Every few months, a story appears in the media about an asteroid on a collision course with Earth. Inevitably, further observations and calculations reveal the asteroid isn’t going to hit us after all. But what is the real risk of an asteroid impact?

We know for certain that Earth has been hit in the past because of the hundreds of impact craters on the surface of the planet, including a few in Canada. Astronomers estimate how often asteroids strike us by studying the number and size of existing craters and by running computer simulations. Smaller objects tend to strike the Earth more frequently than large ones, simply because most of the asteroids out there are small.

Objects about 50 metres across enter the Earth’s atmosphere once every year or so, but objects this small pose no threat because they burn up completely before reaching the surface.

Hundred-metre-sized objects hit the Earth about once per century. In 1908, an object of this size exploded over Siberia, flattening 1,000 square kilometres of forest. Although objects of this size strike with the energy of 100 Hiroshima-type nuclear bombs, they are a relatively small concern because their effects are very localized and are more likely to hit an uninhabited area than a city.

Objects a few hundred metres across are called regional catastrophe objects and strike about every 10,000 years. This kind of asteroid would cause massive tsunami if it hit the ocean, or earthquakes and fires if it hit land. An impact on land would also level a huge area.

The largest near-Earth asteroids can be a kilometre across or larger. These are called global killers because an impact would throw huge amounts of dust into the atmosphere, blocking the sun and causing conditions similar to a nuclear winter. A five- to ten-kilometre asteroid is thought to have wiped out the dinosaurs. Kilometre sized objects strike once every million years, on average.

So how does this translate in terms of an individual’s risk over a lifetime? Only very rough estimates are possible, because no one knows exactly how many deaths an asteroid impact would cause. A recent estimate puts a person’s chance of being killed by an asteroid impact at 1 in 20,000. By comparison, an individual’s chance of dying in a plane crash is about 1 in 5,000 and the chance of dying in a car crash is 1 in 85.

If an asteroid were discovered on a collision course with Earth, it’s likely that a collision could be prevented by deflecting the asteroid with nuclear blasts or conventional rockets. The more warning we have, the easier it would be to deflect it.

Astronomers are currently studying near-earth asteroids, and would like to catalogue at least 90 per cent of the kilometre-or-larger-sized objects in near-Earth orbits. With current efforts it will take several decades to reach that goal.