If you've been to the cinema or watched television in the past 15 years or so, you will probably have seen one of the blockbuster disaster movies that uses, as its pivotal plot point, a space rock a couple of miles wide that's on a collision course with Earth , threatening the very future of mankind.

While the rock has often been credited with more acting ability than some of the humans in these films, it does throw up the question of how realistic this scenario is. What would happen if a giant meteorite ploughed into Earth at several hundred thousand miles an hour? Would we be wiped out? Has it already happened in Earth's distant past? Can it - and will it - happen again, and if so, when? As we found out more about Earth's past, revealing the answers to some of these questions, Hollywood was quick to catch on to the dramatic implications of such events.

In 1908, the world was rather surprised to learn that a remote and virtually uninhabited area of Northern Russia called Tunguska had been unexpectedly demolished by a meteor strike. This was quite a wake-up call for astronomers, and it didn't take long for people to start asking questions like 'What if it had fallen on New York or London?' - the answer being all too obvious. If anything this large landed near a populated area, it would cause massive and horrific destruction. And if anything any larger hit Earth, many theories suggested that it would signal an end to our life on this planet.

Some Brief Definitions

Just so we all know what we're talking about here (and if there's one thing scientists like, it's precise definitions!), here's what we mean when we're talking about Near Earth Objects:

• Asteroid - A large chunk of rock, usually from the Asteroid Belt.

• Meteor/Meteorite - A piece of rock that enters Earth's atmosphere, usually in a fiery extravaganza. The same as a 'shooting star'.

• Comet - A large ball of rock and ice that orbits the Sun with an extremely elliptical orbit. When it's close to the Sun it gets hot, some of the ice melts and its water vapour and dust is lit up in the Sun's rays, visible as a 'tail'.

All of these are called Near Earth Objects, which basically means that their orbits around the Sun can occasionally come within Earth's orbit.