Defending Earth from an invading fleet may be a near impossible feat, but here are some ways humanity can be saved from annihilation.
Alien invasions have taken many forms in sci-fi from the relatively straightforward wholesale extermination of the human race in War of the Worlds, to subversively replacing people with duplicates in Invasion of the Body Snatchers, to simply causing as much damage to a major city as possible in The Avengers. But a real-life attack from outer space would be far more difficult to repel than these movies might suggest.
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We Don’t Come In Piece
Of the various invasion strategies, the “we come to destroy” format is certainly the most spectacular and is often used in movies, television, and books. It’s a formula many sci-fi fans know by heart.
First, aliens arrive in giant interstellar ships as they do in Independence Day, then there’s a period of uncertainty as the ships get into position and coordinate with each other. Things end as several of the world’s monuments are blasted to pieces.
Or, aliens could opt to skip the pleasantries altogether and get straight to the systematic elimination of the human race like they do in The Fifth Wave, Oblivion, and many more. At this point, the only question that remains is whether there’s any way for humanity to defend against such an attack.
The short answer is no. The longer answer is, “it’s very unlikely.”
As the late theoretical physicist and cosmologist Stephen Hawking said, “Meeting an advanced civilization could be like Native Americans encountering Columbus. That didn't turn out so well.”
Essentially, a race that’s capable of traversing the vast expanse of space to find this little world certainly has the technology to destroy or conquer it. Even in sci-fi movies where humanity eventually prevails, it’s usually due to luck.
Examples of this include uploading an alien-compatible computer virus, actual bacterial diseases killing the extraterrestrial invaders, and alien defectors who seek to save humanity by offering support. In most cases, the aliens are able to successfully wipe out a large percentage of the human race and cause untold damage around the world.
Optimistically speaking, there’s a 50/50 chance that aliens will have peaceful intentions like in Arrival, or seek to kill as many as possible. These examples also fail to take invasions with seemingly inexplicable motivations like in Annihilation into account. Either way, there seems to be no way to deal with any of these scenarios.
More Immediate Threats
The main problem is that humanity tends to be short-sighted when it comes to preparing for extraterrestrial threats and would much rather fight with each other. While almost every military power is interested in outer space as “the highest of high grounds,” it’s primarily in service of gaining a strategic advantage over the other nations of Earth.
There’s also no reason for anyone to consider pointing weapons outward, even as a defense against large asteroids, despite theories that one may have caused dinosaurs to go extinct. Even if this weren’t the case, the 1967 Outer Space Treaty – signed by most of the world’s countries – prohibits any nation from placing a weapon of mass destruction in orbit.
The treaty makes sense because having an orbital weapon would cause such an asymmetry of power that it could spark another cold war or possibly a global conflict. These weapons wouldn’t need to be too sophisticated either. Take the theoretical kinetic bombardment weapon “Rods of God” as an example.
A military power could place a satellite in orbit, filled with giant metal rods with thrusters attached to each of them. It could then tactically drop one of these rods from outer space onto a target. The combined speed of gravity and thrusters would be enough to give that ordinary hunk of metal the destructive power of a nuclear weapon. The concept was demonstrated in the 2013 movie G.I. Joe Retaliation using an orbital weapon called "Project Zeus," which destroys the city of London.
Establishing Terrestrial Defenses
We could fire missiles, but it would take days or weeks for one to reach its target, giving the ship plenty of time to destroy it or get out of the way. Another possible solution is to develop terrestrial weapons like massive rail guns. Using the same technology that the CERN Hadron Collider uses to accelerate particles, or similar to how EMALS launches aircraft, electromagnetic rails can speed up a projectile so that it can be fired into space. The US Navy has considered installing hypersonic railguns aboard some of its ships, but they're too expensive right now. With that in mind, one that can fire into space would need to be humungous, and there’s a whole host of problems that come with this solution, the least of which being that one could still spark another world war.
Plus, it takes a lot of time and energy to speed an object that’s larger than a particle to near the speed of light. The projectile will grow in mass as it approaches the speed of light, requiring even more power to push it forward. It might take more energy to fire one shot than any nation could afford. Then there are mind-boggling calculations that would be needed to hit a moving target in space from the surface of a spinning planet that’s orbiting a sun.
There’s also the likelihood that a large projectile moving that fast within an atmosphere will cause air molecules to fuse along its surface, creating a nuclear explosion once it leaves the barrel. So, the projectile has to move fast enough so that an incoming spaceship can’t dodge it, but slow enough so that it won’t blow the whole installation up. That assumes that there’s just one ship. There’s little a single gun can do against a fleet of spaceships.
Earth would need a bunch of massive railgun facilities built around the world to defend itself, including in the polar regions and the middle of the oceans, turning the planet into a battle platform. These stations would have to be manned, monitored, and maintained while being prime targets for hostile human nations and terrorists who either want to sabotage the facilities or gain control of a WMD.
