An engine in outer space operates on the Carnot cycle. The only way in which heat is rejected by the engine to the surroundings is by radiation which is proportional to the product of the fourth power of the absolute temperature of the radiating surface and its area. For a given power output of the engine, hot reservoir temperature T1 and radiator temperature T2 determine the ratio (T1 / T2) for which the area of the radiating surface as a minimum.