Direct calorimetric measurements of a solid state passive switchable radiator for spacecraft thermal control have been performed in a simulated space environment. Dynamic emissivity control is provided by the thermochromic phase change in a multilayer VO thin film based resonant absorber. The measured radiated power difference between 300 K and 373 K was 480 W/m corresponding to a 7× difference in radiative cooling power. We present theoretical and experimental radiator values for both normal and hemispherical as well the optical properties of VO as determined via infrared spectroscopic ellipsometry.