How the world might end in 2012 (or maybe later)

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It's a way off, I know, but don't bother to order a diary for 2013 ? you won't be needing one. On 21 December 2012, the world as we know it will come to an end. It's all been revealed in an old calendar of the Mayans, if you believe how it's interpreted by various self-appointed experts on their slightly whacky websites.

The coming Armageddon could, it's said, take many forms: a disastrous surge in solar activity, a reversal of Earth's magnetic poles, a collision with a black hole, the close passage of a mysterious planet called Nibiru ? the list goes on. It's all very entertaining, and pure nonsense.

The Mayans did keep a calendar, called the "long count", based on a period of 1,872,000 days. It began in August 3114?BC, and so is due to click over to its next cycle late next year. However, there's no evidence to suggest the Mayans saw the switch from one cycle to the next as apocalyptic. More to the point, even if they did, they had no way of foretelling the future.

On the other hand, it's true that we are doomed. Our planet and everything on it won't last. A billion years from now the sun will have brightened and swollen to a point at which the oceans will start to evaporate. A billion years after that, all Earth's surface water will be gone and, with it, all life except for some hardy species that can survive on whatever moisture remains underground.

Incoming!

The finite lifespan of the sun guarantees the demise of planet three. But there are all kinds of other natural calamities that might kill off large swathes of us in the much shorter term ? and we love to talk about them. Asteroid or comet collisions are a big favourite.

The Earth has repeatedly been used for target practice by big dumb objects in the past. An asteroid at least 10 kilometres across barrelled into us just over 65 million years ago and helped wipe out the last of the dinosaurs along with many other groups of animals and plants. A good thing, too, if you're human, because it left mammals free to flourish.

In 1908 a blast with the strength of about a thousand Hiroshima atomic bombs flattened trees over a wide area around the Tunguska river in Siberia. The presumed culprit in this case was a fragment of a comet or a large meteoroid that exploded several kilometres above the surface. Had it happened over a populous city, the effect would have been disastrous.

Other nasty stuff has happened to the Earth. Supervolcanoes have erupted, blanketing vast areas with dust and lava, and plunging the globe into deep volcanic winters. Ice ages have come and gone, and very occasionally, during "snowball Earth" events, it seems that almost the entire planet has frozen over for millions of years.

These kind of events will happen again. We will be hit by asteroids and comets, large and small. More supervolcanoes will erupt. It's inevitable, and some of these events are perfectly capable of decimating the human race, or even driving us to extinction virtually overnight.

Chances are?

The trouble is, the level of danger and the immediacy of the threat is often grossly overstated. Run-ins with asteroids as big as that which put paid to the dinosaurs happen once every 100?million years or so. But that doesn't mean they happen every 100?million years like clockwork. Much smaller impacts, like the Tunguska episode, occur on average about once every thousand years ? a problem if they happen to maliciously target built-up areas, but not so alarming when you think of the great tracts of ocean and wilderness which are much more likely to be on the receiving end.

Speculation about upcoming disasters runs rampant when there's a failure, or unwillingness, to grasp basic facts. It happened this year in connection with an innocuous little comet called Elenin. Google its name and you'll find all kinds of hair-raising stories about how Elenin would come close to or even ram into the Earth, bringing death and destruction on a biblical scale. The hysteria started shortly after the comet's discovery when a few armchair theorists mistook the size of Elenin's coma ? the glowing, almost vacuum-thin shiny fuzz of vaporised particles around the hard nucleus ? for the size of the nucleus itself.

Word quickly spread on the internet, helped by the usual eagerness of tabloids and late-night chat shows to exploit a juicy yarn, that Elenin was as big as a planet and would cause chaos during its close passage of the Earth. In fact, as astronomers knew, Elenin was modest by cometary standards and never going to come any closer than 35 million kilometres, or about 90 times as far away as the moon. In the event, it disintegrated and was lost from view to even the most powerful telescopes.

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