OCO (Orbiting Carbon Observatory) was a satellite that should have joined the A-Train in 2009. It was lost on launch due to a technical failure. It was to have collected data on CO2 levels in the atmosphere and helped scientists to asses the impact of human activity on the climate, as well as the effects of global warming.
The OCO mission OCO was part of the NASA programme Earth System Science Pathfinder (ESSP) dedicated to obtaining knowledge of the Earth's atmosphere, oceans and surface. It was to have helped the scientific community forecast future climate changes.