~500,000

Next Big Asteroid Impact?

It took centuries for geologists to figure it out, but impact cratering is now understood to be a major force of geologic change, not just in the history of the Moon and other heavily cratered planets, moons, and asteroids out there, but also of the Earth. A giant impact between the early Earth and a Mars-sized protoplanet likely formed our Moon. An impact certainly played a major role in the extinction of the non-avian dinosaurs and many other species at the end of the Cretaceous period 65 million years ago, and impacts may have played roles in other mass extinction events in Earth’s history as well. Well-preserved young craters like Meteor Crater in Arizona and small impact events like those in Tunguska in 1908 or Chelyabinsk in 2013 remind us that there are still potentially threatening impacts waiting to happen one day.

So when will an impactor large enough to potentially cause a major climatic and/or biologic catastrophe next hit the Earth, and could we do anything to prevent it? Astronomers, planetary scientists, engineers, and planetary defense experts are working to try to figure that out. An important first step is completing a census of the Potentially Hazardous Asteroids that occasionally pass near the Earth, including their size, composition, and internal strength (are they solid boulders, or rubble piles of pebbles and sand?), so that their threat potential can be assessed. By 2011, surveys had found 90 percent of the asteroids large enough to cause a global catastrophe; continuing observations over the next decade will attempt to expand that survey to include smaller asteroids that could still cause local or regional catastrophes.

Statistically, a potential “dinosaur-killer”-sized asteroid (about 6 miles [10 kilometers] in diameter) is predicted to hit Earth about once every 100 million years, so perhaps we’re safe from that fate for another 35 million years or more. However, an asteroid around 0.6 miles (1 kilometer) diameter strikes the Earth every 500,000 years or so, releasing more than 3,000 times the energy of a World War II atomic bomb and thus causing substantial local devastation and climate disruption. Will we have to wait 500,000 years for that next big impact, or will it come sooner? Will we be ready?

The orbits of known members of the Near-Earth Asteroid population (blue) plotted along with the relatively circular orbits of Mercury, Venus, Earth (white), and Mars, in a view looking down onto the north pole of the solar system.