Page 1 of 2Fragments of an ancient asteroid have revealed new details about asteroid impacts on Earth.

Credit: Don DavisWhat the Morokweng asteroid may have looked like as it hit the Earth 144 million years ago.

In Morokweng, South Africa, lies a crater larger than the city of London. With a diameter of over 70 km, it is one of the ten largest craters on the planet and was created when a huge asteroid collided with the Earth about 144 million years ago. Usually, asteroids large enough to create craters more than four kilometres wide are vaporised by the high temperatures created when they hit the Earth. But recently, an international group of scientists accidentally discovered a fragment of an asteroid in the Morokweng crater that is believed to be a piece of the destructive, ancient space rock.Dr Iain McDonald from Cardiff University and Adrian Boyce from Scottish Universities Environmental Research Centre were two of the researchers responsible for the Morokweng discovery published in Nature in May 2006. The scientists were helping a mining company search for copper and nickel in the crater when by chance, one of the boreholes drilling into the crater struck a 25 cm fragment of what appeared to be a meteorite. "The recovery of the meteorite represents a huge stroke of good fortune," says Boyce. "Had the borehole been sited just a metre away, it might have missed the object altogether."google_protectAndRun("ads_core.google_render_ad", google_handleError, google_render_ad);
By analysing the minerals and chemicals in the fragment, the scientists were able to confirm that it was indeed part of an asteroid - more specifically an LL chrondite which accounts for 9% of meteorites that strike the Earth and originated from the main asteroid belt between Mars and Jupiter. More searching in the crater also revealed that this wasn't the only fragment that had survived the impact: there were many other pebble-sized pieces lodged inside.

Credit: Dr Iain McDonaldInside the Morokweng meteorite: these round melt droplets (chrondules) were formed from dust and gas at the start of the Solar System

This was a big surprise since it contradicts simulations of the behaviour of large asteroids when they hit the Earth. Models generally assume an initial impact velocity of 20 km/s, and under almost all conditions, this would completely obliterate the meteorite. But McDonald says that the fragments found at Morokweng suggest that the initial velocity with which it hit the Earth was much lower - perhaps in the ballpark of 15 km/s. "The velocity of the asteroid as it impacts the Earth is a key variable," says McDonald. "If you change it, you change what might survive impact and what might not."