1. A completely passive, substantially isothermal heat transfer system comprising: a heat receiving surface; a heat rejecting surface; an enclosure connecting said surfaces; means for insulating said enclosure and said heat receiving surface from ambient while exposing said heat rejecting surface thereto; a working fluid including a mass of non-condensible gas carried within said enclosure having liquid and gaseous phases in equilibrium, said fluid tending to vaporize when in contact with said heat receiving surface and to condense when in contact with said heat rejecting surface whereby heat is transported from said heat receiving surface to said heat rejecting surface by said working fluid; means for varying the mass rate of flow of one phase of said working fluid between surfaces as a function of threshold temperature of said system, capillary means for returning said condensed working fluid from the heat rejecting surface to the heat receiving surface, and a radioisotopic fuel capsule thermally coupled to said heat receiving surface and means to pass helium into the enclosure so that said mass of noncondensible gas comprises helium produced in the alpha decay of said radioisotopic fuel.