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This year, in the early morning of 15th February, an asteroid, 20 metre across with a mass of 11,000 tonnes, exploded in an airburst 23 km above the southern Urals just 40 kilometres from Chelyabinsk, a city of 3.5 million people. For 5 seconds the explosion flared up brighter than the sun. Thousands rushed to their windows to view it but were soon lacerated by flying glass from the shock wave that followed a minute later. The total energy released was equivalent to 440 kilotonnes of TNT, over 20 times greater than that released by the atomic bomb over Hiroshima. And though most of the energy was absorbed by the atmosphere, over 7000 buildings across 6 cities were damaged and the asteroid was shattered, sending meteorite fragments streaking through the air to spatter the snow over hundreds of square kilometres.

Just 16 hours later, another asteroid, 30 metres across with a mass of 40,000 metric tonnes passed just 28,000 km above the Earth’s surface, closer than the orbital distance of geosynchronous satellites.

We were lucky not to be hit. Jupiter wasn’t so lucky 19 years ago when the Shoemaker-Lacey-9 comet smashed into the planet and released an energy equivalent of 6 tera-tonnes of TNT.

A pinball lottery rules the solar system.

Asteroids can vary in diameter from a few centimetres to hundreds of kilometres and most of them are confined in orbit between Mars and Jupiter. Gravitational interactions with neighbouring planets, however, can sometimes catapult them into sling shots round the Sun, risking a hairy fly past or terminal collision with Earth.

Further out lies the Kuiper belt, an anarchic region where Pluto resides with his band of fellow dwarf planets, asteroids, planetisimals, rocks, dust and gases. It’s here that comets form which can also be gunned by gravitational forces towards an unsuspecting Earth.

The odds are against being hit, but it’s happened before.

66 million years ago an asteroid of 10 km diameter smashed into what is now Mexico and released energy equivalent to 100 tera-tonnes of TNT, over a million times greater than the largest hydrogen bomb ever detonated. Global fire storms and gigantic tsunamis wrecked the planet and fine aerosols of dust and acid were injected into the stratosphere where they cut out 20% of the sunlight for a decade. A whole food chain from phytoplankton to dinosaurs died out leaving the planet to a few ratty mammals which eventually evolved into us.

It was a science fiction writer who first took the threat of asteroid impacts seriously. In his 1972 novel, Rendezvous with Rama, Arthur C. Clarke envisioned an early warning system called SPACEGUARD to protect the planet from an asteroid impact. In 1992, the name was given to a NASA mission to monitor all asteroids larger than 10 km in near-earth orbit. Following the Chelyabinsk incident, the United Nations has now set up its own action team to investigate the risk of impact from near earth objects while some scientists in California are proposing a massive orbital laser blaster that would channel energy from the sun to a giant laser to vaporize any potential threats.