Safe mode is an operating mode of a modern uncrewed spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.[1]
Safe mode is entered automatically upon the detection of a predefined operating condition or event that may indicate loss of control or damage to the spacecraft. Usually the trigger event is a system failure or detection of operating conditions considered dangerously out of the normal range. Cosmic rays penetrating spacecraft electrical systems can create false signals or commands and thus cause a trigger event. The central processor electronics are especially prone to such events.[2] Another trigger is the lack of a received command within a given time window. Lack of received commands can be caused by hardware failures or mis-programming of the spacecraft, as in the case of the Viking 1 lander.
The process of entering safe mode, sometimes referred to as safing,[3] involves a number of immediate physical actions taken to prevent damage or complete loss. Power is removed from non-essential subsystems. Regaining attitude control, if lost, is the highest priority because it is necessary to maintain thermal balance and proper illumination of the solar panels.[1] A tumbling or cartwheeling spacecraft can quickly roast, freeze or exhaust its battery power and be lost forever.[4]
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