Because of other more urgent threats to our existence, defending the Earth from a possible attack by an extraterrestrial invasion is not a high priority. Until we get rid of the thousands of nuclear weapons on our planet, the likelihood of us blowing ourselves up is much greater than the chance of some aliens doing that. But imagine it is the year 2100, and man has removed the most urgent short-term threats to his existence, by solving the problem of global warming and eliminating the threat of a nuclear holocaust. We might then look at reducing the long term threats to mankind, including the threat of an alien invasion. What technologies might we use to defend ourselves from an alien attack? Below is a list of some of the technologies that could be used.
1. Defensive Space Ships
A defensive space ship is probably the first thing that comes to most minds when they think of defending Earth from alien attack. It is easy to imagine a space ship armed with some kind of laser cannon or guided missiles or even conventional cannons. Such a ship could move close to any invading alien ship, and launch an attack against it. However, such ships might not be the most effective way to defend the planet. The hot rocket exhaust of a spaceship gives off an infrared signature that would be very easy for any invading alien ship to detect. It would probably be very easy for an alien space ship to detect some other ship that was approaching it, and launch a counterattack that would stop it.
2. Earth-to-Space Ballistic Missiles
This type of defensive weapon would be easy to construct using technology only slightly different from existing intercontinental ballistic missile technology. Instead of traveling from continent to continent, the missiles would travel from the ground to an orbit around our planet. The missiles would be designed to blow up a nuclear bomb near an alien spacecraft that had started to orbit our planet. The weakness of this technology is that the hot rocket exhaust of such a missile would give off an infrared signature that would be very easy for an alien ship to detect. So an alien ship might be able to detect the missile's launch, and blast the missile into pieces while it was rising through the atmosphere.
3. Orbiting Nuclear Bombs
There is currently a treaty that prohibits placing nuclear weapons in orbit. But if that treaty could be modified, we could put into place a simple and reliable system of planetary defense by placing a host of hydrogen bombs at various different orbits around our planet. If we were then attacked by an alien ship, we could simply send a radio signal causing the satellite bomb closest to the ship to explode. That would give no warning, and would therefore be hard to defend against.
4. Defensive Lunar Mass Drivers
According to the wikipedia.org article on mass drivers, “A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds.” Mass-drivers were originally conceived as a way of catapulting lunar materials towards the earth, to provide materials that could be used in the construction of space colonies. Robert Heinlein in his novel The Moon is a Harsh Mistress imagined that a lunar colony rebelling against Earth could use a mass-driver to pummel the Earth with rocks that would strike our planet with a devastating kinetic energy.
Lunar Mass Driver Hurtling Rocks Into Space
It is easy to imagine lunar mass-drivers that would serve as defensive weapons to attack invading alien ships. The mass drivers would shoot a hailstorm of small rocks at any ships that were trying to orbit our planet. A mass-driver might be able to launch hundreds of rocks every minute. Each one would hurtle through space with enough kinetic energy to cripple an alien spacecraft. Even if the ship had some kind of defensive technology, or “hull breach sealing” technology, it might not be able to defend against an onslaught of hundreds of rocks being propelled every second from a large group of mass drivers on the moon.
5. Asteroids with Mass Driver Weapons
Just as it would not be very hard to construct a mass-driver on the moon, and turn it into a weapon for destroying invading alien ships, the same technology could be employed on near-Earth asteroids. Imagine an alien ship has reached orbit around Earth. The ship detects an asteroid coming within 20 miles of our planet. This seems like nothing to worry about. But then suddenly the asteroid starts shooting a hailstorm of small projectiles aimed at the alien ship, and a similar rocky hailstorm shoots out over and over again. That type of ambush would be very hard to defend against.
6. Radio-controlled Asteroids with Nuclear Bombs
7. Directed Energy Beam Weapons
Powerful as they may be, the types of weapons mentioned above all would travel much more slowly than the speed of light, possibly allowing an alien ship some chance of defending against the attack. For the purpose of an ambush, the most effective weapon would be one that operated at the speed of light. One such weapon would be a powerful laser beam weapon. Such a weapon might be hidden on the surface of the moon. A laser beam weapon shot from the moon would give an alien ship orbiting Earth only two seconds to defend itself. That would be pretty much the ideal ambush.
Do we have any of these types of weapons now? No. We are sitting ducks, ripe for conquest. So if you ever look up in the sky and see a star in the sky far brighter than any you have ever seen before, you had better say your prayers that the arriving visitors mean well.