On the morning of June 30th of 1908, an asteroid 60 to 190 metres in diameter struck the Podkyamennaya Tunguska River in Siberia, Russia. The impact released 15 megatons of energy, an explosion one thousand times more powerful than the atomic bomb dropped over Hiroshima. The devastation flattened 2000 square kilometres of land, laying waste to the plant and animal life caught in the explosion. No human casualties were reported. If the asteroid had struck a densely populated region of the Earth, it would have gone down in history as the most catastrophic natural disaster in recorded history.
Fast forward to the year 2016. SpaceX has developed the rapidly and fully reusable rocket, thereby significantly cutting the cost to access space. Planetary Resources is building satellites that can detect, survey, and mine asteroids. We are on the cusp of living in an age where humans are no hostage to the capricious whim of the heavens. We will soon have the ability to reach deep into space and bend the heavens to our will. Despite our rapid technological progress, human nature has not changed. Our species is the only one that has perfected the art of war.
Presently, the most powerful weapon in our arsenal is the thermonuclear bomb mounted atop an intercontinental ballistic missile. It has the furthest reach, the greatest impact, and the resulting nuclear fallout would render the enemy territory completely inhospitable for decades or even centuries.
In the future, we will have an even more devastating weapon in our arsenal: the asteroid.
In theory, it is possible to alter an asteroid's trajectory using a small satellite. The satellite's gravity well would pull the asteroid towards it, and over time, this small shift could lead to a radically different trajectory headed straight for Earth. The satellite would only require a small booster to stay in orbit around the asteroid. At any time, the satellite could use its boosters to adjust its relative position, and the asteroid would be tugged along by the satellite's gravity well. Thus, it is possible to finely tune the asteroid's trajectory in order to target a specific location on Earth.
But there are challenges to using asteroids as weapons of mass destruction.
First, the asteroid has to be just the right size and travelling at just the right speed as to not cause a global extinction event. An asteroid matching the one that struck Tunguska in 1908 would be ideally suited for the purpose of laying waste to an enemy city. Since asteroids are difficult to detect, this would require a huge network of satellites to scan the heavens.
Second, if a network of asteroid-hunting satellites already exists, it is more than likely that any attempt to alter an asteroid's trajectory would be detected. This would lead to counter-measures to thwart any attempt to use that asteroid as a weapon. More importantly, any entity that gets caught would be condemned by the global community.
Third, altering an asteroid's trajectory takes time. It could be months or even years before the asteroid strikes its intended target. Thus, an asteroid would not be the ideal weapon to use if rapid destruction of enemy territory is desired. However, if it is anticipated that the war would last for years, then an asteroid might be ideally suited to deliver a devastating blow to the enemy homeland, thereby crippling their ability to wage a sustained war. The benefit of this approach is that it does not create a nuclear fallout. Thus, the enemy territory would remain habitable, which is ideal if invasion and conquest of the enemy homeland is the goal.
In conclusion, it is theoretically possible to use asteroids as weapons of mass destruction in the near future. However, given the challenges and limitations to this approach, it is unlikely that any nation would use this tactic given the availability of other weapons of mass destruction, such as nuclear weapons.
The purpose of this article isn't to suggest that we should use asteroids as weapons. Rather, the purpose of this article is to point out that certain state actors may attempt to use this tactic. Thus, it would prudent to start thinking about counter measures that could be employed against such tactics.
Just as a side note: if we humans ever decide to invade an alien civilization, the easiest way to wipe them out would be to bombard them with asteroids. This doesn't leave any radiation, so we would be able to conquer the planet and live on it without dying of radiation poisoning ourselves. So why don't we ever see this in Hollywood? Why do aliens in movies always attack us with lasers, bombs, and other fancy-dancy doodads? Just use asteroids. They're way more devastating, they don't cost anything, and they don't require any fancy technology. Just saying.