A relativistic kill vehicle (RKV) 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 travelling at a significant fraction of light speed to strike the target. At these relativistic velocities the mass could carry immense amounts of kinetic energy, potentially several times that of its rest mass energy equivalent (ie, the amount of energy that would be released if its rest mass were totally converted into free energy).

RKVs have been proposed as a method of interstellar warfare , especially in settings where faster than light travel or sensors are impossible. By travelling near the speed of light an RKV could substantially limit the amount of early warning detection time. Furthermore, since the destructive effects of the RKV 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 travelling at nearly the same speed and would have little time to disperse. Indeed, some versions of the RKV concept call for the RKV to explode shortly before impact to shower a wide region of space.

Since they would likely be difficult to provide much terminal guidance to, RKVs are usually proposed as a strategic weapon targeted against large and relatively immobile targets such as planets. Accelerating a mass to such velocities in the first place will likely require vast amounts of energy and large, unwieldy accelerators.

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

In some science fiction smaller relativistic projectiles can sometimes be found depending on the technologies imagined in any particular scenario. The movie Eraser, for example, claimed to use man-portable "gauss rifles" that were able to fire bullets at relativistic velocities. Man-portable weapons of this type would have extreme issues with recoil, however; accelerating a 1 gram projectile up to a mere 1% of light speed would produce enough force to send a 100kg (220 pound) man flying backward at 30 meters per second (98 feet per second). A further difficulty is reaching such high speeds over such a short distance; to reach 1% of light speed over the length of a one-meter accelerator would reqire 4.5×10¹²m/s² of acceleration. Space-based RKVs have the advantage of being able to accelerate over a vastly longer distance and period of time.

Examples of RKVs in fiction

Some science fiction novels featuring RKVs are Charles R. Pellegrino 's The Killing Star and Flying to Valhalla . W. Michael Gear 's "Forbidden Borders" trilogy, Requiem for the Conqueror , Relic of Empire and Countermeasures also include RKVs. In A Fire Upon the Deep by Vernor Vinge, the destruction of a human civilization is preceded by a RKV assault. In Iron Sunrise by Charles Stross, attempts are made to obtain the deactivation codes which will stop an automated RKV assault from destroying a world.