Future human spacecraft will venture far beyond the relatively benign environment of low Earth orbit. They will transit through the deep space, but they may encounter warm transient environments such as low lunar orbit. Some spacecraft elements may be launched untended and would operate at relatively low power levels as they transit to their final destination. The combination of extreme environments and high turndown capability will be a major challenge for spacecraft thermal control systems. Thermal switches are among the thermal control devices that are required and can dissipate a wide range of heat loads in widely varying environments while using fewer of the limited spacecraft mass, volume and power resources. This SBIR project proposed by ACT will develop a low mass and high conduction ratio two-phase thermal switch as a thermal control system component for human spacecraft. The proposed device could work in thermal switch regime or in variable conductance regime while the set point can be changed remotely. A trade study will be conducted to define the three features of the low mass two-phase thermal switch and demonstrate the feasability of the concept. A preliminary full scale prototype will be designed, fabricated and tested at the end of the Phase I program.