Although today is a cloudless summer day, the sun doesn't really shine as bright as it ought to. In fact, it's freezing cold down here on Earth. Uh-oh, the Kuiper Belt is at it again!
The Kuiper Belt is a huge disc of cosmic rock, that stretches out across the Universe somewhere between Neptune and Pluto. It is a leftover of the formation of the planets in our solar system. Cosmologist John Gribbin once called its discovery `one of the most dramatic astronomical findings of the 1990s'. For here, lurking right behind our back door, is a real Armageddon in the making.
Okay, so Neptune and Pluto, that's a long way away. The Kuiper Belt sits some forty to fifty times further out from the Sun than we do. But as computer simulations show, that might change. From time to time, the gravity of the outer planets will disturb the Belt, and sling a huge piece of rock straight into the inner solar system. Doom will come rushing straight towards us.
One problem is, we will hardly see it coming. The Kuiper Belt was discovered very late, partly because the comets it contains move slowly, and partly because it isn't made of solid pieces of rock. Most of the supercomets in the Kuiper Belt are `dirty snowballs' made of dust and ice, making them incredibly hard to detect with modern equipment.
Calculations show that there are at least 70,000 of these icy objects out there. And most of them are really big: the first that was detected -- an object dubbed `QB1' -- is about two hundred kilometers across. By comparison: the killer meteor that wiped out the dinosaurs and almost all life on earth some 65 million years ago was only ten kilometers across, twenty times as small as QB1. Some experts argue that Pluto and its moon Charon themselves are to be considered as part of the Kuiper Belt.
Gladly, the chances of such a super comet actually hitting the Earth are vanishingly small. The Earth is only a tiny pea in the vastness of the Universe, and is `protected' against incoming comets by the gravitational fields of other planets.
But then again, full impact is not the biggest threat the Kuiper Belt poses. As a Kuiper Belt comet comes rushing in, its icy surface will heat up. Eventually, when it comes too close to the Sun, it will explode. Its remains will be scattered all over the place. The dust will be attracted by the Sun's gravity, and clot together into a temporary ring of dust around the Sun. The debris will block some of the Sun's heat. And when that happens, we're in big, big trouble.
Temperatures on Earth will drop rapidly. The dust will trigger an Ice Age on Earth. Oh, and then there's the realistic danger our planet is hit by comets after all, with all those chunks of comet debris flying around in our part of the solar system.
Obviously, what grim destiny exactly awaits us depends on a lot of things. Many of them are determined by pure chance. One particular nasty Kuiper Belt scenario involves an all-out Ice Age, in which so much of the Sun's heat is blocked that the entire Earth turns into a barren, frozen planet. The Earth's climate thermostat (being the intricate interplay between oceans, land, vegetation and algae) would be severely deregulated. It would take the planet hundreds of thousands, if not millions of years to recover. Of course, the chances of survival would be slim in a stone-cold world with frozen oceans and no soil to grow your food on.
But here's the good news: with or without humans, the planet eventually will survive. It did so before. In the Pre-Cambrian age (more than 600 million years ago), the Earth survived several super Ice Ages. But then again, in those days the most complicated life forms on the planet were tiny shrimps and snails, crawling around in the deepest depths of the ocean.