An Intertwined History: The Earth Observer and EOS<\/p>\n
The Earth Observer<\/a>, a newsletter issued for nearly 37 years, will release its last online content at the close of 2025. This newsletter evolved in parallel with NASA\u2019s Earth Observing System (EOS). It is almost impossible to speak of this newsletter without mentioning EOS. As\u00a0The Earth Observer\u00a0prepares its final publication, NASA also plans to shutter its three EOS flagship satellites (discussed below) possibly as early as the end of 2026.<\/p>\n While EOS was \u201cmuch more than its satellites,\u201d one cannot deny that the satellite missions and their iconic images provide an entry point to the overarching work conducted by the EOS science teams for almost three decades. These efforts spanned crucial complementary ground- and aircraft-based observations along with focused field campaigns to coordinate observations across multiple levels of Earth system time and spatial scales. The teams worked (and continue to work) closely with the NASA Earth Science Division\u00a0Earth Observing System Data and Information System<\/a>\u00a0(EOSDIS) and related Science Investigator Processing System (SIPS) facilities, as well as developed and enhanced the algorithms that support the satellite products. Readers who wish to learn more about these topics should consult The Earth Observer\u2019s archives page<\/a>, which contains much of the history of this work.<\/p>\n During this point of inflection, The Earth Observer\u2019s publication team felt it important to pause and reflect on the significance of the work detailed in the newsletter throughout this brief slip of time. The result is the article that follows.<\/p>\n A Flagship of an Idea: Almost Four Decades of Science<\/p>\n As described in the article,\u00a0A Condensed History of the Earth Observing System (EOS)<\/a>\u00a0[June 1989,\u00a01:3. 2\u20133], what would become known as EOS had its foundation in the recommendations of an\u00a0ad hoc NASA study group that convened in 1981 to \u201cdetermine what could and should be done to study integrated\u00a0Earth science measurement needs.\u201d Initially, the study group envisioned several large platforms in space, each with numerous instruments that would be serviced by the Space Shuttle, similar to servicing of the Hubble Telescope on several occasions. Known as\u00a0System Z<\/a>\u00a0[Sept.\u2013Oct. 2008,\u00a020:5, 4\u20137], this early vision \u201claid the groundwork for a Mission to Planet Earth\u201d but was reimagined after the tragic loss of the Space Shuttle\u00a0Challenger\u00a0in January 1986. An article written at the end of the Shuttle program included a sidebar that detailed the impracticality of launching shuttle missions into polar orbit to service EOS satellites, see\u00a0Polar Shuttle Launches: The Path Almost Taken<\/a>, [Sept.\u2013Oct. 2011,\u00a023:5, 6\u20137]. Eventually, the large space platform concept morphed into several mid-size flagship satellite missions, known today as Terra, Aqua, and Aura. Smaller satellite missions would supplement and enhance the data gathered by the \u201cbig three\u201d satellites \u2013 see\u00a0Figure 1.<\/p>\n Technological advances further enhanced and refined this vision, allowing satellites to fly in close formation to capture near-simultaneous measurements in much the same way they would if they were on a single platform. The Afternoon Constellation<\/a>, or A-Train, is a shining example of this international effort and is described in more detail below.<\/p>\n NASA released the first EOS Announcement of Opportunity in 1988, and a panel selected the winning proposals. An EOS Project Science Office was established to manage the projects. During this time of rapid development, NASA leadership was keenly aware of the need to keep the international EOS community abreast of the latest information. Enter The Earth Observer newsletter. First published in March 1989, the newsletter was the natural conduit to bridge this communication gap. To set the stage of how things have changed, an early article, titled Direct Transmissions of EOS Data to Worldwide Users<\/a> [July\u2013Aug. 1990, 2:6, 2\u20134], introduced the readership to the World Wide Web, which promoted \u201ca \u2018place\u2019 where scientists communicate with each other and with the data they have collected with the help of their professional colleagues from the engineering and operations disciplines.\u201d<\/p>\n In the more than 1000 printed pages published in the past three decades. The Earth Observer has chronicled the story of EOS and NASA\u2019s broader Earth Science program. The publication has captured \u2013 often in meticulous detail \u2013 the intensive work behind the scenes that has gone into the development of the technologies, algorithms, and data centers that gather data from Earth observing satellites, suborbital observations, and other experiments to inform end users who use the data to address societal issues.\u00a0<\/p>\n In the years before the first EOS missions launched, the newsletter reported in earnest on Investigator Working Group (IWG) meetings, Payload Panel Reviews (reviewing the instruments planned for the EOS platforms), and Mission and Instrument Science Team Meetings. As EOS matured, the newsletter began reporting on the development and implementation of specific science missions, launches, milestones, and research generated from the data collected. The editorial staff began publishing more feature articles to appear along with the meeting and workshop reports. The newsletter shared news stories developed by NASA\u2019s Earth Science News Team and other bimonthly content (e.g., Education Update, Science in the News). “The Editor\u2019s Corner” column in the newsletter gave the EOS Senior Project Scientist a platform to offer commentary on current events in NASA Earth Science as well as on the content of the current issue of the newsletter. While not formally named for the first few issues, an editorial article has been a cornerstone of the publication since the beginning.