If it's .85 lunar distances, that means it's .15 lunar distances from the lunar ... I mean Moon!
The path of this rock cuts inside the lunar orbit. Nasa has an animation that shows the moon going "shit!, shit! shit!" as it skirts out of the way. Click the image at the Nasa page to see the animation.
I hope those guy's calcs are right. It would not take a direct hit on this dirtball of a planet to do us in. All that would have to happen is a collision with the moon. Maybe the Mayan calender isn't bs after all.
Where's my tinfoil hat, my head is cold.
update: Oh here's an encouraging footnote on the Nasa page I missed.
Although classified as a potentially hazardous object, 2005 YU55 poses no threat of an Earth collision over at least the next 100 years. However, this will be the closest approach to date by an object this large that we know about in advance and an event of this type will not happen again until 2028 when asteroid (153814) 2001 WN5 will pass to within 0.6 lunar distances