More than a million asteroids, over 200 of them 100km or more in diameter (but irregular in shape) form belts between the orbits of Mars and Jupiter; though some stray inward, closer to the Sun and so closer to the Earth.
Hardy is a member of Spaceguard UK, an organisation which is dedicated to informing the public about the very real possibility that the Earth will – as it has been millions of times in the past – be hit by an asteroid or comet large enough to destroy a major city.
This would have a massive impact on our planet, creating a kind of 'nuclear winter'. Despite the fictitious portrayals of this scenario in many books (notably by Larry Niven and Arthur C. Clarke) and in films such as Deep Impact and Armageddon, this is not science fiction.
The only question is not 'whether' but 'when?' will this happen, and… 'Can we do anything about it?'