<\/p>\n The Earth Observer has produced several articles reflecting on its interwoven history with EOS, such as The Earth Observer: Twenty-Five Years Telling NASA\u2019s Earth Science Story<\/a> [March\u2013April 2014, 26:2, 4\u201312] and A Thirtieth Anniversary Reflection from the Executive Editor<\/a> {March\u2013April 2019, 31:2, 4\u20136]. These stories expand upon the topics covered in the brief review presented in this article.<\/p>\n Satellite Missions: the Backbone of EOS Science<\/p>\n EOS was originally organized around 24 critical science measurements deemed integral to understand planetary processes and assess variability, long-term trends, and climate change. These science measurements serve as a roadmap for organizing EOS data products and mission objectives. The 24 measurements coalesced into five broad categories that reflect Earth science disciplines:<\/p>\n Atmosphere:\u00a0aerosol properties,\u00a0cloud properties\u00a0(e.g.,\u00a0fraction and opacity),\u00a0atmospheric temperature and pressure profiles,\u00a0water vapor,\u00a0ozone (O3), trace gases [e.g.,\u00a0carbon dioxide (CO2),\u00a0sulfur dioxide,\u00a0and formaldehyde], and\u00a0total solar irradiance;<\/p>\n Ocean:\u00a0ocean color\u00a0(chlorophyll),\u00a0sea surface temperature,\u00a0sea ice cover and motion, ocean surface topography and sea level, and sea surface salinity;<\/p>\n Land\/Cryosphere:\u00a0land surface temperature,\u00a0soil moisture,\u00a0snow and ice cover (extent and elevation),\u00a0land cover and change\u00a0(e.g.,\u00a0forest cover), and\u00a0topography;<\/p>\n Radiation\/Energy Balance: radiant energy balance\u00a0(incoming and outgoing radiation), and\u00a0precipitation (e.g.,\u00a0rainfall,\u00a0snow); and<\/p>\n Solid Earth: static gravity field and synthetic aperture radar observations.<\/p>\n The Grand Vision of EOS: Three Flagships Leading the Earth Observing Fleet<\/p>\n In the late 1980s and early 1990s, a team of scientists envisaged the concept for two missions \u2013 EOS-AM1 and EOS-PM1. The synergy of this system was the ability to make observations in the morning (10:30 AM mean local time, or MLT), a time when cloud cover over the tropical equatorial and other land regions would be at a minimum, and afternoon (1:30 PM MLT), a time when continental convection would peak. The plan was to have two instruments \u2013 the\u00a0Moderate Resolution Imaging Spectroradiometer<\/a>\u00a0(MODIS) and\u00a0Clouds and Earth\u2019s Radiant Energy System<\/a>\u00a0(CERES) \u2013 overlap on the two platforms along with other instruments unique to each mission.<\/p>\n In parallel, the teams envisioned EOS-CHEM1, a satellite platform identical to EOS-PM1 but carrying a payload focused on atmospheric chemistry. Like EOS-PM1, EOS-CHEM1 would be placed in an afternoon orbit but lag slightly in its equatorial crossing time (1:45 PM MLT) to optimize its position for atmospheric chemistry observations.\u00a0<\/p>\n Each mission was slated to be the first in a series that would launch at five-year intervals to ensure continuity of critical Earth science measurements. Budgetary realities and technical advances eventually rendered plans for the second and third series of each satellite obsolete; however, all three flagship missions endured far beyond their planned six-year lifetime and have outlasted the originally proposed 15-year timeframe for each series.<\/p>\n Terra<\/p>\n Terra<\/a>, originally named EOS-AM1, launched in December 1999 \u2013 see\u00a0Figure 2. Terra carries five instruments \u2013 MODIS, CERES (two copies),\u00a0Multiangle Imaging Spectroradiometer<\/a>\u00a0(MISR),\u00a0Advanced Spaceborne Thermal Emission and Reflection Radiometer<\/a>\u00a0(ASTER), and\u00a0Measurements of Pollution in the Troposphere<\/a>\u00a0(MOPITT) \u2013 and was designed to capture information about Earth\u2019s atmosphere, carbon cycle and ecosystems, climate variability, water and energy cycle, weather, and the planet\u2019s surface\u00a0and interior. The Earth Observer\u00a0captured early Terra data in the article,\u00a0Terra Spacecraft Open For Business<\/a>\u00a0[March\u2013April 2000,\u00a012:2, 24].<\/p>\n After over 26 years in service, Terra remains in orbit and continues to gather data; as of this writing all instruments accept MOPITT remain active. However, since 2020 the spacecraft has been allowed to drift from its carefully maintained 10:30 AM MLT equator crossing time toward earlier MLT crossings. This was done to conserve enough fuel to control Terra’s eventual atmospheric reentry. The Terra team also conducted orbital lowering maneuver on the spacecraft in 2022. A more complete history of Terra is available in the online article,\u00a0Terra: The End of An Era<\/a>, published on December 29, 2025.<\/p>\n Aqua<\/p>\n Aqua, originally named EOS-PM1, launched in May 2002 \u2013 see Figure 3. An article in The Earth Observer at the time of launch described the mission, Aqua is Launched!<\/a> [March\u2013April 2002, 14:2, 2]. The second EOS flagship carried six different instruments into orbit \u2013 Atmospheric Infrared Sounder<\/a> (AIRS), Advanced Microwave Sounding Unit\u2013A<\/a> (AMSU-A1 and -A2), CERES (two copies), MODIS (both of which also fly on Terra), the Advanced Microwave Scanning Radiometer for EOS<\/a> (AMSR\u2013E), and Humidity Sounder for Brazil<\/a> (HSB). Aqua\u2019s mission focused on collecting data on global precipitation, evaporation, and the cycling of water. Aqua paired its data with Terra, offering the scientific community additional insights into the daily cycles for important scientific parameters to understand the global water cycle.<\/p>\n The Earth Observer\u00a0article,\u00a0Aqua: 10 Years After Launch<\/a>\u00a0[Nov.