“Rods from God” is the nick-name given to a hypothetical orbital weapon for bombarding targets on the Earth from space. Just how feasible is this concept?
There are undeniable links between the spaceflight and military communities but apart from a few tests of anti-satellite weapons, the odd armed space station and laser battlestation there have been mercifully few weapons in space. Yet the idea of bombarding the Earth from orbit keeps coming up again and again. Thankfully it is ridiculous. I believe the idea is that having weapons hovering menacingly overhead will persuade your enemies to behave themselves. However this idea is stupid- I cannot think of a better word. Anyone who suggests such an idea must have learned their science from Tom Clancy books or Steven Seagal movies.
The current version of this mad scheme is the kinetic energy weapon and is usually described as scores, possibly hundreds, of tungsten (chosen for its high melting temperature and hardness) projectiles orbiting the Earth in formation or attached to a satellite ‘bus’. These could be either relatively small darts (weighing about 100kg) or large ‘phone poles’ (about 8000 kg each). When required these projectiles can be commanded to dive, singly or en masse, at targets on the Earth’s surface, smashing into the victim at orbital speed. As the projectile’s kinetic energy is released, the blast would be equivalent to a large conventional bomb (a 100kg projectile traveling at 7km/s would release about 2.5 gigajoules of kinetic energy, a tonne of TNT releases about 4.2 gigajoules). This would be a non-nuclear precision weapon, essentially a smart bomb that can target anywhere in the world. It is further claimed that the darts would be capable of penetrating deeply into the Earth’s surface enabling non-nuclear attacks on installations deep underground.
This idea is said to have originated in 1964 (but was revised and updated in 1975) in the mind of Jerry Pournelle, engineer, writer and consultant to the US Air Force. He originally named the concept “Thor” after the hammer-wielding Norse god of thunder. Pournelle said each projectile was
…an orbiting element some 20 to 40 feet long. It requires a GPS receiver to locate itself; a means of taking it out of orbit; an atmospheric guidance system, such as a means of changing its center of gravity (moving weights, small fins, etc.), and a communication system to give it a target and activate the system…Achievable accuracy has been estimated at ten to twenty feet CEP (Circular Error of Probability)
Pournelle assumed extremely cheap fully reusable single stage to orbit launch vehicles were just around the corner and would enable his concept to be quickly deployed, sadly no such craft have yet been built. Moving further into fantasy, Pournelle later had a smaller but more accurate optically guided variant of the weapon described as “crowbars” used by invading alien space elephants (really) to devastate the US military in Footfall (1985), a novel he co-wrote with Larry Niven. In this book, Niven and Pournelle introduced the concept
You take a big iron bar. Give it a rudimentary sensor, and a steerable vane for guidance. Put bundles of them in orbit. To use it, call it down from orbit, aimed at the area you’re working on. It has a simple brain, just smart enough to recognize what a tank looks like from overhead. When it sees a tank silhouette, it steers toward it. Drop ten or twenty thousand of those over an armored division, and what happens?
Subsequently similar weapons have appeared in other fictional works where they always work perfectly! A recent example is the movie GI Joe: Retaliation (2013) which is dissected in this video:
This artificial meteorite concept is often nicknamed ‘the rods from God’ even by its supporters, who usually claim it would be relatively cheap to set up (indeed some claim it already exists). They give the impression that at the press of a button, these rods will just fall from the sky on their victims. However it is not that easy. As each rod circles the Earth it is moving at least 7 km/s, to make the rod fall from orbit under gravity, we need to adjust its orbit to intersect the Earth’s surface. To do this each rod therefore needs to be attached to a rocket motor and its fuel tanks (or solid propellant), suddenly each cheap 100kg rod has ballooned into a multi-tonne vehicle, perhaps the size of a Soyuz spacecraft. At least it does not need a heatshield, a tungsten projectile could reasonably be expected to survive the expected heat of re-entry.
