[BenLurkin]

Near-Earth asteroid 2005 YU55 will pass within 0.85 lunar distances from the Earth on November 8, 2011. The upcoming close approach by this relatively large 400 meter-sized, C-type asteroid presents an excellent opportunity for synergistic ground-based observations including optical, near infrared and radar data. The attached animated illustration shows the Earth and moon flyby geometry for November 8th and 9th when the object will reach a visual brightness of 11th magnitude and should be easily visible to observers in the northern and southern hemispheres. The closest approach to Earth and the Moon will be respectively 0.00217 AU and 0.00160 AU on 2011 November 8 at 23:28 and November 9 at 07:13 UT.

http://neo.jpl.nasa.gov/news/news171.html

[Brilliant]

If you look at the second diagram, it appears that it doesn’t actually intersect the Earth’s orbit. It’s skew.

[PapaBear3625]

400 meters cubed is 6.4x10^7 cubic meters of stuff. If we assume an average density similar to ice, then that will be around 6.4x10^10 kg. If it impacts at earth's escape velocity (11,200 m/s) it will release 6.4x10^10 * (11200 ^ 2) = 8x10^18 joules of energy = about 2000 megatons.

Above is all seat-of-the-pants guestimates.