Quick, if you were an alien and had to severely weaken the human race what would you do? Would you release a deadly bio-chemical weapon? Introduce some new species, decimating the native flora? Use some sort of EMP weapon to eliminate electronics? What about a simpler approach?
What about dropping something really big?
What is it?
A kinetic bombardment (sometimes called a mass drop) is a fairly simple concept. Mass is dropped. In other words, the idea behind a kinetic bombardment is to take something massive, and well, drop it.
And wouldn't you know, humanity has been using kinetic bombardments (in a sense) for thousands of years. If it comes from a non-horizontal direction, doesn't use explosives, and has a blunt impact then it's technically a kinetic bombardment. A particularly ancient example occurred during the ice age when hunters would lure mammoths and other prey into lower areas, then drop heavy rocks on them from above. More modern examples can be found in the World War 2 and Cold War eras when multiple bombs were developed that were designed to weigh as much as possible and be dropped from large heights. While the intended purpose of this was to have the bombs penetrate the earth and explode underground, testing revealed that the massive forces involved (imagine 22,000 pounds being dropped from almost 12,000 feat and moving at nearly supersonic speed upon impact) would effectively cause two shock-waves, one when the bomb landed and one when it exploded.
Now imagine, instead of using a bomb or a small load of rocks to attack the earth, you decide to use something a bit more massive.
How would aliens do it?
While the idea of dropping tungsten – a metal with a remarkably high melting point and incredible density – rods the size of telephone poles from orbit has appeared from time to time as a means of precisely striking targets, it seems far more likely that an alien invasion force would simply make use of the supplies in the area rather than haul something that massive across the galaxy.
And conveniently we have plenty of fairly massive, dense object just 56 million miles from earth. Asteroids. All it takes are a few well placed, powerful impacts to knock an asteroid out of orbit and - if properly aimed - directly on a collision course with earth. Furthermore, while the idea of using asteroids as weapons has been explored in the realm of science fiction before, the common saying, “sci-fi writers have no sense of scale” makes itself easily known when looking at the actual science.
What kind of damage could it do?
It turns out you can do quite a bit of damage just dropping something massive enough on something else, especially at a high speed. However, most sci-fi writers tend to drastically overestimate the mass required to cause widespread destruction.
While many sci-fi writers weave terrifying tales of asteroids or even moons hundreds to thousands of miles across, the fact of the matter is that they are drastically underestimating the power of a smaller asteroid. The asteroid we think contributed to the extinction of the dinosaurs? 10 kilometers wide. Sure, that seems pretty large for a single rock, and it is, if you're only looking at rocks here on earth. It turns out there are an expected 12,000 asteroids that are that size or bigger in our asteroid belt alone and that number gets multiplied by 7 if you want to count the asteroids half that size.
And while I could go into excruciating detail into the effects of and size of numerous impacts that have occurred here are some pictures instead:
|Artist's model of the Chicxulub impact. The Crater measures 180 Km across|
|Fallen trees after the Tunguska event. The asteroid a few dozen meters in diameter didn't|
even land, but it caused a blast 1000 times stronger the the Hiroshima nuclear blast.
Depending on the type of asteroid, its size, and where it lands, we could face everything from earthquakes and volcanic activity to slightly more startling events.
If the impact occurs in or near water, we can expect absolutely massive tsunamis reaching miles into the air. In fact, an impact 35 million years ago into the Chesapeake Bay area is suspected of causing a tsunami that topped the Blue Ridge Mountain Range.
Other devastating effects of a sizable impact include fires being started by super-heated fragments of ejecta thrown into the air, an (ever popular) dust cloud -- hindering photosynthesis and devastating vegetation -- and acid rain caused by various sediments lingering in the atmosphere.
Perhaps most terrifying of all is the massive burst of infrared radiation that most life would be subject to. It turns out that if enough heavy ejecta is thrown into the atmosphere then the force and heat generated by re-entry would cause a short but intense burst of infrared radiation.
Altogether this means that, should an alien force decide to attack us with an asteroid, even if we survive the impact, the aftermath will not be pretty.
Unfortunately, without a good deal of warning it can be quite difficult to prevent a collision. While a common movie solution is to simply "blow it up," this would probably makes things much worse. Unless the object is broken up to the point where no pieces large enough to cause damage remain, all blowing it up does is create more than one crater. Worse still, because the amount of energy produced from multiple smaller impacts is the same as one large one, its possible that spreading out the energy could simply heat the entire planet to an uninhabitable level, rather than just a localized area.
Another common idea is to simply adjust the course of an asteroid, thereby causing it to miss Earth. However, most of these ideas would require months of planning and analysis in order to have the slightest chance of succeeding.
So if we can't destroy it, and we can't move it we're pretty much doomed, right? Not quite. The good news is that the same thing that makes the asteroid so difficult to keep from hitting Earth also makes it difficult to move it onto a collision course. Inertia - the tendency for an object to resist any change in motion - means any attempt to get an asteroid to leave its current orbit and set it on a course for earth would require not just a precise application of energy to move it in the right direction, but also a great deal of it.
Of course there's always the small comfort that any civilization capable of reaching our solar system and moving asteroids onto a collision course with earth probably has better ways of killing us all.
Rusty is the co-founder of How To Survive Alien Invasion Novels, and gets up to all kinds of shenanigans planning for any and all kinds of apocalypse when he's not busy reading, writing, or yo-yoing. Keep up with him and Rachel on Facebook and Twitter to get cool, space related news or click here to read more of his thoughts on the terror of alien invasion novels.