Rolf van der Berg was the right man, in the right place, at the right time; no other combination would have worked. Which, of course, is how much of history is made.

He was the right man because he was a second-generation Afrikaner refugee, and a trained geologist; both factors were equally important. He was in the right place, because that had to be the largest of the Jovian moons – third outwards in the sequence Io, Europa, Ganymede, Callisto.

The time was not so critical, for the information had been ticking away like a delayed-action bomb in the data banks for at least a decade. Van der Berg did not encounter it until '57; even then it took him another year to convince himself that he was not crazy – and it was '59 before he had quietly sequestered the original records so that no-one could duplicate his discovery. Only then could he safely give his full attention to the main problem: what to do next.

It had all begun, as is so often the case, with an apparently trivial observation in a field which did not even concern van der Berg directly. His job, as a member of the Planetary Engineering Task Force, was to survey and catalogue the natural resources of Ganymede; he had little business fooling around with the forbidden satellite next door.

But Europa was an enigma which no-one – least of all its immediate neighbours – could ignore for long. Every seven days it passed between Ganymede and the brilliant minisun that had once been Jupiter, producing eclipses which could last as long as twelve minutes. At its closest, it appeared slightly smaller than the Moon as seen from Earth, but it dwindled to a mere quarter of that size when it was on the other side of its orbit.