The THOR system is composed of a thousand or more cheap satellites

Sunday, February 17th, 2019

If you drop something dense and aerodynamic from high enough up, it will hit the ground really, really hard — maybe hard enough to qualify as a kinetic bombardment weapon:

During the Vietnam War, the US used what it called “Lazy Dog” bombs. These were simply solid-steel pieces, less than 2 inches long, fitted with fins.

There was no explosive: They were simply dropped by the hundreds from planes flying above Vietnam.

Lazy Dog projectiles (aka “kinetic bombardment”) could reach speeds of up to 500 mph as they fell to the ground and could penetrate 9 inches of concrete after being dropped from as little as 3,000 feet.

If you drop a telephone pole-sized (20′×1′) tungsten cylinder from orbit, the 9-ton “rod from God” should hit at Mach 10, with the kinetic energy equivalent to 11.5 tons of TNT (or 7.2 tons of dynamite).

Robin Hanson recently mentioned such “rods from God,” and I just happened to be reading There Will Be War, which includes Jerry Pournelle’s “original” 1981 description of Project THOR, which describes something subtly different — a barrage of 20-pound projectiles made of tungsten, less than an inch in diameter and three or four feet long, traveling at Mach 23:

One of the most difficult security missions which the United States must accomplish is the protection of our interests around the globe. Incidents like the North Korean seizure of the USS Pueblo have demonstrated our weakness in not being able to respond quickly and authoritatively in remote locations. Our only solution to this problem so far has been the naval carrier task force. Carrier-based aircraft can project military force to protect our citizens and allies in remote regions of the world. Unfortunately, the high cost and vulnerability of nuclear carriers and their required aircraft and support fleets make them an unattractive solution.


To balance the force of gravity, a satellite two hundred miles above the surface must travel at a speed of seventeen thousand five hundred miles per hour. At this speed, the satellite travels around the Earth once every ninety minutes. With a hundred satellites in orbits near this altitude and traveling in random orbital inclinations, one of the satellites will pass over any given location on Earth every thirty minutes. With a thousand satellites, the timing between satellites overhead is less than ten minutes. The basic physics of orbital motion gives us our global coverage; it also gives us the weapon. The extremely high velocity of a satellite in orbit gives it a tremendous amount of kinetic energy. If a one pound object moving at orbital velocity ran into a stationary target, the energy released in the impact will be the equivalent of exploding almost ten pounds of TNT.


The THOR system is composed of a thousand or more cheap satellites, each made up of a bundle of projectiles, guidance and communications electronics, and a simple rocket engine.


The result is spectacular: a bundle of tens or hundreds of twenty pound projectiles streak down at four miles per second to strike targets with the explosive equivalent of two hundred pound bombs each.


Even if an enemy were to detonate one or more nuclear devices in space in an attempt to destroy THOR, there are a thousand or more widely scattered satellites he must destroy. Because the satellites are at different altitudes and have different orbital inclinations, any holes produced in the global coverage by a nuclear explosion are filled in after several hours by the orbital motions of the satellites.


The satellite can be cocooned in foam, which would be difficult to detect with radar anyway and could be shaped to make detection even more difficult (stealth satellites!).


The foam would insulate the satellite against the heat and shock of nuclear explosions or laser beams.


The jet of metal particles produced when a shaped charge warhead detonates is traveling at about the same velocity as a THOR projectile when striking a target.


The jet of metal from the TOW warhead weighs only a fraction of an ounce; a THOR projectile weighs over twenty pounds!


If the projectile were composed of an outer shell with sand-sized particles inside, it could be designed to explode and disperse the particles just before impact. The metal particles would instantly vaporize, with the resulting shock wave flattening troops, aircraft, or other targets much like the fuel-air explosive bombs presently in service.


The advantages of the THOR weapon system are its low cost, global coverage, quick reaction time, and survivability.


To de-orbit the projectiles and bring them down at an angle of thirty degrees from vertical requires almost as much energy as was required to orbit the projectiles initially, and requires a large quantity of propellant for each THOR satellite.


The individual THOR satellites are most vulnerable while the de-orbit propulsion burn is taking place, when a rocket exhaust plume is a bright beacon marking the location of the satellite for possible destruction by enemy laser weapon satellites. Two solutions are a cold gas propulsion system (high weight of propellant required) or a very fast propulsion impulse which ends before the laser weapon could be brought to bear on the THOR satellite.


With the Global Positioning System navigation satellite network in operation, each satellite could passively receive its own location in space to a very high accuracy while doing nothing to reveal its own position.


Communication by laser beams, which are extremely narrow and almost impossible to intercept, may be possible if the position of each of the thousand or more THOR satellites can be calculated accurately enough to hit the desired satellite.


The projectile could be protected by an ablative nose tip which would vaporize and carry off the heat from atmospheric friction during the few seconds of atmospheric passage.


