On the 7th January 2002 an asteroid of diameter 210 - 460 meters came within 837748 Kilometres of planet Earth, this is only twice the distance of our moon.
If it had hit London, it would have wiped it out,if it had hit water it would cause a 200-500M high tidal wave.
Steller objects with elliptical orbits will pass through the orbits of the other planets.
Apparently the chances of being killed by an asteroid hitting the earth is the same as being killed in an (accidental) aircrash.
The British goverment is to set up a Near Earth Object research centre to help track objects that orbit the sun elliptically and that could be a threat to life on the planet Earth. By gathering more accurate and timely information about NEOs we might be able to do something about them before we get obliterated.
The same asteroid will narrowly miss planet Mercury on September 2005 and again in April 2009.
Looks like it might actually hit Mercury on December 9th 2012 or Mars on Feb 12 2013.
For more details (inluding asteroid path simulation) see http://neo.jpl.nasa.gov/cgi-bin/db?name=2001+YB5
Another asteroid will come close to Earth on the 11th of January 2002 and yet another will pass within 4787131 Km on the 14th so be nice to people - you might not get another chance.
For a list of object currently being tracked by NASA see http://neo.jpl.nasa.gov/
For further information see Impact cratering on Earth