Triton Systems is currently developing an unpowered, switchable emissivity film for application to spacecraft surfaces. We call our approach Phase Change Thermochromic Radiator (PCTR); it self-switches from low to high emissivity above a designed temperature setpoint, causing a surface in space to radiate heat only when it becomes too warm and conserving heat otherwise. Key to the operation of PCTR is the phase change material vanadium dioxide, integrated into a multilayer thin film structure to produce a device which is reflective over the 3-35 um IR band below a transition temperature Tc but strongly absorptive above Tc. PCTR has advantages over competing approaches to dynamic emissivity such as electrochromics, in that it requires no electrical drive power, is relatively simple to fabricate, and contains only stable, rugged, well understood materials. The proposed program will significantly upgrade the performance of PCTR to approach the system requirements of planetary exploration probes and landers, manned and unmanned. Performance targets include high emissivity of 0.9, low emissivity of 0.15, turndown ratio of at least 6:1 and potentially 10:1, and solar alpha 0.1 or less. These advances will be achieved by a new design approach using nanostructural enhancement. Fabrication methods will be developed which can be scaled up to produce 10's of square meters of film. Beyond thermal performance tests (turndown, etc.), key qualification tests include thermal cycling, vibration, peel tests, surface charge and life degradation.