The high speed of the projectile through the atmosphere near the ground where the density of the air is highest would produce a luminous bow shock wave directly in front of the missile. Penetrating such a layer might be a problem, but high frequency radio waves, infrared light, visible light, or ultraviolet light might be effective for targeting. A visible light sensor might have a window covered with a filter which passes light of a wavelength which is not emitted by the ionized air in the shockwave.

The real point of the system, as he points out, is that it could quickly (and cheaply) hit any target, anywhere on earth — which seemed really, really useful, a few months later, when HMS Sheffield succumbed to a French-made Exocet missile in the Falklands. Of course, getting tungsten rods up into space is only economical once you have frequent launches of your newfangled space shuttle.


  1. Bob Sykes says:

    Placing nuclear weapons and other weapons of mass destruction in space or on moons and planets is explicitly prohibited by the 1967 Outer Space Treaty. Whether any weapon can be based in space is ambiguous. The kinetic energy of a telephone pole-sized tungsten rod approaches that of a small tactical nuke, and it can be legitimately categorized as a weapon of mass destruction. There are also numerous UN resolutions condemning weapons is space. But since the US refuses to be bound by inconvenient treaties it probably doesn’t matter.

  2. Isegoria says:

    A small tactical nuke has a yield in the tens of tons, well beyond the kinetic-energy “yield” of even a telephone pole-sized tungsten rod — but the rod could presumably penetrate deep, fortified bunkers just as well.

  3. Bruce says:

    According to Ben Rich’s Skunk Works, they thought of dropping an iron spear off the SR-71 at Mach 3. It would penetrate most bunkers, and be cheap, but they called it a “kinetic energy weapon” when they ran it past McNamara, and he said he didn’t believe in that Buck Rogers energy weapons stuff.

  4. Adar says:

    I think this is Global Strike in the most recent version. A B-52 carries a missile to 40,000 feet and releases. That missile can hit any spot on the planet within a 30 minute span. Downward plunging warhead releases thousand of tungsten rods each having twelve times the amount of energy as a fifty caliber round. Everything within a mile radius of ground zero obliterated. A nuclear like destructive capacity without the nuclear detonation. Those thousands of tungsten rods creating a lot of secondary debris flying all over the place also very deadly.

  5. Freddo says:

    Pie in the sky daydreaming. Air resistance leads to a much lower terminal velocity. Heat friction would severely damage the projectile; the plasma from the heat friction would also prevent sensor packages from operating. The slightest instability would cause the projectile to tumble and shatter.

    If the technology was there you would expect it to show up in current weapon systems at a much higher rate.

  6. Sam J. says:

    I believe it would work. There’s extremely low air resistance to a needle. As for heat shields there’s videos online that show someone figured out how the stuff called starlite, a heat shield made by a hair dresser that protected an object to 10,000 degrees Celsius, worked and what it might have been made of. Turns out, if I remember correctly, it just releases a small amount of carbon dioxide and that gas protects it. I see no reason it couldn’t be made to work.

    There’s also a tactical anti-personal version of the darts dropped off planes.

  7. Wan Wei Lin says:

    Travis Cocoran’s Causes of Separation SF novel uses GPS-guided kinetics loaded with lunar regolith. Deals with air resistance, guidance, and failure modes.

  8. Kirk says:

    The engineering and aerodynamics on these things were already worked out in excruciating detail back in the ’70s by guys like Jerry Pournelle and some others who were working in that arena. The idea works–Issue is the supporting tech, and the economics of it all.

    At some point, this will be a real weapons system we will have to factor in and contend with. Countermeasures will no doubt be found, but the question is how they will work, and what they will consist of.

  9. Edgewise says:

    A cinematic conception? How “accurate”/“plausible” is this?

  10. Isegoria says:

    The two salient flaws in that depiction are that you can’t simply drop something from orbit — you need to propel it out of orbit — and it won’t hit with anywhere near the energy of a nuke.

  11. Kirk says:

    Depends entirely on how you define your terms, though…

    The best write-up on the concept is in the supporting literature for Project Thor; it’s been a few years since I dug into that stuff, but there’s a set of equations somewhere that calculates just how much damage such a kinetic strike would work up to. While it is true that you’d have a hard time getting up to thermonuclear levels of energy with something like a flying telephone pole, the damage is a lot more localized and concentrated, such that you might not flatten a city, but that bunker you were aiming it at would get the effect of having had a nuke go off near it. It’s all about the kinetic energy; mass times speed…

  12. Harry Jones says:

    You can’t get more energy out than you put in. Look at the fuel you burn to get the projectile up there, and imagine using that same amount of fuel in a fuel-air bomb. That’s a basis of comparison.

    Oh, but the rod is more concentrated, you say, to drill through a bunker roof? Yes, only you can’t aim it with any kind of precision, so it’s scattershot or forget it.

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