  Violation of correspondence principle may occur for very macroscopic byt
isolated quantum systems on rather short timescales as illustrated by the case
of Hyperion, the chaotically tumbling moon of Saturn, for which quantum and
classical predictions are expected to diverge on a timescale of approximately
20 years. Motivated by Hyperion, we review salient features of ``quantum
chaos'' and show that decoherence is the essential ingredient of the classical
limit, as it enables one to solve the apparent paradox caused by the breakdown
of the correspondence principle for classically chaotic systems.
