Abstract: A heat dissipating instrument package of a spacecraft, located in a canister having walls in heat transfer relationship with the package, is maintained at a substantially constant temperature. Fixed conductance heat pipes on the canister walls are connected to variable conductance heat pipes, mounted on a radiator structure separated from the canister walls by a thermal blanket. The effective radiating area of the radiator structure is controlled by the variable conductance heat pipes in response to a comparison of a sensed temperature of the instrument package or the canister wall with a seat point value. The comparison controls a heater in a gas reservoir containing a non-condensable gas of the variable conductance heat pipe. To enable the set point to be varied over a relatively wide range, such a 0.degree.-30.degree. C.