Last weeks close Earth flyby of asteroid 2005 YU55 was a reminder that there are many dangerous objects in space and we are exposed to them. As an observational event the flyby was a none starter; despite approaching closer to the Earth than the Moon, this dark as coal aircraft carrier size rock was far too faint to be seen with the naked eye as it zoomed across the heavens. More bad luck for astronomers hoping to catch a glimpse of 2005 YU55 was the presence of an almost full moon, intense natural light pollution that crushed almost any chance of seeing our fleeting visitor without access to a reasonable sized telescope.
To date, the best images of 2005 YU55 have been released by NASA. This image was obtained using radar measurements from the Deep Space Network in Goldstone, California and shows the asteroid at a distance of 1.38 million kilometers (860,000 miles) from Earth.
At closest approach on the 8th November 2011, 2005 YU55 was only 0.85 lunar distances (324,600 kilometers or 201,700 miles) from Earth (apparent magnitude 11). With a diameter of about 400 metres, if it or a similar sized object were to slam into the Earth, the consequences would be devastating. A conservative estimate is that such a collision on land would cause a magnitude 7 equivalent earthquake and leave a 6 kilometre (3.8 mile) sized crater. The event would be devastating with potentially thousands of lives lost and dust and debris thrust high into the atmosphere. Major climatic changes would result and last for months or years to come. An ocean impact would be equally bad, resulting in massive tsunamis with waves tens of metres tall.
The next time a known asteroid this large will come this close to Earth will be in 2028 when 153814 2001 WN5 passes by at a distance of 248,000 kilometres or 154,000 miles. Of course, such close flybys occur seldom and thankfully the chances of a direct strike are extremely rare. NASA recently analysed data from the NEOWISE (Near-Earth Object WISE) space mission and concluded that we are unlikely to be wiped out by a killer asteroid in the next few centuries but precaution prevails as there is still a small chance of a direct hit.
In the very distant past such collisions between asteroids and the Earth were common and astronomers consider 2005 YU55 to be a carbon based C-type asteroid. Such objects are thought to have enriched the early Earth with carbon based materials and water, planting the seeds for life.