Long-term Lunar and Martian systems present challenges to thermal control systems, including changes in thermal load, and large changes in the thermal environment between Lunar (or Martian) day and night. The Lunar thermal environment typically includes long periods in extremely cold thermal environments. A variable conductance heat pipe (VCHP) radiator will be developed that passively accommodates the changing thermal load and environment. In a VCHP, a non-condensable gas is added that blocks a portion of the condenser. The gas charge blocks more of the condenser as the heat pipe evaporator temperature changes. This allows the heat pipe evaporators (and any attached heat exchanger) to remain at an almost constant temperature. In addition to passively controlling the thermal load, the gas allows the fluid in the heat pipe to freeze in a controlled fashion as the heat pipe is shut down, avoiding damage. In addition, the gas in the VCHP will help with start-up from a frozen condition.