The views expressed in this report are those of the authors. Their statements are not necessarily endorsed by the affiliated organisations or the Global Challenges Foundation.

What is at stake?

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.

How much do we know?

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 for the estimated total population. 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.

What are key factors affecting risk levels?

  • It is technologically possible to identify whether an asteroid is on a collision course with Earth long enough in advance, giving humanity time to react. However, many asteroids have not yet been spotted, and shorter reaction times would carry higher risk. Enhanced effort to detect and monitor asteroids would therefore decrease the risk.
  • New technologies that could either deflect the trajectory of an asteroid or reduce its impact would considerably reduce the overall risk level.
  • Systematic monitoring has considerably reduced the estimated risk of impacts from larger objects >1km that would significantly affect the climate. However, to address the remaining risk, resilience building, particularly the potential to rely on food sources less dependent on sunlight – mushrooms, insects, or bacteria – could significantly reduce the death rate among humans.  

The 5 largest asteroid impacts on Earth

  1. Vredefort Crater, South Africa – Estimated impact date: 2 billion years ago. World’s largest known impact structure, with an approximate diameter of 160km.
  2. Chicxulub Crater, Mexico – Estimated impact date: 65 million years ago. Many researchers believe that this was the asteroid that caused or contributed to the extinction of the dinosaurs, with an approximate impact diameter of 150km.
  3. Sudbury Basin, Canada – Estimated impact date: 1.8 billion years ago. Approximate diameter of 130km.
  4. Popigai Crater, Russia – Estimated impact date: 35.7 million years ago. Approximate diameter of 90km.
  5. Acraman Crater, Australia –  Estimated impact date: 590 million years ago. Approximate diameter of 90km.

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.

Tim Spahr

CEO of NEO Sciences, LLC, former Director of the Minor Planetary Center, Harvard-Smithsonian Center for Astrophysics