But microbes might survive if they can escape the Sun's gravity more quickly. And that might happen, says Napier, if the rocks they sit on are first ground to dust1.

The Earth and her sister planets travel through a cloud of grains called zodiacal dust. This is the debris from collisions in the asteroid belt and from the passage of comets.

This dust should sand-blast anything passing through it, says Napier. This process could grind a one-metre boulder down in 20,000-200,000 years, he estimates. If a comet breaks up, thickening the dust, as happens several times each million years, the process could take as little as five centuries.

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Such a grain could travel about six light years from Earth in 70,000 years - far enough to reach other stars. We could be surrounded by a huge 'biodisk' of frozen organisms floating on grains of rock, says Napier, all of which can wander in and out of our solar system quite easily. "The solar system is as leaky as a sieve," he says.

At the time, the president's team would have been putting the final touches to a speech he was due to make the following day at the headquarters of Nasa, the US space agency.

In it he planned to reset the course of manned spaceflight, sending it back to the Moon and on to Mars, but he could have had something very different to say.

He could have begun by warning the world it was about to be hit by a space rock.