Callisto in one of Jupiter's moons, the third largest in the entire solar system and the second largest in the Jovian system (Ganymede being the largest of Jupiter's companions...more about Ganymede later on a forthcoming track).

Composed of mainly rock and ice, it also has traces of silicates and organic compounds and my even have a frozen, subterranean sea down below 100 km from the surface. It has a thin atmosphere of carbon-dioxide and, probably, oxygen molecules.

Callisto orbits Jupiter with the same side facing towards its parent planet, exactly the same way that our moon shows the same face to the Earth. Because this affords the "dark" side a very low radiation level, it has been long-thought that Callisto would be an ideal place for a human base as and when we get around to fulfilling Kennedy's dream of reaching "the very end of the solar system".

Because to do so is a bit like the punch-line from that old Irish joke - it helps, you see, not to start from here. Jupiter (and thus, by association, Calliso) is about 390,000,000 miles from Earth (483,000,000 from the Sun).

At best, that trip would take a couple of years. Starting for the outer rim of the solar system from Callisto would trim all those miles and months off - but the trip to the edge from Callisto would still be a distance of some five BILLION miles.

To save on life-support and the crushing boredom of sitting there watching nothing happen, it's highly likely that the crew of a ship setting out to explore the outer planets would be put into some kind of cryogenic suspension for the majority of the trip.

And, most probably, they'd never come back, due to the travel time, the huge orbits of the outer planets and the simple maths involved. The whole venture would take 20-30 years...add that onto the age of the average astronaut (35-40) and it seems more and more like a one way trip.

So have a listen and sink, if you would, into the cold, long sleep of my imaginary astronaut.