The projectiles experience acceleration force in excess of 100,000 g. To be effective, the fired projectile must first survive the mechanical stress of firing and the thermal effects of a trip through the atmosphere at many times the speed of sound before its subsequent impact with the target. In-flight guidance, if implemented, would require the onboard navigation system to be built to the same level of sturdiness as the main mass of the projectile. In addition to being considered for destroying ballistic missile threats, railguns were also being planned for service in space platform (sensor and battle station) defense. This potential role reflected defense planner expectations that the railguns of the future would be capable of not only rapid fire, but also of multiple firings (on the order of tens to hundreds of shots). Space-based programs Space-Based Interceptor (SBI) Groups of interceptors were to be housed in orbital modules. Hover testing was completed in 1988 and demonstrated integration of the sensor and propulsion systems in the prototype SBI. It also demonstrated the ability of the seeker to shift its aiming point from a rocket's hot plume to its cool body, a first for infrared ABM seekers. Final hover testing occurred in 1992 using miniaturized components similar to what would have actually been used in an operational interceptor. These prototypes eventually evolved into