This article covers weapon systems specifically designed for orbital bombardment. Intentional asteroid and meteoroid bombardment will be dealt with in another article, as will orbital bombardment by spaceships.
Orbital bombardment is just what the name implies--targeting and firing weapons at a planet's surface from orbit. Though not well advertised, it was the fear of the development of this technology which fueled the early years of the Cold War-era Space Race between the US and the USSR. If one side developed advanced spaceflight capabilities and the other didn't, that meant that one side could use orbital technology to target the other with impunity. Neither side was willing to be that vulnerable, and fanatically rushed ahead with rocket technology to make sure they didn't fall behind. It was with a sigh of relief when both sides signed and--suprisingly--adhered to the Outer Space Treaty of 1967, which forbade the placement of nuclear weapons in space. The SALT II treaty of 1979 placed further limitations on such technology by the major players in the Cold War, but does not prevent non-signatories of the agreement from developing such weapons in the future.
One of the major complications with orbital bombardment of a planet such as Earth is its relatively thick atmosphere. Air friction will tend to burn up or deflect incoming projectiles, and many beam weapons such as lasers and particle beams will scatter in the miles of thick atmosphere before it ever reaches the ground. Of course, bombarding worlds with no atmosphere, such as the Moon, will encounter no such difficulties.
One way to get through the atmosphere is to just power your way through it. A big enough projectile or powerful enough beam would be able to just slice through the atmosphere as if it wasn't there. However, for most weapon systems, this takes an enormous amount of energy, equivalent to many nuclear bombs' worth, so such tactics wouldn't be available until relatively high tech levels.
An alternative tactic would be to try for an upper-atmosphere detonation. If the weapon system can't penetrate all the way to the ground, you could instead focus the energy beam/projectiles on a point in the atmosphere you know the weapons can reach. Piling enough energy onto this one point all at once will result in an explosion and shockwave that in turn will slam into the ground.
An example of this, albeit from a natural source, can be found in the Tunguska Explosion of 1908. A comet fragment some 20 meters across was pulled into Earth's gravity well, plummeting toward the ground at over seven miles a second. At those speeds, the thick layers of the lower atmosphere acted pretty much as a physical wall. The comet fragment shattered, unleashing some 10 to 20 megatons of power from its pent-up kinetic energy some six miles above the ground. The resultant explosive shockwave was powerful enough to knock over 80 million trees over 830 square miles around the point of impact.
The remainder of this article mostly addresses weapon systems that can hit the ground directly. However, many space-based weapon systems not discussed here could conceivably attempt the upper-atmosphere detonation strategy.
There's also the issue of which orbit to place such weapons. A Low Earth Orbit will mean that it has a high orbital speed, and will zip over its potential ground target quickly, giving it a narrow firing window. Its low orbit will bring it back into position in as little as 15 to 20 minutes, but in an ongoing conflict that can be an eternity. Higher orbits can mean more time over a potential target and a longer firing window, but the weapon will also have to cover more distance between its position and the ground to reach its target. Geosynchronous orbit will allow a bombardment system to essential hover continuously over its target, keeping it always in sight, but in return any potential projectile will have to cover the 22,300 miles between it and the ground, which could take hours. Designers of orbital bombardment systems will have to take this potential trade-off (strike distance versus open target windows) into account when creating their weapons.
Basically, this is an orbital missile. The weapon is shot into space and placed into low orbit just like any other satellite. The difference here was that this satellite is a nuclear warhead, and would de-orbit itself and plunge through the atmosphere at a planetside target when signaled to do so. Like with warheads of ICBMs, the weapon would be cone-shaped and hardened for fast re-entry, with a small liquid-fueled maneuver rocket and flight avionics attached to allow for a swift de-orbit burn to let it coast to its target. Its orbital capability put no limits on it range, and its orbital trajectory would give only a vague indication of its actual target.
The Soviets actually did develop a Fractional Orbital Bombardment System (FOBS) in the 1960s. It was called "fractional" because it needed not complete an entire orbit to work, only a fraction of one. In the wake of the aforementioned 1967 treaty, it was tested without a live warhead. FOBS became an official part of the Soviet arsenal for nearly two decades, though it was never deployed with an actual live weapon. The SALT II treaty of 1979 explicitly forbade any FOBS system from being deployed, and the Soviets quietly faded the weapon out in the early 1980s.
Still, this is a capability available to any nation or organization with access to nuclear technology and space, which now includes at least a dozen countries. It is one reason why ballistic missile technology is as closely monitored as nuclear capabilities by the international community.
One of the major drawbacks of the Soviet FOBS was it lacked precision targeting. It was good enough to hit somewhere in the vicinity of a city, but could not be maneuvered precisely enough to guarantee a kill on smaller, specific targets, such as early-warning radars or military installations. This was why it was deemed impractical for hitting hardened targets, and remained classified as a strategic as opposed to a tactical military asset.