\u2013Dec. 2012,\u00a024:6, 4\u201317] provides an overview of the mission\u2019s accomplishments during its first decade in orbit. Due to fuel limitations, Aqua completed the last of its drag makeup maneuvers in December 2021. Like Terra, the satellite is now in a free-drift mode, slowly descending below the A-Train orbit and crossing the equator later and at lower altitudes. A more recent newsletter article,\u00a0Aqua Turns 20<\/a>\u00a0[May\u2013June 2022,\u00a034:3, 5\u201312] reflects on Aqua\u2019s accomplishments and legacy after two decades in orbit. As of this writing MODIS, CERES, AMSU, and CERES remain active. <\/p>\n Aura<\/p>\n Originally named EOS-CHEM1, Aura<\/a> was the third and final flagship mission, and was launched in July 2004 \u2013 see Figure 4. The Earth Observer detailed the first post-launch science team meeting, Aura Science Team Meeting<\/a> [March\u2013April 2004, 17:2, 8\u201311]. Aura followed a Sun-synchronous, near-polar orbit, crossing the equator 15 minutes after Aqua. Similar to Aqua, Aura completed its final inclination adjustment maneuver in April 2023 to save its remaining fuel to allow for controlled reentry. As a consequence, the satellite has drifted out of the A-Train orbit, slowly continuing to move to a later equatorial crossing time and lower orbit altitude.\u00a0<\/p>\n Aura\u2019s payload included four instruments: the Microwave Limb Sounder<\/a> (MLS), High Resolution Dynamics Limb Sounder<\/a> (HIRDLS), Tropospheric Emission Spectrometer<\/a> (TES), and Ozone Monitoring Instrument<\/a> (OMI). These instruments gather information on trace gases and aerosols in the atmosphere. The key mission objectives aimed to monitor recovery of the stratospheric O3 hole, evaluate air quality, and monitor the role of the atmosphere in climate change. The article, Aura Celebrates Ten Years in Orbit<\/a> [Nov.\u2013Dec. 2014, 26:6, 4\u201316] detailed Aura\u2019s first decade of accomplishments. The online article, Aura at 20 Years<\/a>, published Sept. 16, 2024, reported on Aura\u2019s status and achievements as it began its third decade of continuous operations. As of this writing MLS and OMI remain active.<\/p>\n Building and Dismantling the \u201cA-Train\u201d<\/p>\n Between 2002 and 2014, a series of satellites joined the A-Train constellation \u2013 see Figure 5. This international effort includes the two EOS flagship satellites with afternoon equatorial crossing times (Aqua and Aura) as well as the Orbiting Carbon Observatory\u20132<\/a> (OCO-2), CloudSat<\/a>, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations<\/a> (CALIPSO). In addition, Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with observations from a Lidar<\/a> (PARASOL) and Global Change Observation Missions with a focus on the water cycle<\/a> (GCOM-W) are two international missions that became part of the A-Train constellation.<\/p>\n In the past decade, many of the satellites in the A-Train have either retired or have been allowed to drift out of the constellation. As of this writing, only two satellites \u2013 OCO-2 and GCOM-W1 \u2013 remain in their positions in the A-Train gathering data.<\/p>\n Three A-Train symposiums have been organized to bring the Earth science community together to discuss the achievements and future synergy of these missions. The outcome from each of these meetings were reported in\u00a0The Earth Observer. The most recent of these was: The Third A-Train Symposium: Summary and Perspectives on a Decade of Constellation-Based Earth Observations<\/a> [July\u2013Aug. 2017, 29:4, 4\u201318]. <\/p>\n Science from the EOS Fleet<\/p>\n The next several sections provide a highlight of science from key missions outside of Terra, Aqua, and Aura. The content has been organized in terms of measurements \u2013 with an overarching focus on water (oceans and fresh water), atmosphere, and land. This summary is far from exhaustive. A record of much of the amazing science conducted during these missions is detailed in the archives of\u00a0The Earth Observer.\u00a0<\/p>\n Interpreting an Ocean of Data<\/p>\n When viewed from space, Earth has been described as a \u201cblue marble.\u201d The planet\u2019s abundance of liquid water is found in the oceans, and while not potable, the oceans play a critical role in regulating Earth\u2019s climate. Satellites provide an unparalleled way to study the global ocean. With each new mission, the process of data collection has been refined and improved. The scientific community can now measure ocean color as a proxy for surface productivity as well as measure subtle changes in surface ocean salinity. These data have improved weather and climate models to increase the accuracy of storm projection and help the scientific community better understand the movement of energy around the planet.<\/p>\n Aqua was the flagship mission dedicated to studying water on Earth, but other missions have contributed and expanded on this data record. For example, Japan\u2019s GCOM-W1 mission, also known as SHIZUKU (Japanese for droplet), continues to gather information on precipitation, water vapor, wind velocity above the ocean, sea water temperature, water levels on land, and snow depths. These data support weather models to improve forecasts to monitor tropical cyclones. The subsections that follow provide examples of how data from these satellites support different science objectives, as well as examples of the science deciphered by both flagship and ancillary platforms within the A-Train. All of these missions and science have been covered in The Earth Observer over the past several decades.