"It will approach earth within 0.85 lunar distances (0.00217 AU) on
November 8, 2011, when it will be an extraordinarily strong radar target."

1. "Asteroid YU55....will approach earth within 0.85 lunar distances (0.00217 AU) on November 8, 2011, when it will be an extraordinarily strong radar target. 

2.Arecibo observations in 2010 revealed that YU55 is a rounded object about 400 meters in diameter. 

3.The 2011 approach is the closest known by an asteroid this large since 1976

4. Due to its size and proximity to earth, the Minor Planet Center has designated YU55 as a "Potentially Hazardous Asteroid."

[link to echo.jpl.nasa.gov] 
Updated by NASA February 4, 2011

3 second video showing trajectory:
[link to ssd.jpl.nasa.gov]