An asteroid the size of a small office building will make the closest approach ever recorded to the Earth on Thursday eveningOr, if not, it's the equivalent of one of the largest H-bombs in existence today (though bigger ones have been in service in the past).
Discovered just two days ago by an automated telescope scanning the sky for near-Earth objects, asteroid 2004 FH will miss the planet by a mere 40,000 kilometres, just over a tenth of the distance to the Moon.
At roughly 30 metres in size, 2004 FH is too small to cause widespread damage should it hit the Earth. It would be more likely explode in the air, releasing about a megaton of energy. However, that energy should dissipate harmlessly if the blast is high enough above the surface.
Jay Manifold does a few back-of-the-envelope calculations on his blog, and gets the answer of 1.1 Megatonnes assuming a specific gravity of 2. If it's Nickel-Iron (as many meteors are) then it's more like 10. In either case, you wouldn't want one to come down in your vicinity.
Does anyone else think that being able to detect these things a bit earlier than 2 days before they're in the neighbourhood might be a good idea?