**Abstract**

Robotic interplanetary spacecraft sent to the outer planets of our solar system face many challenges: maintaining internal health and functionality of spacecraft subsystems handling material stresses from solar heating close to Earth, the cold of deep space once the destination is reached, solar radiation and bombardment of cosmic rays; maintaining adequate power to support engineering devices and science instruments; handling time-critical onboard faults in the presence of the long round-trip light time; and preserving one-time "crucial event" activities such as moon/planet flybys, deployment of the probe, and selected science targets. As an example, this chapter details the strategy implemented on the Cassini Missionto-Saturn spacecraft, how its onboard subsystems are protected and maintained, the advantage of automated onboard fault protection monitor/response routines, protocols implemented to preclude human error in uplinked sequences, and updating onboard flight software as new discoveries are uncovered about the adverse flight environment, so that mission objectives are met under the presence of an ever-increasing delay between ground issued commands and the Cassini spacecraft as it approaches the Saturnian system, safeguarding planetary protection constraints as the spacecraft was deposited into the planet in a final fiery plunge.

**Keywords:** Cassini, spacecraft, Saturn, interplanetary, fault protection
