Sentence:  The spacecraft requirement stems from the need for spacecraft to conserve heat, and thus battery power, when not exposed to sunlight ("<WSD>darkside</WSD>"), to reflect heat when facing the Sun, and, on occasion, to emit excess heat on the <WSD>darkside</WSD>. 
Meaning of the word marked with <WSD> tags:  The side that is in darkness, unlit, or having lesser illumination