Asteroid 2004 MN4 is in an uncertain orbit around the sun. Here is a plot of its current orbit relative to the earth.

It looks pretty harmless and it is, at the moment. The plot of its orbit on Friday 13th April, 2029 looks considerably different. In fact on the scale that this plot is drawn it would show a direct impact of the asteroid with the earth on this date.

Of course this is drawn at a astronomical scale and the current thinking is that it will miss the earth by a astronomical "hairs breadth", 30,000 kms.

So what is the scenario like if it did hit? Well at 320 meters wide it is significantly smaller than the asteroid that is currently being blamed for the extinction of the dinosaurs. It is big enough however to destroy an area roughly the size of Texas or France. If it hit an ocean then the tsunamis would dwarf those recently occuring in the area of Indonesia.

The two initial sightings had Paul Chodas, Steve Chesley and Don Yeomans at NASA's Near Earth Object Program estimating the chances of it hitting at about 1 in 60. Given the level of devastation and death it would cause you would have to say that 1 in 60 is pretty damn scary. More recent analysis indicates that it is unlikely to hit.

The white bar in the image above that runs perpendicular to the trajectory indicates the current estimate of the margin of error in their calculations. That small amount of black space between the end of the margin of error and the earth is supposed to be reassuring.

Read more here.