<\/p>\n Discerning the Ocean\u2019s True Colors<\/p>\n Ocean color data are crucial for studying the primary productivity and biogeochemistry of the oceans. The Coastal Zone Color Scanner<\/a>\u00a0(CZCS), launched on the Nimbus 7 satellite in 1978 and ceasing operations in 1986 \u2013 gave the earliest perspective of the oceans from space. SeaWiFS<\/a>, which served as a follow-on to CZCS, was launched on the privately owned Seastar spacecraft on Aug. 1, 1997 to produce ocean color data and offered a synoptic look at the global biosphere. This mission was a\u00a0data-buy, where NASA purchased the data from Orbital Imaging Corporation. An article in\u00a0The\u00a0Earth Observer, titled\u00a0Sea-viewing Wide Field-of-view Sensor<\/a>\u00a0[March\u2013April 1998,\u00a010:2, 20\u201322] detailed how the satellite gathered chlorophyll-a data that was calibrated to field measurements from a Marine Optical Buoy. The research community have used this information to understand primary productivity in the surface ocean and global biogeochemistry. This data offered an early assessment of the role of the ocean in the global carbon cycle. It also produced one of the first global perspectives of the impact of El Ni\u00f1o and La Nina events around the world. Coastal and fishery managers have used this data to improve the health of these important ecosystems. Launched for a five-year mission, SeaWiFs gathered data until December 2010.<\/p>\n More recently, NASA launched the\u00a0Plankton, Aerosol, Cloud ocean Ecosystem<\/a>\u00a0(PACE) satellite in February 2024 to gather data on ocean and terrestrial ecosystem productivity \u2013 see\u00a0Figure 6. While other missions studied ocean color in the interim between SeaWiFS and PACE (e.g., MODIS on Terra and Aqua), PACE offers an exponential leap forward with its three-instrument payload that includes: the\u00a0Ocean Color Instrument<\/a>\u00a0(OCI),\u00a0Hyper-Angular Rainbow Polarimeter\u20132<\/a> (HARP2), and\u00a0Spectropolarimeter for Planetary Exploration<\/a>\u00a0(SPEXone). The PACE mission aims to clarify how the ocean and atmosphere exchange CO2, a key factor in understanding the evolution of Earth\u2019s climate system. The satellite also examines the role of aerosols in providing micronutrients that fuel phytoplankton growth in the surface ocean. The data gathered extends the aerosol and ocean biological, ecological, and biogeochemical records that were initiated by other satellites. The Dec. 29, 2025 article,\u00a0Keeping Up with PACE: Summary of the 2025 PAC3 Meeting<\/a>, reports on three recent meetings related to the mission.<\/p>\n Mapping the Ocean Surface to Reveal the Rising Seas<\/p>\n The Ocean Surface Topography (TOPEX)\/Poseidon mission, launched on Aug. 10, 1992, was the first in a series of missions that have measured ocean surface topography<\/a>, or the variations in sea surface height. This record now extends more than 30 years. TOPEX\/Poseidon spent more than 13 years in orbit. The data gathered helped to improve the scientific community\u2019s understanding of ocean circulation and its impact on global climate \u2013 including sea level rise. TOPEX\/Poseidon produced the first global views of seasonal current changes, which allowed scientists to forecast and better understand El Ni\u00f1o events. These early efforts to distribute data were captured in The Earth Observer article, Jet Propulsion Laboratory DAAC Begins TOPEX Data Distribution<\/a> [March\u2013April 1993, 6:2, 24].<\/p>\n Jason followed TOPEX\/Poseidon to continue the measure of sea level as well as wind speed and wave height for more than 95% of Earth\u2019s ice-free ocean \u2013 see\u00a0Figure 7. Jason consists of a series of satellites, with Jason-1, launched in 2001, remaining in orbit for 11 years. It was followed by Jason-2, also called the Ocean Surface Topography Mission (OSTM), which was launched in 2008. Jason-2 gathered data for 11 years. Jason-3 launched in January 2016 and remains in orbit, continuing the sea level dataset.\u00a0The Earth Observer\u00a0has reported on meetings of the Ocean Surface Topography Science Team over the years. The online article,\u00a0Summary of the 2023 Ocean Surface Topography Team Meeting<\/a>, was published May 31, 2024 and includes the most recent updates available.<\/p>\n The international partnership between the United States [NASA and the National Oceanic and Atmospheric Administration (NOAA)], the European Space Agency (ESA), and the French Space Agency [Centre National d’\u00c9tudes Spatiales (CNES)] collaborate to create the ESA\u2019s Copernicus Sentinel\u20136 missions. The Sentinel-6B<\/a>, launched Nov. 16, 2025, will follow the path of the\u00a0Sentinel-6 Michael Freilich<\/a>\u00a0(originally called Sentinel\u20136A) satellite, which has been in orbit for five years \u2013 see\u00a0Figure 8. These two Sentinel 6 missions continue the global measurements of sea level, wind speed, wave height, and atmospheric temperature. The data will be used in marine weather forecasts as well as to improve commercial and naval navigation, search and rescue missions, and tracking garbage and pollutants in the ocean. To learn more about Sentinel-6B, see the online article, Sentinel-6B Extends Global Ocean Height Record<\/a>, published Dec. 22, 2025.<\/p>\n While the\u00a0Surface Water and Ocean Topography<\/a>\u00a0(SWOT) mission is fully described in the next section \u2013 with emphasis placed on its novel surface water observation capabilities \u2013 it should be noted that SWOT is also an ocean topography mission that obtains data similar to TOPEX\/Poseidon, Jason, and Sentinel-6 missions. These data will contribute to the long-term time series of the sea surface height record.<\/p>\n Sampling the Salty Seas<\/p>\n Launched June 2011, Aquarius<\/a> was an international collaboration between NASA and Argentina\u2019s Comisi\u00f3n Nacional de Actividades Espaciales (CONAE). The cooperative effort was detailed in the article, Aquarius: A Brief (Recent) History of an International Effort<\/a> [July\u2013Aug. 2010, 22:4, 4\u20135]. The satellite carried a microwave radiometer that was sensitive enough to measure salinity to an accuracy of 0.2 practical salinity units (psu) on a monthly basis. It also carried a scatterometer to measure surface ocean roughness. Pairing data from the two instruments allowed the team to overcome the challenges of measuring salinity from space. This feat is detailed in the article, For Aquarius, Sampling Seas No ‘Grain of Salt’ Task<\/a> [July\u2013Aug. 2011, 23:4, 42\u201343]. The more accurate, global measurements of ocean salinity that Aquarius obtained have helped the research community better understand ocean circulation. The mission ended in 2015, after the satellite experienced a power failure.