Around 65 million years ago, an asteroid of about 10km in diameter struck Chicxulub in Mexico. This impact probably caused one of the three largest mass extinctions in history, abruptly ending the age of the dinosaurs. Large asteroids still exist in orbits near the Earth’s and the impact of an asteroid bigger than 1 km in size would eject enough particles into the atmosphere to dim the sun for a number of months. The resulting cooling of the climate would undermine ecosystems and global agriculture for at least an entire growing season, and could cause a famine leading to the death of hundreds of millions.
Asteroids are small rocks leftover from the formation of our solar system about 4.6 billion years ago. Too small to be called planets, they revolve around the sun, typically along elliptical orbits. The orbits of Earth and the asteroids can occasionally intersect and result in collisions.
The likelihood of asteroid-related risk is better understood than that of many other global catastrophic risks because the underlying dynamics have been well understood for a very long time. Many asteroids have hit Earth in the past, and more will continue to do so. While smaller objects would have only local effects, larger ones could cause a global cooling resulting in large-scale disaster. On the basis of historical evidence, an asteroid impact large enough to cause a global catastrophe is estimated likely to occur every 120,000 years.
In 2011, NASA held a press conference announcing that over 90% of objects larger than 1 km in diameter had now been discovered, and none of those has been estimated likely to enter in collision with the Earth. Currently there are no known objects of any size for which we have well-computed orbits that are predicted to have significant probability of hitting Earth. However, after more than twenty years of survey, the current data for smaller objects of 140 meters up to 1 kilometer in size is only about 30% complete. Further monitoring is required to properly establish risk levels. Although unlikely to directly cause a global catastrophe by cooling the climate, those smaller objects could have significant local impact, and indirectly disrupt social and economic systems.
The impact of an asteroid bigger than 1km in size would release enough particles in the atmosphere to dim the sun for a number of months.
In more recent history, sources indicate that an asteroid impact may have caused the death of up to 10,000 people in the Chinese city of Qingyang in 1490, and an explosion generally attributed to an asteroid impact destroyed 2000km2 of Taiga close to the Tunguska River in Siberia in 1908.
CEO of NEO Sciences, LLC, former Director of the Minor Planetary Center, Harvard-Smithsonian Center for Astrophysics