On Tuesday 8 November 2005 Asteroid 2005 YU₅₅ will pass within 323 500 km of the Earth, roughly 85% of the distance to the moon. This is an unusual event, though not an alarming one. The last time a largish asteroid came this close to the Earth was 1976, and it will not happen again till 2028. The orbit of 2005 YU₅₅ has been mapped for the next 100 years, as far as can be done accurately due to repeated close encounters with Earth, Mars and Venus, and scientists are confident that in tat time it will not hit us.
The orbit of 2005 YU₅₅.
Even if 2005 YU₅₅ were to collide with the Earth, it is not exactly end of the world material. The asteroid has a diameter of ~400 m and is thought to be composed largely of carbonaceous material. Being directly underneath it would be unpleasant, and that much carbon being vaporized at once wouldn't do our climate any favors, but there is no danger of any global firestorms of tsunamis.
However the asteroid is of great interest to amateur astronomers, to whom it represents a once-in-a-lifetime opportunity to actually spot a near Earth asteroid. 2005 YU₅₅ should be visible to viewers with good binoculars or small telescopes for several hours late on 8 November and early on 9 November, and will be best seen from the eastern United States.
Th designation 2005 YU₅₅ indicates that the asteroid was discovered in 2005, after 16 December (the year is split into 24 two-week periods given the letters A to Y), and was the 1395th object discovered in this period. This last bit is derived from the 'U₅₅'; within each period the first object discovered is named 'A' the second 'B' and so on, until 'Z' is reached, indicating 25 (I is not used). The twenty-sixth object is then named 'A₁', the twenty seventh 'B₁' and so on, so that 'U₅₅' implies the 1395th (late December 2005 was obviously a productive period for asteroid discovery).