Smoke and mirrors to deflect asteroids

Scientists at the University of Glasgow have assessed a number of plans for diverting asteroids that could hit the Earth and have concluded that mirrors offer the best solution.

The team checked out nine different methods of diverting asteroids and comets that are dangerously close to hitting the planet.

The most effective plan, the team agreed, is 'solar sublimation' which focuses space-based mirrors onto the object to heat it up and deflect its course.

An object 150 metres across could be diverted by 100 mirrors in just a few days, according to the researchers.

A larger object, which could kill most life on Earth, would take a fleet of 5,000 mirrors focused on the object for around three years.

Project leader Dr Massimiliano Vasile told the BBC: "Asteroid impacts are a real threat. The Tunguska explosion in 1908 devastated an area bigger than Greater London.

"With only 10 spacecraft flying in formation, each with a 20-metre mirror, we could deflect a similar sized asteroid into a safe orbit in about six months. Our studies show that this technology is genuinely feasible."

For smaller objects the team concluded that nuclear weapons could be used to destroy or deflect the object, but such tactics are risky since they could send radioactive shards of rock into the Earth.

Other methods, such as landing on the object and using thrust to deflect its path, were deemed too impractical to be of use.

Several mass extinctions in Earth's history are thought to have been caused by collisions with extraterrestrial objects.

It is believed that asteroids or comets between six and 12 kilometres wide caused mass extinctions 251 and 165 million years ago.

Smaller strikes are more common, however. The most recent major impact was the Tunguska blast of 1908, which hit the Russian tundra with a force of around 10-20 megatons of explosives.