The threat of asteroid impact and global extinction
The Asteroid Belt is the region of the solar system between the orbits of Mars and Jupiter where over 100,000 asteroids can be found. Although the majority of these asteroids are less than 1km in diameter, there are larger ones. The largest asteroid is Ceres with a diameter of 914km. Palas and Vesta are the next biggest with diameters greater than 500km. There are 30 other asteroids with diameters between 200 and 300km, and 200 more which are greater than 100km across. Question marks still remain about where and how many asteroids are over 1km in size. Current estimates state that roughly 1,000 asteroids larger than 1 kilometer (0.6 miles) are thought to have orbits that might one day threaten Earth with planet-wide chaos. So far only 500 of them have been found.
The orbits of most asteroids lie entirely within the asteroid belt at a distance between 2 and 3.5 AU (AU - Astronomical Unit - roughly the distance between the Sun and the Earth). However some asteroids have highly elliptical orbits bringing them into the inner regions of the solar system and around the Sun, sometimes passing quite close to Earth. In 1989, one such asteroid called 1989FC passed within 800,000km - twice the distance between the Earth and the Moon. If this asteroid had collided with the Earth we would have been hit with an explosion equivalent to a thousand 20-megaton hydrogen bombs.
In July 1994, fragments of Comet Shoemaker-Levy 9 released energy equivalent to millions of megatons of TNT and generated fireballs and dark clouds on areas of Jupiter about as large as the Earth itself.
An increasingly popular theory, backed by the evidence of geologists and paleontologists is that an asteroid 10km in diameter struck the Earth 63 million years ago and was responsible for the extinction of half the life forms at that time, including the dinosaurs. The impact would have generated fireballs and enough dust clouds to engulf the Earth and cause a nuclear winter for years. Only animals smaller than today's rats were able to survive such an onslaught.
The Earth has a history of bombardment by space debris, striking at velocities of tens of kilometers per second (11km/s = 25,000mph). Smaller objects burn up in the atmosphere. Objects greater than 50m have the capacity to reach the surface and explode with forces comparable to the most powerful nuclear weapons. Asteroids greater than a kilometer in diameter can inflict greater damage through global environmental effects, threatening the survival of mankind.
Close approach information
63,000,000 BC: Impact?
23,000 BC: Impact
1972: Near miss
1989: Near miss
2002 March: Near miss
2002 June: Near miss
2004: Toutatis - 1,600,000 km (from Earth)