Last year Dec:23. scientists announced that a newly discovered space rock named 2004 MN4 had about a 1-in-300 chance of striking Earth on April.13, 2029.
Last Friday the risk was upgraded to an unprecedented 1-in-40, as the latest observations rolled in, which means it warrants careful monitoring. The odds at various times since its discovery have been, 1-in-63, then 1-in-45, until last Friday the odds shortened to 1-in-40.
Slow changes might come in within the next few weeks, but nothing too dramatic until the Arecibo fixed Radar observatory gets it in line of sight early Feb: 2006.
2004 MN4 is about a quarter mile (400 meters) wide, large enough to cause considerable local or regional damage were it to hit the Earth. It is larger than the asteroid that carved Meteor Crater in Arizona thousands of years ago, and much bigger than one that exploded in the air above Siberia in 1908, flattening thousands of square miles of forest.
Like most asteroids, MN4 resides in a belt between Mars and Jupiter, but its 323-day orbit lies mostly within the orbit of Earth.
Scientists surprised to see such a high probability of impact, state that there are still 39 chances in 40 that the object won't impact.

http://www.space.com/news/asteroid_defense_041227.html