Aqua, Latin for water, is a NASA Earth Science satellite mission named for the large amount of information that the mission is collecting about the Earth's water cycle, including evaporation from the oceans, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, land ice, and snow cover on the land and ice. Additional variables also being measured by Aqua include radiative energy fluxes, aerosols, vegetation cover on the land, phytoplankton and dissolved organic matter in the oceans, and air, land, and water temperatures.

The Aqua mission is a part of the NASA-centered international Earth Observing System (EOS). Aqua was formerly named EOS PM, signifying its afternoon equatorial crossing time. A timeline of Aqua on-orbit progress through the initial 120 day check-out period can be found here.

Aqua was launched on May 4, 2002, and has six Earth-observing instruments on board, collecting a variety of global data sets. Aqua was originally developed for a six-year design life but has now far exceeded that original goal. It continues transmitting high-quality data from four of its six instruments, AIRS, AMSU, CERES, and MODIS, and reduced quality data from a fifth instrument, AMSR-E. The sixth Aqua instrument, HSB, collected approximately nine months of high quality data but failed in February 2003.

Aqua was the first member launched of a group of satellites termed the Afternoon Constellation, or sometimes the A-Train. The second member to be launched was Aura, in July 2004, the third member was PARASOL, in December 2004, and the fourth and fifth members are CloudSat and CALIPSO, in May 2006, and the sixth member was GCOM-W1 in May 2012. In December 2013, PARASOL completed its mission and exited the A-Train. The latest addition to the A-Train was OCO-2, launched in July 2014. Now complete, the A-Train is led by OCO-2, followed by GCOM-W1, then Aqua, CALIPSO, CloudSat and, in the rear, Aura.