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Worried that an asteroid will wipe out life on Earth? Then get online, where you can now keep up with the top 40 lumps of rock most likely to wallop the planet. NASA has activated an automated impact-monitoring system called Sentry, which lists all known near-Earth objects and their risk of impact (). Just days after the site was launched, a space rock designated 2002 CU11 topped the charts. From current observations, the asteroid, which measures 800 metres across, has a 1 in 100,000 chance of hitting the Earth in 2049. As more observations are made, the odds of hitting Earth are likely to drop for most of the asteroids.