Arthur C. Clarke's popular 1972 novel Rendezvous with Rama, in which an asteroid collides with the earth on September 11, 2077, was a hit in more ways than one. In the book, the impact wipes out the Italian cities of Padua and Verona and plunges Venice into the sea. The coincidence of date with the terrorist attacks on the twin towers in New York helped revive interest in it.
To avoid similar disasters and detect earth-impact events ahead of time, the inhabitants of Clarke's fictional world created Project Safeguard. Someone at Nasa must have been reading that book, because 20 years later the space agency launched its own Project Safeguard to monitor the skies and assess risks.
Twelve years into the project, three astronomers at the Kitt Peak National Observatory in Arizona - Roy Tucker, David Tholen and Fabrizio Bernardi - discovered something worrying: a small comet called 2004 Mn4 seemed to be on a collision course with us. News spread that the comet would strike the earth in 2029, and a more disquieting name was soon found: Apophis, a character in the Stargate television series and also an evil ancient Egyptian god.
New observations have since prompted other scientists to scale back assessments of the comet's potential danger and size, and revise its impact date to around April 13, 2036. That just also happens to be Easter Day.
According to the latest data, Apophis is about 350 metres in diameter and travels at more than 30 kilometres per second. Its density and composition, keys to forecasting its behaviour, remain a mystery but its speed suggests that it could pack quite a punch: 510 megatons. Compare that to the 50-megaton Soviet Tsar Bomba, the biggest atomic bomb yet exploded.
The comet's closest passage to our planet before impact will be on April 13, 2029, according to Nasa and researchers at the universities of Pisa and Valladolid. It will have an astronomical magnitude of 3.4, which will make it visible to the naked eye, as it will travel at the same altitude as some of our telecommunications satellites.
If everything goes to plan it should pass through a 600-metre patch of the sky known to astronomers as the keyhole. That's when observers will be able to get the best data about its trajectory, but it will also leave just seven years to organise its deflection before a possible catastrophic impact.