A Trail Of Clues

"The Snows of Ganymede."

We have learned to look for clues when reading so here are some.

As the Planetary Engineers approach Ganymede, their radar detects an orbiting metallic object, larger than an ordinary spaceship, that does not respond to radio. The colonists had broken up their ships for parts and were not known to to have launched a satellite.

When researching in the library on Ganymede, the Engineers find hints of a colony on Callisto. They are told that a small group had gone to Callisto from Earth during the Humanist troubles but was too small to survive on its own so had then come to Ganymede.

Chapter V, which summarizes earlier history, ends by stating that, during the Humanist Revolution, many members of the Psychotechnic Institute went to Mars or Venus but that another group fled in an old ecological-unit spaceship to an unknown destination.