On November 8, 2011 at 23:28 UT (5:28 p.m. CST) the asteroid safely passed 0.85 lunar distances (324,600 km or 201,700 miles) from the Earth. November 9, 2011 at 07:13 UT (1:13 a.m. CST) the asteroid is expected to pass 0.00160 AU (239,000 km; 149,000 mi) from the Moon. During the close approach the asteroid reached about apparent magnitude 11.
The asteroid would have been visible to experienced observers using high-end binoculars with an objective lens of 80+ mm if it were not for bright moonlight preventing a true dark sky. Since the gibbous moon did interfere with the viewing, observers trying to visually locate the asteroid required a telescope with an aperture of 6 inches (15 centimeters) or larger.
2005 YU55 traveled the closest known approach by an asteroid with an absolute magnitude this bright since 2010 XC15 approached within 0.5 lunar distances in 1976.
2005 YU55 is expected to return, but not get as close to earth, in 2041.