The potentially hazardous asteroid 2005 YU55 passes 0.85 lunar distances from Earth (about 324,600 kilometres or 201,700 miles), the closest known approach by an asteroid of its brightness since 2010 XC15 in 1976.

A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and which is large enough to cause significant regional damage in the event of impact. They are conventionally defined as having a minimum orbit intersection distance with Earth of less than 0.05 astronomical units and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters. More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years. As of February 2025, just 21 of the known potentially hazardous objects listed on the Sentry Risk Table could not be excluded as potential threats over the next hundred years. Over hundreds if not thousands of years though, the orbits of some "potentially hazardous" asteroids can evolve to live up to their namesake.

(308635) 2005 YU₅₅, provisionally named 2005 YU₅₅, is a potentially hazardous asteroid 360±40 meters in diameter, as measured after its Earth flyby. Previously it was estimated to be 310 meters or about 400 m (1,300 feet) in diameter. It was discovered on 28 December 2005 by Robert S. McMillan at Steward Observatory, Kitt Peak. On 8 November 2011 it passed 0.85 lunar distances (324,900 kilometers; 201,900 miles) from Earth.

The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. In contrast, the Lunar distance, or Earth–Moon characteristic distance, is a unit of measure in astronomy. More technically, it is the semi-major axis of the geocentric lunar orbit. The average lunar distance is approximately 385,000 km (239,000 mi), or 1.3 light-seconds. It is roughly 30 times Earth's diameter and a non-stop plane flight traveling that distance would take more than two weeks. Around 389 lunar distances make up an astronomical unit.