The asteroid 4179 Toutatis zipped past the Earth at 7:05 pm International Standard Time on Wednesday, September 29th, 2004. At the time, people thought this would be the the closest approach of any asteroid this big in the 21st century!
Toutatis whizzed past us at 40,000 kilometers per hour, coming within 1.5 million kilometers at its closest approach - only 4 times the distance of the moon. It will come even closer in 2562: only 400,000 kilometers. Unlike almost everything in the Solar system, it spins in a complicated way, not along its principal axis of inertia. If you have Realplayer or can handle mpegs, you can watch animated movie of this, made by Scott Hudson!
Will Toutatis eventually hit the Earth? Can we predict when this will happen? Alas, its interaction with the Earth is chaotic, and we can't accurately predict its motion after our close encounter with it in 2562.
If and when it hits, it'll create quite a stir, since it's 4.6 kilometers long and about 2 kilometers across. For comparison, the asteroid that doomed the dinosaurs was 10 kilometers in diameter.
But right now, people aren't worried about Toutatis. They're more worried about 99942 Apophis, an asteroid which should whiz past us and miss in 2029 - but has a chance of hitting Earth in 2036. Its diameter is only 400 meters, but that's still big enough to demolish a city, or cause a tsunami if it hit the ocean. Luckily, if it seems to be headed for us in 2036, a gentle gravitational nudge should be enough to deflect it - at least if we act early. But, we need to wait until 2029, since its orbit will be affected in a somewhat unpredictable way when it passes Earth then: we shouldn't nudge it until we know precisely what effect the nudge will have!
If you like asteroids, you might like my webpage on Lagrange points, with links to information about 3753 Cruithne - a near-earth asteroid with an incredibly complicated orbit.