A probe near Jupiter or beyond can be 40+ light-minutes from Earth — by the time a fault report arrives and a ground command is sent back, the spacecraft could already be dead. So flight software must handle faults itself, in three steps:
- Detect — a watchdog notices a sensor reading leave its expected envelope or a subsystem stop responding to heartbeat polls.
- Isolate — the faulty unit is switched off the power/data bus immediately, before its bad behaviour (a short, a stuck actuator, corrupted data) can damage or confuse neighboring subsystems.
- Recover — a redundant backup unit is switched in, or the spacecraft drops into a safe, degraded operating mode, and mission operations continue without waiting for a ground round-trip.
Without FDIR, an undetected fault can cascade around the subsystem ring: a power fault starves attitude control of voltage, which lets the spacecraft drift off-pointing, which knocks out comms, which then loses the thermal system its housekeeping data — a single component failure snowballing into total mission loss. This 2D panel plots that same state machine as a flat ring diagram instead of a 3D model, so every transition (fault → detect → isolate → recover, or fault → cascade) is readable as a single glance at node color and the connecting lines.