Electrical power (EPS), thermal control, attitude determination and control (ADCS), communications, and propulsion subsystems, and why spacecraft design differs fundamentally from aircraft design once maintenance access disappears.
An aircraft with a failing system can usually land and get fixed. A spacecraft cannot — whatever redundancy it launches with is the entire redundancy it will ever have for a mission that can run fifteen years or more, in a vacuum environment with no convective cooling, extreme orbital thermal cycling, and a hostile radiation environment the previous module's aircraft never has to contend with. This module works through electrical power, thermal control, attitude determination and control, communications, and propulsion in turn, then closes on the redundancy architecture — cross-strapping, graceful degradation, triple modular redundancy — that no-maintenance-access operation demands.
By the end of this module you should be able to explain why reaction wheels need periodic desaturation, why free-space path loss makes deep-space communications a harder problem than low-Earth-orbit communications in degree rather than kind, and why the Tsiolkovsky rocket equation from this program's earlier propulsion coverage directly explains the modern shift toward electric propulsion for station-keeping — the same telemetry-only diagnostic reasoning this program's troubleshooting module (Module 17) applies to a real spacecraft attitude control anomaly.