NASA has downgraded the threat of a massive asteroid slamming into Earth in 2036.Apophis, which was discovered in 2004 and thought to pose a threat to Earth, is about 270 metres in diameter.<br/><br/>There is now a one-in-250,000 chance of a collision with Earth in 2036, sharply down from an earlier estimate of a one-in-45,000 chance, according to scientists at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. <br/><br/>At first, astronomers worried the asteroid had a 2.7 per cent chance of colliding with Earth in 2029. Later calculations ruled out a collision in that year. Apophis is slated to sail no closer than 29,450 kilometres above Earth's surface on April 13, 2029.<br/> <br/>