An asteroid striking the earth is a premise often examined in fiction, but what would the result of such an impact really be?
This is not a theoretical threat. It’s happened before and there are craters all over the planet to prove it. In fact, the last significant asteroid strike, 65 million years ago, caused the extinction of the dinosaurs.
Any significantly sized asteroid falling from the sky would unleash a tremendous amount of energy. For example, a mile-wide asteroid hitting the earth at 30,000 mph would cause an explosion equal to a 1 million megaton bomb. That's enough to wipe out virtually every living thing on the planet.
An asteroid that big would throw up enough dust to completely block out the sun. Anyone who survived the initial impact would soon perish. And if the asteroid were to land in the ocean it would cause massive tsunamis hundreds of feet high that would devastate coastal cities around the globe.
With the end of the Cold War, the threat of a global nuclear war may have receded somewhat, but before we start congratulating ourselves at having dodged the bullet, let's remember that there are at least 20,000 active nuclear weapons remaining, enough to destroy the planet many times over.
And the possibility of malicious use of these weapons has not receded completely. The fear of deployment by a rogue state or terrorist group is very real, but what is even more frightening is a showdown between the US and rising world power, China.
The effects would be devastating. Quite aside from the havoc wrought by these weapons, we'd be facing radiation borne illnesses, nuclear winter and destruction of the ozone layer allowing solar radiation to penetrate our atmosphere.
Plagues and pandemics are nothing new. In the 14th century the Black Plague killed half of the Chinese populace before heading west and devastating the population of Europe. In 1918, an outbreak of Spanish Flu wiped out up to 100 million people, or 6% of the world's population.
But surely, with the advances in modern medicine such a pandemic couldn't happen these days? The scary truth is that it could, and perhaps it is even likely. The problem is that viruses and disease causing microbes mutate and become resistant to antibiotics. We've seen this in recent years with the panic caused by avian flu and swine flu, and it wouldn't take much for such a pandemic to spiral out of control.
And quite aside from the risk of a naturally occurring epidemic is the constant threat of biological warfare. Creating killer bugs is cheap and they are easy to produce, conceal and transport. All it would take is one motivated and demented terrorist group. A frightening thought.
Think about it, there are billions upon billions of planets in our universe and it is almost impossible to believe that Earth is the only one that houses intelligent life. And as our technology improves, allowing us to look deeper into the universe, our chances of encountering extraterrestrial life increase exponentially.
While we search the skies for them through initiatives like the SETI project, who is to say they aren't looking for us too? And if they find us, what are the chances they'll come in peace and not to colonize Earth or exploit our natural resources.
It is for this reason, that scientists like Stephen Hawking warn against trying to make contact with alien civilizations. They reckon that if a race has the technology to traverse the light years to reach us, they'll also be more than capable of exterminating us.
When Mount Tambora erupted in Indonesia in 1915, it pumped so much ash into the air that it blocked out the sun, changing the global climate for an entire year. 1816 became known as the year without a summer as temperatures plummeted, crops failed and famine and starvation were widespread.
Now imagine a volcano many times more powerful than Tambora, a super-volcano. There are five of these around the globe, and the biggest of them is sitting right on our doorstep in Yellowstone National Park.
The famous geysers and hot springs are powered by a sea of magma 30 miles wide and another 6 miles deep, and when it blows it will not only devastate North America, but cause every other volcano on the planet to erupt as well causing a global cataclysm that will wipe out everything on the planet.
And the other bad news is that Yellowstone is overdue. It has a cycle of eruption every 600,000 years and now it has now been 640,000 years since it last erupted.