Carl Sagan, from his book Billions and Billions:
Some mass extinctions of life in the past are now understood by immense mantle plumes gushing up through the surface and generating lava seas where solid land once stood. Others are due to the impact of a large comets or near-Earth asteroids igniting the skies and changing the climate. In the next century, at the very least we ought to be inventorying comets and asteroids to see if any of them has our name on it.
From ESA News:
Based on the recommendations of asteroid experts, ESA has selected two target asteroids for its Near-Earth Object deflecting mission, Don Quijote.
The current scenario envisages two spacecraft in separate interplanetary trajectories. One spacecraft (Hidalgo) will impact an asteroid, the other (Sancho) will arrive earlier at the target asteroid, rendezvous and orbit the asteroid for several months, observing it before and after the impact to detect any changes in its orbit…
While the eyes of the world were on the Asian tsunami last Christmas, one group of scientists were watching uneasily for another potential natural disaster – the threat of an asteroid impact.
On 19 December 2004 MN4, an asteroid of about 400 m, lost since its discovery six months earlier, was observed again and its orbit was computed. It immediately became clear that the chances that it could hit the Earth during a close encounter in 2029 were unusually high. As the days passed the probability did not decrease and the asteroid became notorious for surpassing all previous records in the Torino and Palermo impact risk scales – scales that measure the risk of an asteroid impact just as the Richter scale quantifies the size of an earthquake.