Aqua

Aqua (also designated EOS PM-1) is a NASA Earth Science satellite mission launched on 4 May 2002 from Vandenberg Air Force Base aboard a Delta II 7920-10L rocket [^aqua-home]. Managed by NASA Goddard Space Flight Center (GSFC), it was designed as the afternoon complement to the Terra (EOS AM-1) mission, carrying six science instruments for observations of Earth's water cycle, energy balance, atmosphere, and land surface [^aqua-home].

Aqua operates in a 705 km sun-synchronous near-polar orbit at 98.2 degree inclination, originally crossing the equator on the ascending node at 1:30 PM local time [^aqua-home]. This afternoon timing, reflected in its original designation EOS PM-1, made Aqua the anchor of the A-Train (Afternoon Constellation), a formation of satellites coordinated to observe the same ground swath within minutes of each other [^atrain-page].

In December 2021, after exhausting its station-keeping propellant, Aqua completed its final drag makeup maneuvers and entered free-drift mode [^aqua-home]. The satellite began descending below the A-Train orbit and drifting to progressively later equatorial crossing times -- approximately 50 minutes later than nominal by January 2025 -- exiting the A-Train formation by early 2022 [^atrain-page]. As of May 2026, Aqua remains operational, having completed more than 126,000 orbits and contributed to over 30,000 peer-reviewed scientific publications [^aqua-home]. Spacecraft passivation is scheduled for Fall 2026 [^aqua-home].

Of its six instruments, four remain operational: MODIS, AIRS, AMSU-A, and CERES [^aqua-home]. MODIS (Moderate Resolution Imaging Spectroradiometer), carrying 36 spectral bands from visible to thermal infrared at 250-1000 m GSD and a 2,330 km swath, provides daily global coverage underpinning vegetation indices, burned-area detection, and active fire products [^modis-specs]. AIRS (Atmospheric Infrared Sounder), with 2,378 infrared channels from 3.74-15.4 um, provides atmospheric temperature and humidity profiling [^eoportal-aqua]. CERES (FM4+FM5) measures Earth's broadband radiation balance in three spectral channels at 20 km nadir resolution [^ceres-instr].

Two instruments are permanently non-operational: AMSR-E (Advanced Microwave Scanning Radiometer for EOS), a 12-channel passive microwave radiometer contributed by JAXA, experienced a major anomaly in October 2011 ending science data collection, and was powered off in March 2016 [^aqua-home]. HSB (Humidity Sounder for Brazil), a passive microwave humidity sounder contributed by INPE, failed in February 2003 after nine months of operation [^eoportal-aqua].

Users of Aqua MODIS time-series should account for overpass-time drift since December 2021. Long-term continuity assessments require drift-correction methodology equivalent to that applied to Terra MODIS (drifting since February 2020).