It's not hard to imagine the devastation to life on
Earth, should an asteroid hit us.
Objects up to about 1 km in diameter will cause extreme
local damage, but pose no global threat. However objects
greater than 1 km in diameter, resulting in 1 million megatons
of TNT, would be large enough to perturb the
Earth's climate on a global scale. About 25% of humanity
would be wiped out. How often do such events occur?
Tunguska
In 1908, something exploded about 8 km above the ground, over an
extremely remote section of Northern Russia. Witnesses much
farther south only saw the trail of a fireball going north,
then a tremendous rumbling, like an earthquake.
Today most scientists believe it was a comet or a stony
asteroid, about 50 m in diameter. It had the energy of
20 megatons of TNT (1000x Hiroshima bomb), destroying
an area the size of Rhode Island. In 1929 a scientist
investigating stories from that incredible night,
found and photographed the site:
PEEKSKILL (October 9, 1992)
On October 9, 1992 a fireball brigh like the full moon (-12) was observed in Northern America. The cosmic body ended
its flight on a car in Peekskill (NY, USA) and a meteorite (H6) of 12.4 kg was recovered.
Another great impact/meteor website, here