Large current uncertainties in the characteristics of aerosols in the Earth’s atmosphere preclude meaningful climate model evaluation. The FAST mission will contribute to the characterization of aerosols and their relation to climate change through a synoptic evaluation of local, regional and global aerosol data and altitude profiles of the cryosphere. The Dutch-Chinese mission will implement this objective through a unique combination of payloads onboard of two cooperating micro-satellites flying in formation in a Low-Earth Orbit. The FAST payload suite is unique in that it allows an unprecedented characterization of aerosols through a spectro-polarimeter instrument, which, when combined with radar and laser altimeter data, contributes to the characterization of the indirect climate forcing due to aerosols. Finally, flying instruments on two satellites in formation and, in a later mission phase, in a train configuration, allows an exceptional flexibility to study the temporal and spatial distribution and evolution of aerosols in the Earth’s atmosphere.