We know of many scary viruses such as the smallpox virus which killed many millions. Smallpox thankfully now exists only in two well-guarded laboratories. But it is all too possible that genetic engineering technology will allow man to one day create an artificial virus as deadly as smallpox.
Virologist Ron Fouchier of the Erasmus Medical Centre in the Netherlands and his colleagues discovered that a mere five mutations to the avian H5N1 virus was enough to make it spread far more easily. Fouchier knows of some way to tweak H5N1 to make it bad enough to kill many millions. He wanted to publish a paper describing his technique, but the publication was blocked. We can only wonder about whether future technology will allow scientists to create super-lethal viruses or bacteria capable of wiping out a large fraction of humanity.
The threat of terrorists attacking with a homemade nuclear bomb has existed since at least the time of the breakup of the Soviet Union in 1991. Making a nuclear bomb from scratch is extremely difficult, but making a nuclear bomb is fairly easy if you have acquired 100 kilograms of highly enriched uranium. The problem is that there is a huge supply of highly enriched uranium scattered in places all over the world. Supposedly outside of the world's nuclear weapons there is enough highly enriched uranium to make 40,000 additional nuclear bombs. Some of this nuclear material exists in sites that are not adequately secure. If terrorists wanted to create a nuclear weapon, they would merely need to steal a blob of highly enriched uranium equal to about the size of a softball. Given that amount of nuclear material, it is fairly easy to make a small nuclear bomb. We therefore have a threat of a nuclear 9/11 that might kill many times more people than the attacks of September 11, 2001.
EMP stands for electromagnetic pulse. An EMP bomb would be a bomb exploded in the upper atmosphere. The purpose of the bomb would not be to kill people directly but to wipe out a country's electrical systems and computer systems. Exploding a nuclear bomb in the upper atmosphere creates an effect called the Compton Effect. The gamma rays from the nuclear bomb causes the ionization of huge number of atoms in the air, and that ionization releases a huge amount of electrons. The electrons are capable of wiping out all electrical and electronic systems for hundreds of miles. It has been estimated that a single nuclear bomb exploded sufficiently high in the atmosphere could wipe out a large fraction of the United State's electrical power and computer systems.
An asteroid missile would be a devastating weapon that might even be capable of making the entire planet uninhabitable. To create an asteroid missile, a nation would need to land a rocket on an asteroid that passes fairly close to Earth. The nation would then need to change the orbit of the asteroid, causing it to crash into our planet. Once you have landed a rocket on an asteroid, it is not that difficult to change its orbit, through techniques such as burning rocket fuel, or ejecting some of the mass of the asteroid into space. Basically the technical skill needed to create an asteroid missile is very similar to the technology needed to mine an asteroid, and perhaps even simpler, as the creator of an asteroid missile doesn't have to worry about extracting useful materials from the asteroid.
What is frightening is how devastating an attack can be launched with a small asteroid missile. Online calculator's such as Purdue University's Impact Earth allow you to calculate the blast effects of asteroids. Below is a screen shot showing the effects of a 20-kilometer asteroid hitting Earth. Even 2000 miles away, multistory buildings will collapse. Such an asteroid collision would send so much dust into the atmosphere that it would create a nuclear winter effect that would wipe out almost all agriculture for years.
How to defend against asteroid missiles? You would need a system to detect and divert incoming asteroids, which could also be used to defend against purely random unintentional asteroid collisions.
Scientists know that for each type of regular subatomic particle such as the proton there is also an antiparticle. For example, the antiparticle of a proton is the antiproton, and the antiparticle of an electron is the positron. Matter consisting of antiparticles is called antimatter. In our universe there is very little antimatter, but scientists can make very small amounts of antimatter in nuclear laboratories.
Antimatter is incredibly dangerous, because when even a tiny amount of antimatter is combined with regular matter, it produces incredible amounts of energy. In theory, you could make an antimatter bomb that would be much more powerful than a nuclear bomb. A single bomb might be powerful enough to blow up much of our planet. Thankfully it would require incredible amounts of technical expertise and research dollars to create even a very tiny amount of antimatter. So it is not appropriate to lie awake at night worrying about dying from an antimatter bomb.
Microwave Crowd Controllers
The US military has an existing weapon called the Active Denial System, which was designed for controlling crowds by beaming microwaves at them. It is called a non-lethal system, although such a system might cause eventual cancer for those to whom the beam was directed. A future version of the Active Denial System might work by beaming microwaves at a crowd, causing them intense pain, and low-level burns. People in the crowd might suddenly feel as if they had a bad case of sunburn. The current version of the Active Denial System supposedly makes you feel as if a light bulb has been placed on your skin.
Although the damage caused by such a system is trivial compared to some of the other weapons discussed here, this weapon is very scary because it is so easy to imagine it being deployed by a future government trying to disrupt protesters. Plus there is something particularly creepy about being microwaved just as if you were a chicken nugget.
Orbiting Charged Particle Beams
An orbiting charged particle beam is basically a death ray in space. Ever since the advent of Star Wars technology in the 1980's (officially called the Strategic Defense Initiative or SDI), the US military has spent billions working on similar weapons, designed to shoot down or disable incoming missiles. Similar technology could be placed in orbit for the purpose of destroying ground based targets: buildings, vehicles, or people.
The creepiest thing about such weapons is their potential for instant death: one minute you're alive and well, the next minute you're burnt toast.
The United States uses drone aircraft in Afghanistan and other places. These are large aircraft that work by remote control, with one person acting as the controller. Future drone weapons may be much smaller, and may work in a swarming fashion, with many drones acting in unison to accomplish a goal. Particular units in a drone swarm may communicate with each other by radio, exchanging information. For example, a swarm of small drones may attack a car, stripping it to pieces like a pack of hungry hyenas attacking and devouring an animal.
Imagine a future government that wanted to disrupt a mass protest. The government might send two types of drone swarms to attack the protesters: one swarm of wheeled drones that would attack the feet and legs, and another swarm of aerial drones that would attack from the air. If the government also used a microwave bombardment such as the Active Denial System, one can only imagine how quickly the crowd would disperse. The use of any such weapons against peaceful protesters would be morally deplorable.
Artificial Supernova Weapons
Alexander Bolonkin and Joseph Friedlander have published a scientific paper claiming that it may be possible to create a weapon that causes the sun to explode in an artificial supernova that would kill everyone on Earth and everyone in the solar system. That would be pretty much the ultimate doomsday weapon. Others doubt that any such thing is possible. Let's hope they are right, as we have quite enough terrifying weapons to worry about.