Hypervelocity Rod Bundles are a leading candidate. More colloquially known as Rods From God, they are long, slim, dense metal rods, typically of tungsten or uranium, each weighing perhaps 100 kilograms and deployed from an orbiting platform. Once a rod is released by the platform, a large two-stage rocket would bring it to a stop, after which orbital dynamics determine the projectile's trajectory to a terrestrial target [see illustration, [link|http://ieeexplore.ieee.org/iel5/6/30455/1402717/1402717f2_v20.html|"Space Arrows"]]. The slender rods would eventually reach a speed of several kilometers per second if dropped from LEO, their length facilitating the penetration of hard or buried targets.

Because the rods' trajectory paths from LEO would be many hundreds of kilometers long, they would require about 5 minutes to reach their targets, so it would be difficult to use them against moving objects. Since no target is likely to be directly under the platform's orbital path, each rod would have to be equipped with a rocket or some other means to move it from that path. Also, the rods would need shielding to keep them from burning up during reentry. The shielding and rocket both add weight and thus increase the cost of putting these weapons into orbit in the first place. Once the rod has reentered Earth's atmosphere, it could be maneuvered by shifting an internal mass or by ejecting gas.

How destructive could such a weapon be? A 100-kg rod of tungsten falling from an altitude of 460 km and reaching an impact velocity of roughly 3 km/s would have the destructive force of a similar amount of conventional high explosives delivered by bomb or missile. The rod would be more effective than conventional high explosives at penetrating to a buried target, because the rod's force would be concentrated and directed in the line of motion. Higher orbits would deliver greater energies but would take even longer to strike a target--about 6 hours, for instance, from geosynchronous orbit.