The 27-metre diameter asteroid 2008 AF3 will tonight pass by the Earth as close as the Moon (see its orbit here).

In fact, it is passing so close that you can see it for yourself (from a dark site, with a large telescope). At magnitude 14, you will need a 12" diameter telescope to spot it as it passes through Ursa Major (the Plough) this evening (you can plot its position from your locations using Tom's Asteroid Flybys Webpage).

But the worry is that it was only discovered 3 days ago, highlighting the difficulty of finding asteroids that are coming straight for us!

If an asteroid is larger than 100m, and is expected to pass the Earth within 20 times the Earth-Moon separation, then it is considered a potentially hazardous asteroid. Fortunately, none are expected to hit the Earth at the moment, but new ones are being discovered on a regular basis.

According to SpaceWeather.com there are currently 917 potentially hazardous asteroids out there. A full list is maintained by the Centre for Astrophysics at Harvard.