"That could ruin your day"

Apocalyptic thought has a tradition that dates to the Persian prophet Zoroaster in the 14th century BC. Recently, anxiety has grown over the prediction of the end of the world in the Mayan calendar.

It's true that the Mayan odometer will hit zeros on 21 December 2012, as it reaches the end of a 394-year cycle called a baktun. But this baktun is part of a larger 8,000-year cycle called a pictun, and there's no evidence that anything astronomically untoward will happen as the current baktun slides into the next. However, that hasn't stopped the feverish speculating that sells books and cinema tickets.

What kind of catastrophe would it take to end the world? Astronomical intruders provide a potentially serious threat. Impacts can be caused by stray rubble from the Asteroid Belt and the rocky snowballs that travel in highly elliptical orbits in the comet cloud. There are many fewer large bits of debris than small bits, so the interval between large impacts is much longer than the interval between small impacts.

That's good news. Every century or so, a 10-meter meteor slams into the Earth with the force of a small nuclear device. Tunguska was the site of the last, in 1908, and it was pure luck that that meteor landed in the uninhabited wilderness of Siberia. Every few thousand years, Earth can pass through unusually thick parts of the debris trail of comets, turning the familiar light show of a meteor shower into a deadly firestorm. Roughly every 100,000 years, a projectile hundreds of meters across unleashes power equal to the world's nuclear arsenals. The result is devastation over an area the size of England, global tidal waves (if the impact is in the ocean), and enough dust flung into the atmosphere to dim the Sun and kill off vegetation. That could ruin your day.