During the Cold War, it was generally accepted the superpowers’ nuclear arsenals were the ultimate weapons available at the time. However, thanks to their space programs, it would have been possible for either side to escalate the level of destruction available by at least an order of magnitude. Given the aggression and paranoia both superpowers exhibited at times, it was probably fortunate that this option was never taken seriously, or if it was, it was never enacted upon.
The option in question is deliberate asteroid bombardment. Though it has only sporadically been studied in the real world as a means of warfare, variations of it has been seen in numerous science fiction sources. Links to articles detailing the exact effects of asteroid and comet impacts are at the bottom of this article.
In the related article Redirecting Asteroids (link at the end of this article), a number of methods was discussed for nudging both asteroids and comets onto new trajectories, either to avoid a catastrophic impact with Earth or to push such bodies into more advantageous orbits for economic exploitation.
However, if one wanted to, one could use the same techniques to hurtle one of these flying mountains deliberately into Earth’s path. If done with precision, such a massive impactor could be made to hit any specific spot on the planet, including an enemy stronghold. This is actually a capability that has been available since at least the late 1960s, but no one has yet taken advantage of it.
For most of the Cold War era, the scientific community was largely unaware of the role played by such collisions in the history of Earth, and of how truly devastating an asteroid impact could be on a large scale. More importantly, they only had rudimentary theories on the composition and nature of this cosmic debris. Even though they could reach nearby asteroids with spacecraft and effect a change in course via nuclear impulse weapons, it would have also taken months using the computational methods at the time to work out a new trajectory for any impactor, and pinpoint accuracy was by no means guaranteed.
Today, any power that can boast interplanetary spaceflight capability can use asteroid impactors as weapons. Besides the US and Russia, today this includes the member nations of the ESA, and in the near future will likely include China, India, and Japan as well.
A potential asteroid weapon would have to be carefully chosen, and its trajectory laid out with equal attention to detail. Ideally, a candidate impactor should be just large enough to cause tremendous local damage, but not so massive as to create global environmental havoc and risk the redirecting nation’s existence.
Composition of the impactor will be a factor to consider. Asteroids composed mostly of nickel-iron are ideal, as they are the objects mostly likely to make it through the atmosphere intact. Carbonaceous asteroids and comets are thought likely to break up high in the atmosphere, their relatively fragile structures shattering as they hit the thicker layers of the lower atmosphere at the tremendous velocities orbital deadfalls generate. This is not necessarily a detriment to their use, as the Tunguska Explosion of 1908 showed. That impact of a small comet or asteroid shattered six miles above the ground, unleashing some 20 megatons of explosive force that felled trees for nearly a thousand square kilometers around the point of impact.
The exact size of the asteroid chosen would probably depend on the desired level of destruction and the estimated speed with which it would impact. The higher the velocity, the less mass needed to cause a target goal of collateral damage. Smaller asteroids would cause the same amount of destruction as nuclear weapons, but would not have the accompanying radiation and radioactive fallout, making them ideal for taking out enemy strongpoints and cities in a region that an aggressor may plan on capturing at a later point.
Asteroids capable of delivering Hiroshima-like blasts would usually measure between 10 and 50 meters across, depending on its final impact speed. Impactors measuring 100 meters or more across are capable of delivering multi-megaton level damage. Asteroids larger than one kilometer across would unleash catastrophic environmental damage across an entire continent and plunge the world into a nuclear winter with gigaton or even teraton-level power. With impactors much larger than this an aggressor risks triggering a guaranteed mass extinction event.
Availability of weaponizable rocks becomes another factor in this kind of warfare. Even though there are some one million or so usable asteroids (neither too small so that they would burn up completely in the atmosphere, nor too large as to trigger major global damage) near Earth's orbit, finding the ones that are of the right composition, in the proper position, and on a trajectory that can be altered favorably is another matter. Comprehensive full-sky surveys would be needed to find which ones can be put to optimal use.
One tactic that can be deployed here is not diverting the asteroids from their original orbits directly to their targets. Instead, a small population of them can be diverted into "holding" points closer to Earth until they are needed. Potential stable holding points for impactors include the Earth-Moon L4 and L5 LaGrange points, Lunar orbit, or even the Earth-Sun L4 and L5 LaGrange points. In fact, putting the clusters of impactors near Earth where they can clearly be seen in the sky with the naked eye would tend to have a tremendous psychological impact on a potential rival.
This is also not an option for the impatient. Even the nearest asteroids would take weeks or months to properly divert. Acquiring ammunition from even farther out in the solar system may take years to accomplish.
Of course, the entire purpose of such an attack may indeed be total global devastation after all, in which case larger and more massive impactors will be used. Such a capability may be used by one faction or another as a potential doomsday option, a threat to frighten rivals into submission. A demonstration on another celestial body, such as the moon or a nearby planet, may be arranged to prove that they have such a capability.
In the future, as human civilization becomes an interplanetary and perhaps even an interstellar entity, bombardment by doomsday rocks may be used deliberately to remove entire worlds from the strategic equation.
However, asteroids as weapons have two serious drawbacks that have so far prevented them from being considered seriously as potential weapons: cost and targeting accuracy.
At current launch and spaceflight costs, sending a mission to divert a nearby asteroids would cost at least hundreds of millions of dollars and would require constant monitoring of a year or more. Nuclear ICBMs, which do a comparable level of damage and can be deployed much quicker, are a bargain in comparison.
This may not always be the case, however. As major infrastructures are built up in orbit and beyond in the decades to come, access to space rocks may become much more common and economical. Their potential use as weapons will likely increase as well in these circumstances.
