Mandie Deuchrass, of Port Chalmers School, asks :-

How were the moon's craters made?

Duncan Steel, an astronomer at the Anglo-Australian Observatory in Canberra, Australia, and who has an interest in comet impacts, responded.

The craters of the Moon were almost all produced by high-speed impacts by asteroids and comets over the past 4.5 billion years, since the time that the sun and planets condensed from a gigantic cloud of gas and dust. In that process not all of the lumps of material were swept up into the planets, with the result that from time to time some of the left-over debris may hit a planet, or one of its moons.

The typical impact speed of an asteroid or cornet hitting the Moon is about 30 kilometres per second, and at that velocity a one kilometre-sized asteroid releases energy equivalent to 100,000 million tonnes of TNT as it strikes. Such an asteroid will excavate a crater about ten km wide and several km deep.

All solid surfaces in the solar system - such as Mars, Mercury, and the moons of the outer planets - are densely cratered from past impacts. The Earth is also struck from time to time by asteroids and comets, although weathering by the wind, rain and snow, and the Earth's active geology (earthquakes, volcanoes and so on), tend to erode away the evidence over millions of years. There are 150 known impact craters in the world. Nineteen of them are in Australia but as of yet there have been no impact craters identified in New Zealand. The best-known terrestrial crater is Meteor Crater in Arizona, which is 1.2 km wide. It was formed 50,000 years ago by an asteroid about 30 to 40 metres in size. The hypothesis that the dinosaurs were all killed 65 million years ago by a gargantuan asteroid impact received a boost in 1990 when a crater more than 180 km in size was identified in Mexico, and found to be of the correct age.

A one km asteroid collides with the Earth about once every 100,000 years, and we believe that over 25 per cent of humans would be killed either by its direct effects, or subsequent starvation. Smaller asteroids strike much more often, such as the 50 metre body which blew up over Siberia in 1908. On February 1st this year a 10 or 20 metre asteroid blew up on hitting the atmosphere over the western Pacific. Luckily no-one was hurt.