For a while there in 2004, the newly-discovered asteroid 99942 Apophis looked like it had Earth's number. Then scientists crunched some numbers, and the odds of a terrestrial bullseye dropped to 1 in 45,000, where they stand today. Sort of. It turns out that there are a few things we still don't know about the orbit of Apophis, which could change its projected course by millions of miles, according to an article yesterday in New Scientist. Are we going to get slammed by the 270-meter long hunk of rock? We probably won't know for sure until we get a closer look at its close-ish Earth flyby in 2013.
We already know that Apophis is due for a close pass by Earth in 2029, and if things go just right (or wrong), April 13, 2036 could be a very bad day for us.