Katherine Eastall, of Ridgeway School, asks :-

Where was the largest known meteorite found?

Duncan Steel, an astronomer and the author of the book 'Rogue Asteroids and Doomsday Comets', responded.

The largest meteorite known is a nickel-iron specimen of mass over 60 tonnes, which was found near Grootfontein in Namibia in 1920. The second most massive is a 34 tonne nickel-iron body found in the Antarctic and which now stands in the American Museum of Natural History in New York.

Although nickel-iron meteorites consist of only a small proportion of incoming meteoroids, they represent a large fraction of recovered meteorites - especially of the larger sizes - due to their strength and also greater likelihood of being recognized. Stony meteorites tend to fragment in the atmosphere, and are thus less likely to reach the ground intact. The largest known stony meteorite is a 2 tonne specimen which fell in China in 1976. However, in 1908 an object thought to have been a small stony asteroid with a mass of about 200,000 tons (diameter about 50 metres) blew up high above Siberia. No large meteorite reached the ground, but the shock wave caused devastation of the forest below.

If one considers even larger rocks from space, these are not significantly impeded by the atmosphere, and so hit the ground with their original speed (typically 100,000 km per hour). At such a speed these objects have more energy than a hundred times their own mass of TNT, and so can cause great devastation and large craters. The best-known impact crater on Earth is the 1.3 km wide Meteor Crater in Arizona, formed 49,000 years ago. There are over 150 known craters on the Earth, some of them hundreds of kilometres across.

There are no known impact craters in New Zealand, but there are 23 in Australia; in fact four of the Australian craters are less than 6,000 years old. In 1990 it was recognized that a heavily eroded crater on the Yucatan Peninsula of Mexico is likely to be linked to the extinction of the dinosaurs 65 million years ago. The crater is over 180 kilometres in diameter, and was formed by an asteroid or comet about 20 kilometres in size. Astronomers believe that there are at least 2,000 asteroids, and many comets, on orbits that could lead to massive impacts on the Earth and thus environmental catastrophes that could end civilization as we know it. Moves are now afoot to start an international search programme to discover these and determine whether an impact is due in the forseeable future, in which case it would be possible to divert the offending projectile so that it misses our planet.