(TELEGRAPH) — Considering the dangers lurking out there, it’s a wonder that our little planet is not in the firing line more often. We are just 93 million miles from a star that, while mostly well-behaved, occasionally has temper tantrums that could bring our civilisation to its knees. Our solar system is home to a swarm of comets, rocks, boulders and flying mountains, tens of thousands of which are big enough to wipe out anything from a small city to the entire biosphere. And further out lurk delinquent stars whose death explosions are the largest since the Big Bang. If one of these went off nearby, it would be curtains for all of us.
In fact, Earth can be considered rather lucky to have not suffered a total cataclysm in at least 3.5 billion years – the period during which we have an unbroken record of life existing on the Earth’s surface. Before then, global sterilisation events, caused by collisions with huge space rocks, almost certainly took place many times – perhaps once every few hundred years. Each may well have wiped out early versions of life. Then, after one final cataclysmic impact, it was plain sailing all the way.
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That was what we used to think, anyway. But in recent decades, it has become clear that our cosmic neighbourhood, in more recent times, has not been as benign as was thought. In the 1980s, for example, it was confirmed that Earth has been hit several times in its history by objects from space – none big enough to sterilise the planet completely, but a handful packing enough of a punch to change the course of life forever. The most famous of these was our collision with a six-mile-wide asteroid 65 million years ago, whose fiery passage into the Mexican coast has been blamed for killing off, or at least delivering the coup de grâce to, the dinosaurs.