Not all killer asteroids found yet

BBC NEWS | Science/Nature | UK to join 'killer' asteroid hunt

I wonder whether people are starting to feel that most major asteroids with potential to hit the earth are already identified, given that every couple of years there is mention of one that might hit the earth sometime in the future. (With later calculations often showing it shouldn't happen after all.)

The story above warns that in fact there is a lot of small rubbish still not spotted:

"The current generation of search telescopes are designed for the objects about 1km across and larger, because if one of those hits, it could cause instant global climate change," said Alan Fitzsimmons, a professor of astronomy at Queen's University Belfast.

"The smaller objects need a larger telescope and a more efficient camera system - they're the kinds of objects Pan-Starrs has been designed to detect.

"Even though they're smaller and don't cause as much damage, there are more of them and they hit more frequently."

Although sub-1km asteroids might not cause devastation on a global scale, they could cause death and destruction at a local and regional level, potentially wiping out millions of lives.

The last significant event like this occurred in 1908, when an asteroid or comet exploded above the Tunguska region of Siberia. The area was sparsely populated and, as a result, did not cause extensive loss of life.