Today, with far superior electronics and positioning systems available, this limitation can be easily overcome. A modern FOBS-like system could land a nuclear warhead anywhere on Earth, probably with a variance of a few hundred meters from its target at most.
Another disadvantage is its lack of stealth. When first conceived, FOBS would have allowed the Soviets to launch a surprise attack on the US, allowing the missiles to approach North America from the south, as opposed to going over the north pole, where much of the US's early-warning radar arrays were located. Today, however, with world-wide monitoring provided by satellites and a large array of ground-based radars monitoring objects in space, it would be near impossible to disguise the launch and trajectory of a FOBS warhead. Unlike submarines, which can remain undetected underwater even within a few miles of shore, tracking a FOBS warhead would be a simple enough matter from the ground.
However, it would be possible today to use modern stealth technology to help hide the warhead, provided one could find a way of employing it in orbit without being detected. FOBS warheads could also be disguised within a more normal satellite until the time came to strike.
This is a larger satellite sporting numerous ground-targeted missiles. It is the orbital equivalent of a nuclear missile submarine; it would be capable of hitting a large number of surface targets from a single vehicle. It would allow devastating first strike and rapid counter-offensive capabilities within the span of a single orbit.
When the prospect of space stations was first being bandied about by both the US and the USSR in the 1960s, it was in part their potential use as orbital missile platforms that justified their further development. Unlike a FOBS, a manned orbital missile platform such as a space station could be manually targeted and would have had a somewhat better chance to hit a hardened target than the automated systems of the time.
Unlike a FOBS warhead, a larger integrated missile platform would be much harder to hide. If constructed today, it can of course be fully automated and have near-pinpoint satellite-guided accuracy. But unlike a FOBS, it would be more difficult to disguise and stealth, because of its larger size.
This system has been informally called the "Rods From God." The idea originated in the 1950s from the RAND corporation, who suggested putting clusters of iron rods on ICBMs to act as kinetic energy weapons to use against conventional targets. Science fiction writer Jerry Pournelle expanded the idea into its current orbital satellite version. The US Military has researched the concept and several papers and reports on it still appear from time to time.
The current incarnation of the idea involves a dual satellite system, where two or more satellites are deployed in very close proximity to each other. One satellite would contain the sensors and targeting equipment, while the other would contain the actual bombardment projectiles and de-orbit launch system. The advantage of using this system is two-fold; the vibrations of the launch of the projectiles won't skew the precision targeting necessary to make the system practical, and keeping the projectile magazine separate would allow the system to be easily "reloaded" without having to replace the entire system. In fact, a single targeting satellite might support a number of nearby projectile-magazine satellites.
The actual projectiles themselves are dense tungsten alloy rods a foot in diameter and up to twenty feet long, similar to the dimensions of a telephone pole. The rods are re-inforced and tapered to allow fast re-entry, with stabilizing fins to keep it on target. The projectiles would be ejected from the launcher and plunge to the ground at over 36,000 feet per second, the sheer force exerted by its orbital deadfall making explosive warheads superfluous. The projectiles would also have a very high penetration factor, allowing them to be used to take out targets even buried deep underground.
The main barrier to developing this system is not so much technical as it is economic. It currently costs $10,000 to lift a pound of payload into orbit. The 'Rods from God' system would weigh dozens of tons, especially the projectile-carrying satellites, and the system would have to be deployed in large numbers to be effective. Many analysts simply believe it isn't worth the cost compared to many other, cheaper alternatives that can give comparable results. Only when launch costs become significantly cheaper would this weapon system become practical.
Aside from costs, there are a few technical issues that still need to be grappled with. The first is precision targeting. A variance of a few hundred meters is not a big deal for a nuclear weapon bombardment system such as FOBS. However, kinetic deadfall weapons such as the rods need to hit their target precisely dead-on in order to be optimally effective, a capability that's still very iffy at orbital speeds from 200+ miles up. In order to overcome this limitation, the rods would most likely need to be used in clusters instead of as individual projectiles. This is why they would need to be deployed in orbit in large numbers, and this in turn is why the system is likely to remain prohibitively expensive until launch costs come down by a large margin.
Others have expressed doubt that the rods could really be used for deep-penetration targeting. They may be travelling so fast that they would completely vaporize on impact with the ground, doing a great deal of surface damage but leaving buried targets shaken but intact. If this is indeed the case, the rods can be re-engineered to overcome this (perhaps by having a frangible outer layer that would absorb the majority of the impact energy, while a denser inner core retains enough of the momentum of the fall to continue the plunge deep into the surface), but this would only drive the cost of the already expensive weapon system up more.
The Tunguska Explosionhttp://www.qsl.net/w5www/tunguska.html
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