One maverick scientist had put forward the theory that Mount Zeus was a 'cosmic iceberg' – a cometary fragment that had dropped upon Europa from space; battered Callisto gave ample proof that such bombardments had occurred in the remote past. The theory was very unpopular on Ganymede, whose would-be colonists already had sufficient problems.

They had been much relieved when van der Berg had refuted the theory convincingly; any mass of ice this size would have shattered on impact – and even if it hadn't, Europa's gravity, modest though it was, would have quickly brought about its collapse. Radar measurements showed that though Mount Zeus was indeed steadily sinking, its overall shape remained completely unaltered. Ice was not the answer.

The problem could, of course, have been settled by sending a single probe through the clouds of Europa. Unfortunately, whatever was beneath that almost permanent overcast did not encourage curiosity.

ALL THESE WORLDS ARE YOURS – EXCEPT EUROPA.

ATTEMPT NO LANDINGS THERE.

That last message relayed from the spaceship Discovery just before its destruction had not been forgotten, but there had been endless arguments about its interpretation. Did 'landings' refer to robot probes, or only to manned vehicles? And what about close flybys – manned or unmanned? Or balloons floating in the upper atmosphere?

The scientists were anxious to find out, but the general public was distinctly nervous. Any power that could detonate the mightiest planet in the Solar System was not to be trifled with. And it would take centuries to explore and exploit Io, Ganymede, Callisto and the dozens of minor satellites; Europa could wait.