The ground-penetrating effects of such projectiles is grossly over-stated too- do falling meteorites of this sort of size always bury themselves hundreds of metres under the ground? Laboratory experiments show that objects striking the surface at speeds greater than 1 km/s are melted by their own kinetic energy before they penetrate the ground, effectively liquefying on impact. Rather than slamming into the target at 20 times the speed of sound, the rods may need to be slowed down to fast aircraft speeds to prevent them disintegrating on impact.
The problems of guiding each rod is usually dismissed with handwaving references to GPS, although some armchair space marshals also follow Pournelle’s fictional lead to suggest each rod would have its own imaging sensor to find and steer onto moving targets like tanks or warships. I have no doubt that the electronics are feasible but the rod now needs control surfaces hooked to its guidance system and sounds more like a missile than a cheap metal rod. Do these now complex projectiles require maintenance in orbit?
Finally, it is said that the rods can hit any target on Earth minutes after the KILL button is pressed. Once again, this doesn’t seem properly thought out. The rods can only hit targets on or near their orbital track, for weeks at a time some parts of the world would be invulnerable as their potential attackers would never come within hundreds of kilometers from their positions. The only way around this limitation is to have hundred of rods waiting ready in multiple orbits, requiring a ludicrous number of launches. Even if the target is directly under the rod’s orbital track, the attack may not be instantaneous, as those who order the attack wait perhaps 90 minutes for the rods to move around the Earth into position. Even the Joint Chiefs of Staff cannot overrule Sir Isaac Newton.
The number of launches needed to deploy even a few dozen individually weighty weapons is glossed over by Rods enthusiasts. Assuming they are deployed, every rod (or their carrier satellite) will move around the Earth on a regular and predictable orbit where they will be observable from the surface by radar and optical sensors, so potential enemies will always know where they are. “Dropping” the projectiles from orbit is no actual advantage by the way, by the time they reach the surface they will have no more kinetic energy than was imparted to them by their original launch vehicle. After considering all this, to be honest, it would make more sense to launch each rod from the Earth’s surface directly to the target. To make them less vulnerable to preemptive attacks, perhaps the rods should be based in hardened underground silos or hidden in submerged submarines. Congratulations, we have just reinvented the ICBM!
The most recent unclassified mention of the concept is in a USAF document called The US Air Force Transformative Flight Plan (2003) which talks of “hypervelocity rod bundles” as a potential weapon in the post 2015 period. Note this mention in an official document does not mean the US military can magic this weapon into existence. There is a history among militaries world-wide of wasting billions on R&D into projects which are hopelessly impractical or even completely ungrounded in reality (see hafnium excimer bomb, atomic-powered aircraft, camouflage uniforms which make the wearer more conspicuous and using ESP for espionage). As of 2015 no kinetic-energy orbital bombardment system has officially been proposed or tested, never mind deployed.
The older version of the concept is the idea of putting nuclear bombs in geostationary orbit over a potential enemy country is still brought up from time to time. This is even more ill-conceived than the Rods from God. When you think about it, it is obvious that a geostationary orbit must be above the Earth’s equator. Now there are only thirteen nations on the equator and it hard to see why any would want to suspend a bomb over say, the Maldives or Gabon.
Even though they are sometimes put forward by apparently sensible people or organizations, “Rods from God” and other schemes for bombing the Earth from space are half-baked science fiction concepts. The cost of developing an orbital bombardment system would make the F-35 project look cheap in comparison. They are militarily pointless and hopelessly implausible. Similar damage could be inflicted more cheaply and easily by conventional ground-based weapons.
A cynic would say that clever diplomacy would avoid the need for the weapons altogether.
(Update: since I wrote this I have discovered the 2003 paper Space Weapons: Not Yet by Richard L. Garwin which discusses this concept and others. Garwin’s conclusions about how believable this concept is are very much in agreement with my own.)
(Article by Colin Johnston, Science Education Director)