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A kinetic bombardment is the act of attacking a planetary surface with an inert projectile, where the destructive force comes from the kinetic energy of the projectile impacting at very high velocities. The concept is encountered in science fiction and is thought to have originated during the Cold War.
Project Thor is an idea for a weapons system that launches kinetic projectiles from Earth orbit to damage targets on the ground. Jerry Pournelle originated the concept while working in operations research at Boeing in the 1950s before becoming a science-fiction writer.
The most described system is "an orbiting tungsten telephone pole with small fins and a computer in the back for guidance". The weapon can be down-scaled, an orbiting "crowbar" rather than a pole. The system described in the 2003 United States Air Force (USAF) report was that of 20-foot-long (6.1 m), 1-foot-diameter (0.30 m) tungsten rods, that are satellite controlled, and have global strike capability, with impact speeds of Mach 10.
The time between deorbiting and impact would only be a few minutes, and depending on the orbits and positions in the orbits, the system would have a world-wide range. There is no requirement to deploy missiles, aircraft or other vehicles. Although the SALT II (1979) prohibited the deployment of orbital weapons of mass destruction, it did not prohibit the deployment of conventional weapons. The system is not prohibited by either the Outer Space Treaty nor the Anti-Ballistic Missile Treaty.
The idea is that the weapon would inflict damage because it moves at orbital velocities, at least 9 kilometers per second. Smaller weapons can deliver measured amounts of energy as small as a 225 kg conventional bomb. Some systems are quoted as having the yield of a small tactical nuclear bomb. These designs are envisioned as a bunker buster.
In the case of the system mentioned in the 2003 USAF report above, a 6.1m x 0.3m tungsten cylinder impacting at Mach 10 has a kinetic energy equivalent to approximately 11.5 tons of TNT (or 7.2 tons of dynamite). The mass of such a cylinder is itself over 8 tons, so it is clear that the practical applications of such a system are limited to those situations where its other characteristics provide a decisive advantage - a conventional bomb/warhead of similar weight to the tungsten rod, delivered by conventional means, provides similar destructive capability and is a far more practical method.
The highly elongated shape and high density are to enhance sectional density and therefore minimize kinetic energy loss due to air friction and maximize penetration of hard or buried targets. The larger device is expected to be quite good at penetrating deeply buried bunkers and other command and control targets. The smaller "crowbar" size might be employed for anti-armor, anti-aircraft, anti-satellite and possibly anti-personnel use.
The weapon would be very hard to defend against. It has a very high closing velocity and a small radar cross-section. Launch is difficult to detect. Any infra-red launch signature occurs in orbit, at no fixed position. The infra-red launch signature also has a small magnitude compared to a ballistic missile launch. One drawback of the system is that the weapon's sensors would almost certainly be blind during atmospheric reentry due to the plasma sheath that would develop ahead of it, so a mobile target could be difficult to hit if it performed any unexpected maneuvering. The system would also have to cope with atmospheric heating from re-entry, which could melt the weapon.
While the larger version might be individually launched, the smaller versions would be launched from "pods" or "carriers" that contained several missiles.
Perhaps the earliest examples of kinetic bombardment come from E. E. "Doc" Smith's 1930s and 1940s Lensman series. In these books, however, planetary masses were used rather than smaller projectiles. It was in the mid-1960s that popular science interest in orbital mechanics led to a number of science fiction stories which explored their implications. Among these was The Moon Is a Harsh Mistress by Robert A. Heinlein (which has been adapted into a screenplay of the same name by Tim Minear circulated widely over the Internet among Heinlein fans) in which the citizens of the moon bombard the earth with rocks wrapped in iron containers which are in turn fired from an electromagnetic launch system at Earth-based targets.
In the 1970s and 1980s this idea was refined in science fiction novels such as Footfall by Larry Niven and Jerry Pournelle (the same Pournelle that first proposed the idea for military use in a non-fiction context), in which aliens use a Thor-type system. During the 1980s and 1990s references to such weapons became a staple of science fiction roleplaying games such as Traveller and Shadowrun as well as visual media including Babylon 5's "mass drivers" and the film Starship Troopers, itself an adaptation of a Heinlein novel of the same name.
One of the first references to the "crowbar" is in David's Sling by Mark Steigler (Baen, 1988). Set in the Cold War, the story is based on the use of (relatively inexpensive) information-based "intelligent" systems to overcome an enemy's numerical advantage. The orbital kinetic bombardment system is used first to destroy the Soviet tank armies that have invaded Europe and then to take out Soviet ICBM silos prior to a nuclear strike.
In the 2000s and early 2010s, kinetic weapons as science fiction plot devices appeared in video games, featuring prominently in the plot of Mass Effect 2 or the MAC from the Halo franchise, for example. In the 2004 military science fiction novel Orphanage by Robert Buettner, the Earth's major cities are being wiped out from kinetic projectiles from an exo-solar alien race's base on the Jupiter moon of Ganymede.
In the 2008 novel Tom Clancy's Endwar, kinetic strikes, known as "Rods from God", have replaced intercontinental ballistic missiles as US military's most destructive strategic weapon. They were used to destroy a large Russian armoured convoy in Alberta.
This weapons system was also described and used in Dan Simmons' 2011 novel Flashback.
Also in Clive Cussler's 2008 novel Plague Ship a satellite called Stalin's Fist is used by the protagonist to destroy a cult's underground fortress.
Neal Stephenson's novel Anathem contains an incident in which an orbiting spaceship attacks a planet with a rod, striking and activating a dormant volcano and causing it to destroy everything in the vicinity downhill.
In the science fiction game of Renegade Legion, the concept of the Thor system is used very close to the original, real world concept, including the name itself: Thor Missiles. The weapons are stored in space aboard satellites, the size of tree trunks, made of solid metal, and equipped with computers and guidance fins to steer them as they descend towards their target, and do not use explosives. Impact velocity and mass are sufficient to destroy any known tank in the game.
In John Varley's book Red Lightning a ship traveling at near-lightspeed impacts the Earth causing a massive tsunami in the Atlantic and devastating the East coast of the United States. The ship was believed to be piloted by members of a doomsday cult who were aiming at Washington DC, but missed.
This system was also described and used in "Stony Man #81, Sky Hammer".
In Peter F. Hamiltons The Night's Dawn Trilogy, "kinetic harpoons" are used to bombard the surface of a planet. The book in which the event occurs also specifies how the staggering of the harpoons impact caused the shockwaves from the impacts to contract and result in an artificial earthquake.
The trailer for the upcoming film G.I. Joe: Retaliation appears to show a kinetic bombardment of London. The space based weapon appears to be a pole shaped object with a motor, very much in line with the speculated design of such a weapon. Instead of an explosion, a violent earthquake appears to take place, which is again a characteristic of the weapon.