Abstract: A passive cooling device especially suitable for application where unattended high performance is required for long periods, such as with space satellite sensors, optics and electronic systems, contains multiple stages of heat radiators insulated from each other and from the supporting structure, with heat pipes thermally attached to some or all of the stages of the radiators to transport heat from the heat producing sources. The multi-stage radiator system utilizes intermediate radiator stages to intercept heat loads conducted through the insulation and supports in order to reduce such conductive loads on the outermost stage and to permit that stage to reject substantial heat loads at extremely low temperatures. These intermediate stages can also provide efficient thermal rejection at the different temperature levels of various elements to be cooled.