Well, Chandler told himself, it would be fifty years before this particular package reached its destination, so a delay of a week would hardly make much difference. But all the calculations about rotation, centre of mass, and thrust vectors would have to be redone, and radioed back to Mars for checking. It was a good idea to do your sums carefully, before nudging billions of tons of ice along an orbit that might take it within hailing distance of Earth.

As they had done so many times before, Captain Chandler's eyes strayed towards the ancient photograph above his desk. It showed a three-masted steamship, dwarfed by the iceberg that was looming above it – as, indeed, Goliath was dwarfed at this very moment.

How incredible, he had often thought, that only one long lifetime spanned the gulf between this primitive Discovery and the ship that had carried the same name to Jupiter! And what would those Antarctic explorers of a thousand years ago have made of the view from his bridge? They would certainly have been disoriented, for the wall of ice beside which Goliath was floating stretched both upwards and downwards as far as the eye could see. And it was strange-looking ice, wholly lacking the immaculate whites and blues of the frozen Polar seas. In fact, it looked dirty – as indeed it was. For only some ninety per cent was water-ice: the rest was a witch's brew of carbon and sulphur compounds, most of them stable only at temperatures not far above absolute zero. Thawing them out could produce unpleasant surprises: as one astrochemist had famously remarked, 'Comets have bad breath'.