This could be it, folks. This could be the one. This might be the one we've all been waiting for. Based on the most recent observatons as of the time of this writeup, there is a 1-in-45 chance that Asteroid 2004 MN4 will hit the Earth on Friday, April 13th, 2029. This is not a drill.

2004 MN4 was first observed on June 19th, 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi at Kitt Peak, Arizona. The asteroid was next seen 6 months later on December 18th, and soon after that it was realized that there was a chance of impact in 2029. The risk of impact was first assessed as being approximately 1 in 233. This is a relatively high probablility of impact — in fact, it's higher than any previously observed asteroid of this size — but generally, these things turn out to be nothing in a day or so. The funny thing about this asteroid is that instead of the odds decreasing, they've increased significantly over the past few days. On December 24th, the odds were upped to 1 in 62, and on December 25th, the odds were further raised to 1 in 42, then lowered slightly to the current estimate of 1 in 45 (i.e. 2.2%) later that day.

Size and Destructive Power
It's hard to be sure of an asteroid's size, since the albedo of asteroids varies widely, but currently the best estimate of 2004 MN4's size is approximately 380 meters across, with a mass of about 7.5 x 1010 kilograms. This is down from the initial estimates, which were 440 m for the diameter and 1.2 x 1011 kg for the mass. Current NASA estimates for the energy of the impact, if it does occur, are around 1.4 gigatons of TNT (1400 megatons) — for comparison, the bomb dropped on Hiroshima was about 15 kilotons. This wouldn't cause worldwide extinctions, but it would cause severe "regional devastation," as NASA puts it. But what region? Well, if it does hit, it will be on the night side of the Earth, towards the end of the day in UTC, implying that an impact would probably be somewhere in the Eastern Hemisphere.

More on the Odds
Saying that there's a 1-in-45 chance of the asteroid hitting the Earth in 24 years is somewhat misleading. Its orbit is set, and either it will hit us or it won't — we just don't know exactly what its orbit is yet. The issue is one of observation. The more the asteroid is observed, and the greater the period of time over which it is observed, the more precisely its orbit can be calculated. So right now, there's a range of orbits that 2004 MN4 may have, and 2.2% of those are orbits which will lead to an impact. What astronomers are currently trying to do is narrow down the possibilites so they can determine the path of the asteroid with greater precision. (Here is a wonderful animation from the Spaceguard Foundation in Italy explaining how the orbits of asteroids are determined from observations.)

Sources and More Information:

Update, December 27th, 2004: The odds have increased again. There is now a 2.7% chance that 2004 MN4 is on an impact trajectory, making the odds about 1 in 37. Also, NASA has changed their estimate of the asteroid's size slightly — they now believe it is about 390 m across, with a mass of 7.9 x 1010 kg. The estimated energy of impact has changed correspondingly as well, to 1500 megatons. I'll post more information here as I become aware of it.
IT'S OVER: Later in the day on the 27th, previously unrecognized images of 2004 MN4 from March 15th were uncovered at the Spacewatch Observatory in Tuscon, Arizona. When this data was taken into account, the possibility of an impact in 2029 disappeared entirely (though the asteroid will still pass within the moon's orbit) and the cumulative probability of any impact at all anytime in the 21st century was lowered to .0018%, or approximately 1 in 56,000. Its estimated size has changed too — it's now believed that its diameter is about 430 m, its mass is about 1.1 x 1011 kg, and the energy of impact would be 2100 megatons. (Note: While I was writing this update, the cumulative odds changed to .0041%, or about 1 in 24,000. The most probable impact is now April 13, 2053, with odds of 1 in 45,000. This is still pretty darn low, and I doubt it will get bigger.)
WE WILL SEE IT! (and it might hit after all): Radar observations of 2004 MN4 made at Arecibo Observatory have made it possible to predict the path of the asteroid with great precision until its flyby in 2029. It turns out that it's going to make a very close approach — closer than geostationary orbit! In fact, it'll be close enough for it to be visible with the naked eye. The night of Friday, April 13, 2029, observers in the Eastern Hemisphere will be able to see 2004 MN4 moving across the sky at quite a clip, about 42 degrees per hour. (There are 180 degrees in the sky given a totally flat horizon, so this translates to 180/42 = about 4 hours for the asteroid to make it across the sky.) There's no chance that it can hit the Moon, though it will be heading in that general direction.
For more information: It should be noted that due to the closeness of the approach, it's very hard to predict the asteroid's orbit after the 2029 flyby. As a result, there is a chance that 2004 MN4 will hit Earth at some point after 2029. (See this Slashdot post for more information.) Current odds on that can be found at the JPL Earth Impact Risk site listed above, but these may well change after the 2029 approach. Also, the Arecibo observations have altered estimations of the size and mass of the asteroid. It's now believed that 2004 MN4's diameter is approximately 320 m and its mass is 4.6x1010 kg. This means that if it were to hit, the impact energy would be about 860 MT. But once again, the odds on it hitting the Earth at any point in the forseeable future are very low.