<\/p>\n Focusing on Freshwater<\/p>\n While most water on the planet is housed in the ocean, fresh water is a primary concern for life on the planet. Fresh water accounts for ~3% of the total amount water on the planet. Of that small amount, a significant portion is locked in ice on land and as sea ice. The remaining water flows on Earth’s surface and underground. Maintaining a supply of fresh water is critically important to our survival. The location, status, and purity of this precious resource continues to be an on-going focus for many of the missions.<\/p>\n Monitoring Rain and Snow<\/p>\n The joint NASA\/National Space Development Agency of Japan (NASDA \u2013 which is now known as the Japan Aerospace Exploration Agency, or JAXA)\u00a0Tropical Rainfall Measuring Mission<\/a>\u00a0(TRMM) carried\u00a0a\u00a0Microwave Imager, Visible Infrared Scanner, and Precipitation Radar to gather\u00a0tropical and subtropical rainfall\u00a0observations\u00a0(and\u00a0two related instruments) \u2013 see\u00a0Figure 9. These data filled a critical knowledge gap \u2013 to understand the interactions between the sea, air, and land. Over the years, these data were incorporated into numerous computer models to clarify the role of tropical rainfall on global circulation and formed the basis for experimental quasi-global merged satellite precipitation products.\u00a0The Earth Observer\u00a0detailed the early data collection in the article titled\u00a0TRMMing the Uncertainties: Preliminary Data from the Tropical Rainfall Measuring Mission<\/a>\u00a0[May\u2013June 1998,\u00a010:3, 48\u201350].\u00a0The mission was extended twice but eventually the satellite\u2019s maneuvering fuel was exhausted, resulting in a slow decline in the orbital altitude beginning in 2014, with reentry in 2015. Data from TRMM have improved understanding of storm structure of cloud systems, produced reliable global latent heating estimates to improve water transfer estimates within the atmosphere, and continue to be used in calibrating modern precipitation products for the TRMM era.\u00a0<\/p>\n To continue the efforts that began with TRMM \u2013 and extend coverage to most of the globe \u2013 NASA and JAXA launched the\u00a0Global Precipitation Measurement<\/a>\u00a0(GPM) mission in 2014. This satellite aims to advance our understanding of water and energy cycles, improve forecasting of extreme weather events, and extend current capabilities to use accurate and timely information of precipitation to directly benefit society.\u00a0The Earth Observer\u00a0detailed the accomplishments of this mission in the online article,\u00a0GPM Celebrates Ten Years of Observing Precipitation for Science and Society<\/a>, published Oct. 3, 2024.<\/p>\n Surveying Earth\u2019s Surface Water<\/p>\n Introduced briefly in the previous section, the SWOT mission is a joint venture between the United States and France. Launched in December 2022, SWOT is conducting the first global survey of Earth\u2019s surface water \u2013 see Photo. The mission was introduced to the EOS community in The Earth Observer article, Summary of the 2022 Ocean Surface Topography Science Team Meeting<\/a> [May\u2013June 2023, 35:3, 19\u201323]. SWOT carries the Ka-band Radar Interferometer<\/a> (KaRIN) \u2013 the first spaceborne, wide-swath, altimetry instrument capable of high-resolution measurements of sea surface height in the ocean and freshwater bodies. SWOT covers most of the world\u2019s ocean and freshwater bodies with repeated high-resolution elevation measurements. This data have been applied to monitor rivers across the Amazon basin, simulate land\/hydrology processes, and predict streamflow. A more comprehensive overview of SWOT applications is detailed in online article, Summary of the 10th SWOT Applications Workshop<\/a>, published Sept. 20, 2024.<\/p>\n Gracefully Tracking Water Movement<\/p>\n The twin GRACE satellites were launched on March 17, 2002. The mission,\u00a0a partnership between NASA and the\u00a0German <\/a>GeoForschungsZentrum<\/a> <\/a>(GFZ) Helmholtz Centre for Geosciences\u00a0was developed to measure Earth\u2019s shifting masses \u2013 most of which comes from water \u2013 and map the planet\u2019s gravitational field using a K-band microwave ranging system and accelerometers. Some early results of the satellites appeared in\u00a0The Editor\u2019s Corner<\/a>\u00a0column [Nov.\u2013Dec. 2002,\u00a014:6, 1\u20132]. GRACE enabled groundbreaking insights into Earth\u2019s evolving water cycle as the satellites tracked monthly mass variations in ice sheets and glaciers, near-surface and underground water storage, the amount of water in large lakes and rivers, as well as changes in sea level and ocean currents.\u00a0<\/p>\n GRACE\u2019s mission was extended with the\u00a0GRACE-Follow On<\/a>\u00a0(GRACE-FO) mission launched in 2018 \u2013 see\u00a0Figure 10. GRACE-FO continues comprehensive tracking water movement across the planet, including groundwater measurements that have important applications for everyday life. The most recent developments of the GRACE-FO science meeting was detailed in an online article,\u00a0Summary of the 2023 GRACE Follow-On Science Team Meeting<\/a>, published March 30, 2024 \u2013 and also published in the final print issue [Jan.\u2013Feb. 2024,\u00a035:7, 19\u201326]. The data gathered during the GRACE-FO mission details large-scale changes in Earth\u2019s groundwater reservoirs, Greenland and Antarctica\u2019s sensitivity to warming ocean waters, and even subtle shifts deep in Earth\u2019s interior that reveal how large earthquakes can develop.<\/p>\n In 2028, NASA will move into a third-generation of gravity observations with the launch of\u00a0GRACE-Continuity<\/a>, or GRACE-C,\u00a0which will further expand the foundational observations of global mass change and expand the societal and economic applications that have been created from these data.