Relativistic kill vehicle

A relativistic kinetic kill vehicle (RKKV) or relativistic bomb is a hypothetical weapon system sometimes found in science fiction. The details of such systems vary widely, but the key common feature is the use of a massive impactor traveling at a significant fraction of light speed to strike the target. Therefore, the weapon would be an extreme example of the real-life concept of a kinetic bombardment.

RKKVs have been proposed as a method of interstellar warfare, especially in settings where faster than light travel or sensors are impossible. By traveling near the speed of light, an RKKV could substantially limit the amount of early warning detection time. Furthermore, since the destructive effects of the RKKV are carried by its kinetic energy, destroying the vehicle near its target would do little to reduce the damage; the cloud of particles or vapor would still be traveling at nearly the same speed and would have little time to disperse. Indeed, some versions of the RKKV concept call for the RKKV to explode shortly before impact to shower a wide region of space.

As providing terminal guidance for such a high-speed object would likely be difficult, RKKVs are usually proposed as a strategic weapon targeted against large and predictable targets such as planets. However, they can still be used against smaller targets like spaceships, by aiming the weapons in the area they are in, and detonating a fuse in advance to shatter the mass into swarms of smaller particles, all traveling at nearly the same speed. This would cover a much larger area, and destroy smaller targets in space. Accelerating a mass to such velocities in the first place will likely require vast amounts of energy and large, unwieldy accelerators.

An RKKV could theoretically be launched using any of the spacecraft propulsion techniques that are capable of accelerating starships to relativistic velocities, such as antimatter rockets, Bussard ramjet systems, nuclear pulse propulsion or reactionless drive (see also relativistic rockets). Since an RKKV would be unmanned, higher accelerations could be used (though with most propulsion methods high acceleration may not be the most efficient approach).

...
Wikipedia