Project Thor is an idea for a weapons system that launches kinetic projectiles from Earth orbit to damage targets on the ground. It is said that the concept originated in a classified study for the United States Air Force in the 1950s.
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 as small as several meters long, an orbiting "crowbar" rather than a pole.
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 weapon inflicts damage because it moves at orbital velocities, at least 9 kilometers per second. The amount of energy released by the largest version when it hits the ground is roughly comparable to a small nuclear weapon or very large conventional bomb. Smaller weapons can deliver measured amounts of energy as small as a 500 lb conventional bomb.
The "pole" shape is optimal because it enhances reentry and maximizes the device's ability to penetrate 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 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.
While the larger version might be individually launched, the smaller versions would be launched from "pods" or "carriers" that contained several missiles.
It was most recently popularized by Jerry Pournelle, on his website, under the title "Project Thor."