1979’s Titan is the first novel in John Varley’s Gaea trilogy.
Ringmaster is the first crewed spacecraft to visit distant Saturn. The crew discovers a twelfth moon of Saturn, the first discovered in the six decades since Janus and Epimethius were detected in the 1960s1. What at first appears to be a small object turns out to be huge, over 1300 kilometres in diameter2. By rights, any moon that large should be spherical and comparable in mass to our Moon. This object, which the astronauts initially call Themis, is a torus of low mass which is spinning rapidly enough that any object on its surface would be flicked off into space. The obvious conclusion is that the object is artificial, possibly an alien generation ship.
When Ringmaster approaches the object, they discover something else about Themis. It knows that Ringmaster is nearby and is able to reach out and tear the spacecraft apart. The crew — Cirocco Jones, Bill NLN, April and August Polo, Gaby Plauget, Calvin Greene, and Gene Springfield — are grabbed by immense tentacles and borne off into the depths of the vast moon.