Killer asteroids: how safe are we, really?

A new study suggests we're safe from big impacts, but it's the small ones we have to worry about

- COLIN STUART

It's the ultimate cosmic catastrophe. A killer space rock is locked on a collision course with Earth. When it hits, the curtain comes down on humanity as we fade into the shadows of history, just like the dinosaurs before us.

Despite being the subject of a string of apocalyptic Hollywood blockbusters, there is some good news. A recent study found that we're unlikely to be hit by any of the nearly 1,000 known near-Earth asteroids above 1km in diameter within the next millennium (the asteroid that unleashed hell upon Tyrannosaurus rex and co 66 million years ago is thought to have been between 10-15km wide).

The study, led by Oscar Fuentes-Muñoz from the University of Colorado Boulder, is a marked improvement on previous work, which could only forecast a century ahead. Although, according to Prof Phil Bland, an asteroid expert at Curtin University in Australia, the $1,000 impact-free years' claim comes with some important caveats. Most notably, it only applies to the big asteroids we already know about.

"It doesn't speak to the 5% that are still out there waiting to be discovered," he says. "It doesn't include comets either, which we'll never be able to constrain."

This could be important, as many comets, which can be as big as asteroids, fly in from the outer solar system having never entered the inner solar system before. We have no way of tracking them until they're already very close to us.