The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat, launched November 11, 2016, is a pathfinder for a constellation to measure the Earth’s energy imbalance, which is the single most important quantity for predicting the course of climate change over the next century. RAVAN demonstrates small, accurate radiometers that measure top-of-the-atmosphere Earth-leaving fluxes of total and solar-reflected energy. The radiometers rely on two key technologies. The first is the use of vertically aligned carbon nanotubes (VACNTs) as radiometer absorbers. VACNT forests are some of the blackest materials known and have an extremely flat spectral response over a wide wavelength range. The second key technology is gallium fixed-point black body calibration sources that serve as stable and repeatable references to track the long-term degradation of the sensors. Absolute calibration is maintained by regular solar and deep space views. The RAVAN payload flies on a 3U CubeSat that combines stellar attitude determination, sub-degree pointing, and UHF communication. We present the scientific motivation for the NASA-funded mission, key technologies tested in space, payload design, the 3U CubeSat bus, mission operations, instrument calibration, and the first results on-orbit