<\/p>\n Assessing the Atmosphere from Above<\/p>\n Earth has a unique atmospheric makeup that maintains a stable temperature allowing life to thrive. As far as we know, our atmosphere is unique in the universe.\u00a0Satellites provide an unparalleled perspective to study variability in the column of air extending from Earth\u2019s surface.\u00a0While Aura has a suite of instruments making a wide range of atmospheric chemistry measurements, other missions also measure the abundance and impact of atmospheric constituents that, while often invisible to the unaided eye, can have profound impacts on Earth\u2019s air quality and climate.\u00a0These data have also improved climate models and help the scientific community better understand how energy is emitted into space.<\/p>\n Tracking Tiny Particles with Big Impacts<\/p>\n France\u2019s\u00a0PARASOL mission was an original member of the international A-Train constellation. Launched in 2004.\u00a0PARASOL sought to capture the radiative and microphysical properties of clouds and tiny atmospheric aerosol particles using a unique multiangle imaging POLDER polarimeter. <\/p>\n NASA\u2019s Glory mission was intended for operation in the A-Train; it carried a multiangle polarimeter as its instrument. Unfortunately, the spacecraft failed to separate from the Taurus rocket due to a fairing separation failure during its launch in 2011. As a result, POLDER on PARASOL was the only atmospheric polarimeter to fly in space until two (SPEXone and HARP2) launched as part of NASA\u2019s PACE mission.\u00a0Researchers gathered information from POLDER and other A-Train instruments about how aerosols affect the formation of precipitations and clouds, the movement of water around the planet, and the reflection and absorption of radiative energy that impact overall planetary climate. PARASOL was deactivated in 2013 after nine years in service.<\/p>\n Cloud particles form when water vapor nucleates onto aerosols; changes in one can impact the other. After many years and conversations, it was decided to pair two NASA Earth System Science Pathfinder (ESSP) missions \u2013\u00a0CloudSat<\/a>\u00a0and CALIPSO \u2013 and fly them in coordination with each other and with other A-Train satellites. By combining the two datasets, it was possible to explore cloud and aerosol processes. This information helped the community drill into the larger climate questions. The two satellites were launched on the same Delta-II rocket from Vandenberg Air Force Base in California on April 28, 2006. CloudSat used a 94 GHz cloud profiling radar that is\u00a01000 times more sensitive than a typical weather radar,\u00a0capable of distinguishing between cloud particles and precipitation. CALIPSO contained a Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), Wide-Field Camera, and Imaging Infrared Radiometer to detect and distinguish between aerosol particles and cloud particles.\u00a0<\/p>\n The Earth Observer captured the early data collection of the two satellites in the article, CloudSat and CALIPSO: A Long Journey to Launch\u2026But What a Year It\u2019s Been!!<\/a> [May\u2013June 2007, 19:3, 7\u201312]. The later article, A Useful Pursuit of Shadows: CloudSat and CALIPSO Celebrate Ten Years of Observing Clouds and Aerosols<\/a> [July\u2013Aug. 2016,\u00a028:4, 4\u201312] provided a review of the accomplishments of the missions after 10 years in orbit. CALIPSO and CloudSat were both deactivated in 2023 after 17 years of service.<\/p>\n An Oracle of High-Altitude Wisdom<\/p>\n The\u00a0Stratospheric Aerosol and Gas Experiment<\/a>\u00a0(SAGE) has experienced several iterations, extending back nearly half a century. The initial SAGE mission launched on Feb. 18, 1979, aboard the Applications Explorer Mission-B (AEM-B) to measure vertical distribution of aerosols and important gases in the upper troposphere and stratosphere (UTS). The satellite failed after three years in orbit. In 1984, SAGE II began collecting data on\u00a0stratospheric O3, producing a stable record of this important greenhouse gas from 1984\u20132005.\u00a0SAGE III<\/a>\u00a0was launched on\u00a0\u041c\u0435\u0442\u0435\u043e\u0440-3\u041c<\/a>\u00a0(SAGE III\/M3M). The third-generation satellite\u00a0produced an\u00a0accurate measurement of the vertical structure of aerosols, O3, water vapor, and other important trace gases in the upper troposphere and stratosphere. The satellite was terminated on March 6, 2006, following a power supply system failure, resulting in loss of communication with the satellite.\u00a0<\/p>\n Another version of SAGE III was launched to the International Space Station (ISS) on\u00a0Feb. 19, 2017, where it was installed on the EXpedite the PRocessing of Experiments to Space Station (ExPRESS)\u00a0Logistics Carrier [ELC-4] \u2013 an\u00a0unpressurized attached payload\u00a0platform for ISS. SAGE III\/ISS, which is shown mounted on ELC-4 in\u00a0Figure 11, has completed its prime mission after three years of operation. NASA granted approval to extend the\u00a0SAGE III\/ISS<\/a>\u00a0mission through at least 2026 \u2013 meaning the instrument will continue to provide the public and science community with world-class vertical profiles of\u00a0O3, aerosol, water vapor, and other trace gases, e.g., nitrogen dioxide (NO2) and nitrate (NO3), data products for at least another year.\u00a0An article titled,\u00a0Summary of the 2024 SAGE III\/ISS Meeting<\/a>, published May 26, 2025, details the latest findings from SAGE.<\/p>\n Watching Earth Exhale<\/p>\n The Orbiting Carbon Observatory (OCO) was launched into space in February 2009, but it failed to separate from the Taurus rocket during its ascent, leading to mission failure and loss of the satellite. Undaunted, the EOS community began again and assembled\u00a0OCO-2<\/a>, which was successfully launched into orbit, joining the A-Train on July 2, 2014 \u2013 see\u00a0Figure 12. The satellite\u2019s mission focused on making precise, high-resolution measurements of atmospheric CO2. OCO-2 measures reflected sunlight that interacts with the atmosphere. Using diffraction gratings to separate the reflected sunlight into spectra, OCO-2 measures the absorption levels for the different molecular bands to calculate CO2\u00a0concentration. This information is invaluable for the quantification of CO2\u00a0emissions and can characterize both sources and sinks of this critical greenhouse gas. The mission was detailed in an article, titled\u00a0Orbiting Carbon Observatory-2: Observing CO2 from Space<\/a>\u00a0[July\u2013Aug. 2014,\u00a026:4, 4\u201312].