The other major obstacles is pinpoint targeting. Asteroids usable as weapons often mass millions of tons, and that's on the low end of their size scale. On top of this, they are usually very irregularly shaped and can have wildly varying density throughout. As one can imagine, these are very difficult objects to wrestle about with precision. The variations in shape and densities can throw off course corrections very easily. Also, unforeseen fault lines within the rock could cause portions of it to fracture or break away during or after a course alteration, or even release outgassing that can change its trajectory.
All this adds up to a projectile that is by no means guaranteed to hit the target you aim it for, especially with the techniques available at lower tech levels. One would need a redirecting spacecraft to stay with the impactor for most of its journey to make the necessary nudges as needed, right up to the point where its entering the atmosphere, to ensure a direct hit. Asteroid-anchored rockets and dedicated asteroid tug spacecraft are ideal, perhaps coupled with other redirecting techniques.
Asteroid-sized artificial masses can also be used similarly. In the long-running anime science fiction series Gundam, renegade colonies in orbit unleashed their ultimate weapon against the nations of Earth: dropping an asteroid-sized O'Neill Colony on the planet with apocalyptic results.
Instead of hitting a planet with one asteroid, you hit it with multiple large impactors all at once. This can be accomplished in two ways: actually gathering many asteroids in one place and then set them hurtling together toward the target world, or by detonating a single large asteroid so its many fragments will impact the world in a short amount of time. While Cold War Era technology is up to the task of redirecting single asteroids, herding them accurately in large numbers or knowing enough about their structure to fracture one with precision is the task of more advanced periods.
While the results of single catastrophic impact are fairly well known at this point in time, no one is really sure of the exact effects of large numbers of impactors hitting a world like Earth all at once. Imagine several hundred or even several thousand Tunguska-like events striking all over the globe within a period of several hours. Besides the expected global dustclouds and land devastation and tsunamis, hundreds of large impact events could super-heat the atmosphere over large swaths of territory, perhaps sparking off a world-wide firestorm.
Asteroid swarms are definitive doomsday weapons for inhabited worlds. It is hard to imagine any nation or organization today would attempt such an attack. However, in the future when interplanetary or interstellar societies become possible, asteroid swarm bombardment may become a viable tactic.
Though arranging the impactors to strike within minutes of each other is theoretically doable, given the scale of such a bombardment, it will likely have to be carried out over a period of several hours or days or weeks, as the mountain-sized rocks are slowly maneuvered about over distances of many thousands (or even millions) of kilometers.
Asteroid swarm bombardments can be seen in two very different on-screen science fiction properties: This Island Earth and Space Battleship Yamato, both of which depict one civilization trying to grind another under its heel by bombarding their rivals' homeworlds with swarms of asteroids. In the real world, a very dramatic example of a multiple asteroid bombardment was seen in 1994, when fragments of comet Shoemaker-Levy 9 crashed spectacularly one after another into the atmosphere of Jupiter.
A tactic that requires significant more tactical and technological finesse is using space debris such as meteoroids to attack smaller targets without causing any kind of major environmental damage. Their effect on the battlefield would be the equivalent of artillery shells launched from orbit. Since acquiring, launching, and aiming enough small impactors to make this scheme practical is a much harder task than just redirecting a single large impactor, this weapon system's tech level is that much higher.
The size of these impactors would be significantly smaller than those previously discussed, perhaps several to a dozen meters across at most. Nickel-iron asteroids or similar heavy metal compositions which could endure fiery re-entry all the way to the ground would be a must; the atmospheric detonation strategy simply would not work with projectiles this small.
Even with highly advanced technology, it would be quite impractical to hunt down appropriately-sized projectiles from all over the solar system. It would be much easier just to mine the rocks from a larger source. This way, not only would an aggressor have a very large supply of potential ammunition, but the impactors could even shaped and sized to specification.
Two ways have been proposed for providing meteoroid impactors in quantity: one is with a large asteroid redirected into orbit about Earth, and the other is from a base on the Moon. In both instances, mining of the impactors will likely be carried out by automated machines and shot into holding orbits with electromagnetic launchers. There, a launcher spacecraft or satellite will rendezvous with and gather the meteoroids for use in the actual bombardment. These in turn will also likely be electromagnetic launchers. Alternately, many small, cheap, expendable flight packages--composed of a small rocket to break orbit, radio transceiver, and flight avionics--will mate with individual rocks and await a signal to launch them at a target.
Problems with precision targeting will persist as with the larger asteroids, complicated even more now by the effects of atmospheric entry on these smaller projectiles. Since meteoroids would not be ideal for very deep penetration attacks, such tasks would likely be left to other weapon systems while this bombardment system concentrates on surface damage. Because of targeting issues, these are most effective when employed in swarms of half a dozen or more.
Meteoroid bombardment is most effective against non-mobile installations such as airbases and radar installations, but can be used to target slow-moving vehicles or vehicles moving in a predictable pattern, such as tank columns, naval surface ships, and trains.
For an example of tactical meteoroid bombardment warfare used in science fiction, refer to the novel Footfall by Larry Niven and Jerry Pournelle.
http://impact.arc.nasa.govhttp://www.lpl.arizona.edu/impacteffects/ http://www2.jpl.nasa.gov/sl9/ http://orbitalvector.com/Solar%20System/Asteroids%20And%20Comets/Redirecting%20Asteroids/REDIRECTING%20ASTEROIDS.htm http://22.214.171.124/search?q=cache:-LeNiihg1m0J:www.rand.org/pubs/monograph_reports/MR1209/MR1209.appc.pdf+asteroid+as+weapon&hl=en&ct=clnk&cd=3&gl=us
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