Then, there’s an ethical issue with these weapons in that anything launched into space would travel indefinitely until it hit something. Even if it takes thousands of years, Earth could accidentally fire the shot that destroys life on some other planet. Even by our own standards, having these weapons at all would be justification enough for an alien race to invade and wreck the place.
Setting Up Orbital Defenses
So, it’s clear that in order to stop an alien invasion from reaching Earth, we need defenses set up in space. To get that, we’d need a global organization to oversee and maintain these defenses. Ideally, that would mean world peace and a single planetary government like in the Star Trek universe.
Even if humanity could achieve that monumental milestone, it would be a small step in Earth’s global defense. We would still need a way to get things to and from space in a cheap and safe way, and that’s when the real problems start.
The planet is a completely vulnerable rock floating through space, with nothing but an orbiting moon to use as a natural shield. Unlike ground wars where mountains and horizons act as barriers, the planet itself has none of that, which is why large asteroids are significant threats.
Orson Scott Card describes the issue with Earth defense in detail in Ender’s Shadow, the first book in a spin-off series of Ender’s Game. Earth would need a network of orbital forts to defend the planet. Each would have to be manned and maintained to keep a constant watchful eye in all directions, consuming massive resources in doing so.
Even if we could afford the cost, orbiting stations would be weapons of last resort, since the invading fleet would have to be fairly close to the planet for these fortresses to be effective. Or hostile aliens could just attach thrusters and navigation systems onto some far-off asteroids before sending them our way as kinetic weapons.
Blasting these asteroids wouldn’t stop them. They’d just break apart into a bunch of smaller asteroids that would devastate large parts of the planet. We’d need to set up bases further out, like on the moons of Jupiter and along the asteroid belt. Of course, the further you go out, the more complex things become.
You can’t establish defenses around all points of the solar system. Whatever you do set up needs to be manned, monitored, and maintained. Eventually, defending Earth would require more resources than anyone could possibly afford.
Sending Ship Patrols
Having our own fleet patrol the solar system wouldn’t help much either. Invaders can split up and attack from all directions. Even at relatively high speeds, moving ships across the great distances of space requires a great deal of time (it currently takes about three days to reach the moon from Earth).
Positioning ships to deal with one threat may leave the planet vulnerable in a different direction. We ultimately end up with the same problem as the orbiting space stations. It would require far too many resources to maintain an effective defense.
Even if we were to destroy 99 out of 100 enemy ships, the one that got away could cause massive damage by wiping out most of the world’s population centers. Ender’s Shadow doesn’t resolve this strategic conundrum, other than to surmise that the best defense is a strong offense. Specifically, strike the alien homeworld before they have a chance to reach us.
Fortunately, there are solutions besides turning into an evil empire that wipes out alien civilizations with unprovoked attacks, thus becoming the thing we fear. One is to colonize other planets and solar systems. It wouldn’t stop an invading fleet from destroying Earth, but the human species would have a better chance at surviving if it wasn’t conveniently located on one single planet.
Hawking proposed that we avoid contact with alien races altogether, which may seem easy given the enormity of space. But Earth has been broadcasting radio and television signals into space for decades and will likely continue doing it for decades to come. Aliens would simply need to catch some of those signals and figure out where they are coming from.
Creating a Planetary Shield
Ultimately, the best solution might be to set up a planetary shield like the one seen in Rogue One and Star Wars: The Force Awakens. An impenetrable force field like in those movies may not be possible with our current technology, but there is a way to cheaply make a defensive barrier around the planet. However, like with many cheap and easy solutions, it’s not a very good idea.
It’s called the Kessler syndrome, and it’s what happens in the movie Gravity. The chain reaction involves orbiting high-speed space debris that destroys everything in its path. This includes satellites, the International Space Station, and the Hubble Telescope. In destroying these structures, the cloud of fast-moving space junk will grow until it takes up most, if not all, of Earth’s orbit.
That cloud of killer garbage will stay in orbit for hundreds of years, shielding the planet from a multitude of extraterrestrial threats. It will also prevent humanity from launching anything into space. Everything that relies on satellite coverage, including navigation, weather tracking, and communications will come to an end.
A junk shield may be a method of last resort, but we could already be on our way toward creating one. In 2007, the Chinese government destroyed one of its own inactive weather satellites to test its missile system.
The act created thousands of pieces of space debris, adding to the already crowded orbital space we have around Earth. The mess drew immediate condemnation from the international community, but maybe they’ll be more forgiving if the garbage ever repels a hostile alien advance. Then again, we may never know because we wouldn’t have satellite-based monitoring or communications systems to detect or speak to these aliens and learn their intent.
How do you think Earth can be saved from an extraterrestrial attack? Let us know in the comments!