Prof. Mark Bailey's (Armagh Observatory) public lecture at NAM 2008 celebrated the 100th anniversary of the Tunguska event, when a large meteoroid exploded 5 miles above a remote region of Siberia.

The eyewitness reports of the event are, quite frankly, terrifying.

...the sky split in two and fire appeared high and wide over the forest. The split in the sky grew larger, and the entire Northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire;

Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them.

The event was even observed from Northern Ireland, some 3,500 miles away from Tunguska. The astronomer working at the Armagh Observatory that night noted a "nocturnal glow" in the log book (even though the Moon was not present), which was bright enough to read a newspaper and play cricket, even after midnight!

Fortunately, a Tunguska type event is only expected to occur, on average, once every century. And a meteor crater type impact is only expected once per million years.

But it is really difficult to estimate precisely how frequently such events occur. If you were to ask an astronomer in 1993 what the chance was of a large comet hitting Jupiter, they would have said maybe once every 100 million years... but just a year later, Comet Shoemaker-Levy 9 did hit Jupiter.

And astronomers are discovering new Near-Earth Objects (asteroids with orbits that come relatively close to the earth) all the time, as the graph below shows (courtesy NASA/JPL).

Impact frequency

But, while such events may scare us, Mark pointed out that our entire existence was dependent on the impact that wiped out the dinosaurs. If that collision had not have happened, we would not be here today.

The asteroid that wiped out the dinosaurs originated in the outer solar system, where a chance gravitational tug from another asteroid set it on a collision course with Earth. If that tug had been slightly different by a minuscule amount, the asteroid would have missed Earth, the dinosaurs would have continued their reign, and tiny mammals (from which we evolved) would never have got started - and so humans would not be here today.

Prof. Bailey then began to discuss the financial implications of ignoring the asteroid threat. If the Tunguska impact was centred on London, it would have destroyed everything within the M25, killing millions. If it hit anywhere else in the UK, it would have still killed tens of thousands. If we are lucky, and such an asteroid strikes land just once every ten attempts, that is still a major town disappearing every thousand years.

But the real risk is from the small, more frequent asteroids. We expect a 10m meteoroid to hit every year, causing a 100m crater. Maybe only 1 in ten would hit a populated area - but considering the average density of England, that once-a-decade strike could still kill 12 people.

Mark argued that this represents an intolerable risk, and using economic reasoning suggested that significant investment is required in order to research the threat posed.

Today, almost a hundred years after the event, little can be seen of the aftermath in Tunguska. Only a few felled trees remain visible, and even the small rise at the epicentre (upon which a totem pole to the Siberian fire god Agby stands) will be flat in a decade. One member of the audience suggested that, even though the fallen trees have almost disappeared, new growth may follow the lines of fertiliser provided by the fallen trees, providing at least some evidence to what happened nearly a century ago.