The successful launch of the Aquarius/Satélite de Aplicaciones Científicas (SAC)-D satellite on June 10 marks the beginning of an international mission to study ocean circulation and Earth’s water cycle via ocean salinity. The mission is a joint project between NASA and Argentina’s space agency, Comisión Nacional de Actividades Espaciales. Aquarius/SAC-D microwave radiometers will measure minute differences in thermal radiation from the ocean’s surface, which correlate with salinity. Ocean surface salinity affects the density of the water, and salinity differences help drive ocean circulation. In 2009, the European Space Agency launched the Soil Moisture & Ocean Salinity (SMOS) satellite. Salinity measurements from SMOS and Aquarius/SAC-D will augment data from other Earth-observing satellites, providing insights into how fresh and salty water affect global systems such as the atmosphere and sea ice.