(4)Added to the more than 20,000 known asteroids such as Gaspra(1) (pictured left), is an object dubbed 2004 AS1, which was recently revealed(2) to have had a one-in-four chance of hitting our planet within a 36 hour period. An animation(3) of 2004 AS1 put together by the Great Shefford Observatory, shows the trajectory of the asteroid. As Space.com reports, there is a new call for planetary defense from such objects.
1. http://www.solarviews.com/eng/gaspra.htm
2. http://news.bbc.co.uk/2/hi/science/nature/3517319.stm
3. http://www.birtwhi.demon.co.uk/Gallery2004AS1.htm
4. http://www.solarviews.com/eng/gaspra.htm