\u00a0<\/p>\n On May 4, 2019, NASA launched the third iteration in the OCO group to the ISS. It was subsequently installed on the Japanese Experiment Module\u2013Exposed Facility (JEM-EF). Constructed from parts left over from OCO-2,\u00a0OCO-3<\/a>\u00a0continues the mission of making CO2 measurements with a focus on daily variability. In particular, the measurements explore the role of plants and trees in the major tropical rain forests of South America, Africa, and Southeast Asia. As of today, both OCO-2 and OCO\u20133 remain operational and gathering data.<\/p>\n The science team reflected on both these missions in a recent article posted in the online article, A Tapestry of Tales: 10th Anniversary Reflections from NASA\u2019S OCO-2 Mission<\/a>, published Aug. 12, 2025.<\/p>\n Tracking the Sun\u2019s Output<\/p>\n In December 1999, NASA launched the\u00a0Active Cavity Radiometer Irradiance Monitor Satellite<\/a>\u00a0(ACRIMSAT)\u00a0satellite to extend the more than two-decade record of total solar irradiance (TSI). Scientists use this important measurement to quantify the\u00a0solar energy input to the planet and thereby its interactions\u00a0with Earth\u2019s oceans, land masses,\u00a0and atmosphere. It is also a critical component to understand variations of the planet\u2019s climate. The Active Cavity Radiometer Irradiance Monitor 3 (ACRIM3) instrument onboard combined the best features of the ACRIM I (flown on the Solar Maximum Mission), ACRIM II (flown on the Upper Atmosphere Research Satellite), and SpaceLab-1 ACRIM (flown on Space Shuttle<\/a> Columbia, STS 9). ACRIM3 improved on its predecessors by incorporating a new electronics and package design.\u00a0The Earth Observer\u00a0captured the initial information from this mission in the article,\u00a0The ACRIMSAT\/ACRIM3 Experiment \u2014 Extending the Precision, Long-Term Total Solar Irradiance Climate Database<\/a>\u00a0[May\u2013June 2001,\u00a013:3, 14\u201317]. ACRIMSAT spent 14 years in orbit and ACRIM3 extended the TSI record to 36 years (i.e., building on measurements from previous ACRIM missions).<\/p>\n NASA continued its quest to observe the incident solar energy budget with the launch of the Solar Radiation and Climate Experiment<\/a> (SORCE) in January\u00a02003. SORCE focused on measuring\u00a0solar radiation incident to the top of the Earth\u2019s atmosphere. The Total Irradiance Monitor (TIM) onboard continued the TSI record that the ACRIM series of satellites established. In addition to TIM, the satellite carried a Spectral Irradiance Monitor (SIM), an Extreme Ultraviolet (XUV) Photometer System [XPS], and a stellar observation from the Solar\u00a0Stellar Irradiance Comparison Experiment (SOLSTICE). The satellite has produced\u00a0groundbreaking TSI and spectral solar irradiance (SSI) measurements \u2013 two key inputs for atmosphere and climate modeling.\u00a0<\/p>\n Early results from SORCE are detailed in the article, The SORCE (SOlar Radiation and Climate Experiment) Satellite Successfully Launched<\/a> [Jan.\u2013Feb. 2003, 15:1, 16\u201319]. The article, The SORCE Mission Celebrates 10 Years<\/a> [Jan.\u2013Feb. 2013,\u00a025:1, 3\u201313] details the most significant results from a decade of SORCE observations. Designed for a five-year mission, SORCE gathered data until 2020 \u2013 although a degradation of a battery power that began in 2008 increasingly hindered data collection for the remainder of the mission. During its time in orbit, SORCE captured two of the Sun\u2019s 11-year solar cycles and observed the solar cycle minimum in both 2008 and 2019. SORCE\u2019s orbit will decay and re-enter Earth\u2019s atmosphere in 2032.<\/p>\n To continue the crucial long-term TSI and the SSI record that SORCE originated,\u00a0NASA launched the\u00a0Total and Spectral Solar Irradiance Sensor<\/a>\u00a0(TSIS-1) to the ISS on Dec. 15, 2017, which was installed on JEM-EF ELC-3. The satellite\u2019s mission set out to measure the total amount of sunlight that falls on the planet\u2019s surface \u2013 see\u00a0Visualization 1. This data will clarify the distribution of different wavelengths of light. TSIS-1 was introduced in\u00a0The Earth Observer\u00a0article,\u00a0Summary of the 2018 Sun\u2013Climate Symposium\u00a0<\/a>[May\u2013June 2018,\u00a030:3, 21\u201327]. Similar to SORCE, TSIS-1 carries a\u00a0TIM and SIM. The instrument\u00a0extends the multidecadal SSI record and provides highly accurate, stable, and continuous observations that are critical to understanding the present climate conditions and predicting future conditions.\u00a0The most recent efforts from this mission were detailed in the online article,\u00a0Summary of the 2023 Sun\u2013Climate Symposium<\/a>, published July 18, 2024.\u00a0TSIS-1 has been extended by at least three more years as part of the Earth Sciences Senior Review process. A follow-on mission, TSIS-2, is under development to extend the long-term observational record through continued TSI and SSI measurements.<\/p>\n<\/p>\n Visualization 1. NASA\u2019s Total and Spectral solar Irradiance Sensor (TSIS-1) measures the total amount of solar energy input to Earth as well as the distribution of the Sun\u2019s energy across a wide range of wavelengths. The animation illustrates the various wavelengths of light that are partially reflected into space at different places in the column of atmosphere above the ground. Chronicling the Changing Land Surface<\/p>\n Along with Terra, other satellites also provide global estimates about the land. Each new mission provides the scientific community more information to refine these measurements. These data have improved climate models as well as improved our understanding of how the planet\u2019s interior is altering the surface of the planet.