 What and where is Ganymede?
The glowing semi-disk of Jupiter was more than half a million miles distant. It had been built for Ganymede, but not for Jeffers. Tolliver began to have his doubts the next day; which was "Tuesday" by the arbitrary calender constructed to match Ganymede's week-long journey around Jupiter. Sometimes there's no reason but them for sending a ship back with a crew instead of in an economy orbit. They found they could send back loaded ships by 'automatic' flight—that is, a long, slow, economical orbit and automatic signalling equipment. Then he headed the tractor across the frozen surface of Ganymede toward the permanent domes of the city. "Any square inch of Ganymede is likely to be dangerous."
 It’s a moon close to Jupiter.
 It’s a planet close to Earth.  It’s a planet close to Jupiter.  It’s a moon close to Mercury.  It’s a moon close to Jupiter.