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.
Rationale
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, or nuclear pulse propulsion (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).
In some science fiction smaller relativistic projectiles can sometimes be found depending on the technologies imagined in any particular scenario. In the movie Eraser, for example, characters used man-portable "gauss rifles" that were able to fire bullets at relativistic velocities.[1] Man-portable weapons of this type would have extreme issues with reaching such high speeds over such a short distance; to reach 1% of light speed over the length of a one-meter accelerator would require 4.5 · 1012 m/s2 (or over 450 billion g) of acceleration. Space-based RKKVs have the advantage of being able to accelerate over a vastly longer distance and period of time.
Calculating energy content
Newton's formula for kinetic energy, given as 
, is only an approximation for the kinetic energy of an object, reasonably accurate for speeds well below c, approximately 3 × 108 m s−1. For higher speeds, Einstein's formula for kinetic energy, Ek, must be used.
Where:
m is the object's mass in kg,
c is the speed of light in m s−1,
is the Lorentz factor, given by:
Where v is the velocity of the object in question.
Therefore, expanded the equation is:
Example
A 1 kg mass traveling at 99% of the speed of light would have a kinetic energy of 5.47×1017 joules. In explosive terms, it would be equal to 132 megatons of TNT or approximately 75 megatons more than the yield of Tsar Bomba, the most powerful nuclear weapon ever detonated. 1 kg of mass-energy is 8.99×1016 joules or about 21.5 megatons of TNT.
In fiction
- An example of RKKV use in science fiction novels is in Charles R. Pellegrino and George Zebrowski's The Killing Star and Flying to Valhalla, wherein aliens exterminate all human life on Earth by kinetic weapons traveling at relativistic velocities called "R-bombs".
- One of earlier examples of RKKV, although that term was not used, was in PK Dick's 1953 novella, The Variable Man. Here the Earth is fighting a war against Proxima Centauri and is constructing a bomb based on relativistic principles.
- W. Michael Gear's Forbidden Borders trilogy, Requiem for the Conqueror, Relic of Empire and Countermeasures also include RKKVs.
- In Iron Sunrise by Charles Stross, attempts are made to obtain the deactivation codes which will stop a destroyed planet's automated RKKV retaliation from destroying another world.
- In Joe Haldeman's The Forever War, a comparatively lightly armed human spaceship uses two drones traveling at relativistic speeds to destroy a much larger enemy battlecruiser and, accidentally, part of a nearby moon.
- In the universe of the video game series Halo, over three hundred orbital defense platforms have been positioned by the United Nations Space Command in defense of Earth. These Super-MAC stations fire magnetically accelerated projectiles which weigh hundreds of tons and travel at approximately 4% of the speed of light.
- In the Mass Effect series, most spacefaring species utilize spinal-mounted mass accelerators in their warships to accelerate ferro-slugs to 1.3% of light speed. Mass Effect 2 has a scene of a drill instructor citing Newton's first law and warning a pair of trainees that "eyeballing" such a weapon of mass destruction without a firing solution must be avoided at all costs since a RKKV will not stop until it hits something; in the instructor's own words, "that could be a ship, or the planet behind that ship; it might even go off into deep space and hit somebody else in ten thousand years". In fact, there is a planet with a gigantic canyon on its surface, said to have been dug millennia ago by a glancing blow from a mass accelerator of absurd power (compared to what is available with the protagonists' technology) fired light-years away; the protagonist uses it to track down the location of the battle where the weapon was fired. The Reapers use a more powerful version firing molten streams of metal. In Mass Effect 2, the Turians reverse-engineer the Reaper version into the smaller Thanix Cannon that can optionally be mounted on the protagonist's ship for use in the final battle.
- In the "Cerberus News Network" online snippets used as story bridge between Mass Effect installments 2 and 3 and to expand the lore, one series of "news articles" discusses a group of separatists using an FTL-equipped starship to relativistically bombard a planet. Even though the starship was the size of a small shuttle, it was driven into the planet at a considerable fraction of c by disengaging navigational safeties and initiating an FTL transition on a trajectory intersecting the planet. The resulting impact was in the gigaton range and devastated a considerable portion of the planet.
- In Larry Niven's Known Space series (a.k.a. the Ringworld series), particularly in the "Fleet of Worlds" companion series coauthored with Edward M. Lerner, relativistic weapons play a significant plot role as, at various times, diplomatic or perceived threats, fears, and attempted aggressive acts. RKKV's in this series include a range of objects, like asteroids, placed in the path of or in trajectory toward planets. Other uses involve the threat of relativistic ship fired weaponry used in a similar manner or even ships themselves.
- In John Varley's book, Red Lightning, something traveling at near-lightspeed bounces off Earth causing a massive tsunami in the Atlantic. Authorities believe that the object is a space-ship that is deliberately directed at planet Earth; luckily the ship is not correctly aimed and bounces off.
- The role-playing game GURPS features the Azrael (named after the Jewish apocryphal name for the Angel of Death), a massive missile with multiple smaller missiles attached and able to reach relativistic speeds, that impacts its target with the force of 42 million megatons of TNT (on the order of the asteroid/comet that caused the Cretaceous–Paleogene extinction), while the smaller missiles have an impact force of 700 megatons each.