Pushing Ice by Alastair Reynolds

"A thrilling ride in the new era of well-written space adventure"(The Denver Post) from the author of the Revelation Space series.
2057. Bella Lind and the crew of her nuclear-powered ship, the Rockhopper, push ice. They mine comets. But when Janus, one of Saturn's ice moons, inexplicably leaves its natural orbit and heads out of the solar system at high speed, Bella is ordered to shadow it for the few vital days before it falls forever out of reach.

In accepting this mission she sets her ship and her crew on a collision course with destiny-for Janus has many surprises in store, and not all of them are welcome...