Hannah Calder, Jessica Wells and Amanda Watts of Cashmere High School, asks :-

Is it likely that a large asteroid will hit Earth within the next hundred years?

Duncan Steel, a space researcher in Australia and the author of the book "Rogue Asteroids and Doomsday Comets", responded.

The Earth orbits the Sun amongst a hail of debris. The larger objects (called asteroids and comets) are capable of causing ecological catastrophe when they strike our planet. The best-known such catastrophe occurred about 65 million years ago, and seems to have resulted in the extinction of the dinosaurs. That asteroid was about 10 to 20 kilometres in size. There are many more smaller rocks in space which can thump into us, the last decent-sized one being a boulder about 50 metres across. It entered the atmosphere above Siberia in 1908, laying waste to over 2000 square kilometres of largely-uninhabited forest. Three minutes later and it could have squashed Tokyo.

There are craters scattered all over the Earth which are testament to past impacts. Asteroids or comets of one or two kilometres in size, large enough to cause a global catastrophe, run into our planet at a rate of between once per 100,000 years and once per million years. Thus the chance of such an event within the next 200 years is small, but big enough to be taken seriously (because of the phenomenal consequences). On the other hand, the probability is well over 50:50 that a small asteroid, like the Siberian one, will impact the Earth within the next couple of centuries. For either large or small asteroids the next impact could be due tomorrow: we just don't know. The tragedy is that Australia and New Zealand are not looking for these dangerous objects.

Small meteoroids the size of a grain of sand rain down into our atmosphere all the time. We see these as shooting stars. Meteor showers occur when the Earth passes through the trail of debris left behind by a comet in its orbit. Around November 17th each year a strong shower is seen, the Leonid shower. If you go outside soon after midnight on the nights of both November 17 and 18 and look Northeast, you should be able to see shooting stars through to dawn at a rate of some hundreds per hour. Some might knock out satellites.