The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat mission demonstrates an affordable, accurate radiometer that directly measures Earth-leaving fluxes of total and solar-reflected radiation. The radiometer exploits two key technologies: vertically aligned carbon nanotubes used as the radiometer absorber and a gallium fixed-point blackbody as an internal calibration source. RAVAN will fly on a 3U CubeSat, with a launch likely at the end of 2016. Our ability to understand and predict future climate is limited by our ability to track energy within the Earth system. RAVAN will enable the development of an Earth radiation budget constellation that could provide the global, diurnal measurements needed to significantly advance our understanding of ongoing and future climate change.