Aqua (EOS PM-1) is a NASA scientific research satellite in orbit around the Earth, studying the precipitation, evaporation, and cycling of water.

The satellite was launched from Vandenberg Air Force Base on May 4, 2002, aboard a Delta II rocket.

Aqua operated in a Sun-synchronous orbit as the third in the satellite formation called the "A Train" with several other satellites (OCO-2, the Japanese GCOM W1, PARASOL, CALIPSO, CloudSat, and Aura) for most of its first 20 years; but in January 2022 Aqua left the A-Train (as Cloud Sat, CALIPSO and PARASOL had already done) when, due to its fuel limitations, it transitioned to a free-drift mode, wherein its equatorial crossing time is slowly drifting to later times, from its tightly controlled orbit.

It has demonstrated a very high level of precision in making the primary long-term measurements of the mission.

Thousands of scientists and operational users from around the world have made use of the Aqua data to address NASA's 6 interdisciplinary Earth science focus areas: Atmospheric Composition, Weather, Carbon Cycle and Ecosystems, Water and Energy Cycle, Climate Variability and Change, and Earth Surface and Interior.