Nick Wilson, of Kings High School, asks :-

Why are there shooting stars?

Jack Baggaley, a physicist at the University of Canterbury, responded.

Many billions of small rocks (meteoroids) orbit around the sun. They are part of the sun's family (the solar system) just like the earth and are made out of minerals similar to those we find in earth rocks. Some of these meteoroids collide with the earth during its journey around the sun. A 'shooting star' is a common name for what is observed when a meteor hurtles into the earth's atmosphere.

Impacting with the air heats the minerals (a sort of friction) to a white heat and it is this bright rock and its debris that we see. Although the meteoroids are small (the faintest you can see is initially about the size of a marble and the very brightest, as bright as the moon, is initially the size of a football), they have a lot of energy because they are travelling into our atmosphere very fast, about a 100 times faster than a jumbo jet aircraft.

Most of those we see are completely vapourized by friction with the air. This occurs at a height of about 100km. Those initially larger than about a football do survive and hit the earth's surface as a meteorite.

There are all sorts of sizes of meteoroids. There are very many very small ones and relatively few very large ones. Objects larger than about a football hit the total area of New Zealand about once a month. Ones you can see by eye are seen at a rate of about five each hour. We operate a research radar which tracks very small grains (about one tenth the size of a full stop on this page). We measure about 300 each hour. You are welcome to visit our research facility and see how we do this. By measuring their trajectories we have learnt that some meteoroids come from comets and this tells us something about the evolution of our Solar System.