http://www.newscientist.com/home.ns 15 July 2008David ShigaMagazine issue 2664
The solar system's most dangerous rock, Apophis, could be more of a threat to Earth than thought. YOU'D think that by now we'd have a firm grip on the trajectory of the solar system's most worrisome chunk of rock. In fact we have only a hazy understanding of how likely the asteroid Apophis is to strike Earth. What's more, budget cuts may shut down the telescope that could clarify the situation.
Since Apophis was discovered in 2004, asteroid-watchers have known that it has a slim chance of hitting Earth in 2036. At 270 metres wide, it is too small to rival the object that wiped out the dinosaurs, but it could cause devastating tsunamis were it to hit the ocean. Worrying as this is, we have been able to take comfort in the computed probability of impact, which is just 1 in 45,000.
Now it seems the true risk is unclear, thanks to minute effects that the calculations didn't take into account.