<\/p>\n Measuring Ice and Vegetation Heights<\/p>\n NASA launched ICESat in 2003 on a three-to-five-year mission to provide information on ice sheet mass balance and cloud properties. It carried the Geoscience Laser Altimeter System (GLAS), which combines a precision surface lidar with a sensitive dual-wavelength cloud and aerosol lidar. ICESat was decommissioned seven years after launch. The science team began efforts for the follow-on mission,\u00a0ICESat-2<\/a>, which launched on Sept. 15, 2018 \u2013 see\u00a0Figure 13. Data collected during a series of\u00a0Operation IceBridge<\/a>\u00a0field campaigns to the Arctic and Antarctic helped to fill the data gap between the two satellite missions \u2013 allowing for continuity of measurements. ICESat-2 carries a payload of a photon-counting laser altimeter on its three-year mission. The laser is split into six beams capable of measuring the elevation of the cryosphere, including ice sheets, glaciers, and sea ice, down to a fraction of an inch. The laser altimeter also gathers the height of ocean and land surfaces, including forests, snow, lakes, rivers, ocean waves, and urban areas.\u00a0The mission objective includes quantifying polar ice sheet contribution to sea-level change, estimating sea-ice thickness, and measuring vegetation canopy height. The mission was detailed in\u00a0The Earth Observer\u00a0article,\u00a0ICESat-2: Measuring the Height of Ice from Space<\/a>\u00a0[Sept.\u2013Oct.. 2018,\u00a030:5, 4\u201310]. The research community has been using this information to investigate how the ice sheets of Antarctica and Greenland are changing as the planet warms.<\/p>\n NASA\u2019s\u00a0Global Ecosystem Dynamics Investigation<\/a>\u00a0(GEDI \u2013 pronounced \u201cjedi\u201d) mission was launched to the ISS on Dec. 5, 2018 and was subsequently installed on the JEM\u2013EF ELC-6. From that vantage point GEDI produces high-resolution laser ranging observations of the three-dimensional (3D) structure of Earth that can be used to make precise measurements of forest canopy height and canopy vertical structure \u2013 see\u00a0Visualization 2. These measurements have improved understanding of important atmospheric and water cycling processes, biodiversity, and habitat. Upon completion of its prime mission, which lasted from December 2018 to March 2023, GEDI was moved from the ISS\u2019s EFU-6 to EFU-7 (storage). Since April 2024, the GEDI instrument has been back in its original location on EFU-6 and continues to collect high-resolution observations of Earth\u2019s 3D structure from space. The GEDI research team hopes the mission can continue collecting data until 2030.<\/p>\n The GEDI mission has been covered in\u00a0The Earth Observer\u00a0through summaries of periodic meetings of the GEDI Science Team. The online article,\u00a0Summary of the 2025 GEDI Science Team Meeting<\/a>, is the most recent installment of GEDI\u2019s progress, published on Aug. 18, 2025. This article includes discussion of \u201cthe return of the GEDI\u201d from hibernation and the science results since then.<\/p>\n Monitoring Earth in Intricate Detail<\/p>\n The\u00a0Soil Moisture Active Passive<\/a>\u00a0(SMAP) mission was designed to measure the amount of water in surface soil across Earth. The satellite was launched from Vandenberg Air Force Base on Jan. 31, 2015. The satellite payload consisted of both an active microwave radar and a passive microwave radiometer to measure a swath of the planet 1000-km (~621-mi) wide. The radar transmitter failed just nine months after launch on July 7, 2015. Although the loss of the radar was unfortunate, the nine months where both instruments functioned provided an invaluable dataset that established the dependence of L-band radar signals on soil moisture, vegetative water content, and freeze\u2013thaw state. Two of these variables (surface soil moisture and freeze\u2013thaw state) are critical variables that influence the planet\u2019s water, energy, and carbon cycles. The three variables influence weather and climate. Furthermore, the SMAP team quickly turned a setback into a success. They repurposed the channels that had been dedicated to the radar to record the reflected signals from the\u00a0Global Navigation Satellite System<\/a>\u00a0(GNSS) constellation in August 2015, making SMAP the first full-polarimetric GNSS reflectometer in space for the investigation of land surface and cryosphere.<\/p>\n The Earth Observer article, SMAP: Mapping Soil Moisture and Freeze\/Thaw State from Space<\/a> [Jan.\u2013Feb. 2015, 27:1, 14\u201319] offered a preview of SMAP that was published shortly after its launch. A more recent online article, A Decade of Global Water Cycle Monitoring: The Soil Moisture Active Passive Mission<\/a>, published Aug. 18, 2025, reflects on the achievements of SMAP after a decade of operations.<\/p>\n More specific to vegetation water content, NASA launched the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station<\/a> (ECOSTRESS)\u00a0to ISS on June 29, 2018. It was subsequently installed on the JEM\u2013EF ELC 10, placing it in close proximity to GEDI (installed on ELC 6) and enabling combined observations. While GEDI focuses on the canopy height and related characteristics, ECOSTRESS monitors the combined evaporation and transpiration of living plants \u2013 known as evapotranspiration (ET). ECOSTRESS determines ET indirectly through measurements of the thermal infrared brightness temperatures of plants.<\/p>\n As with GEDI, The Earth Observer has reported on the activities of the ECOSTRESS mission. The most recent coverage was in the article, ECOSTRESS 2019 Workshop Summary: Science, Applications, and Hands-On Training<\/a> [July\u2013Aug. 2018, 31:4, 15\u201318.]<\/p>\n Last, but certainly not least, the most recent Earth observing satellite to launch is a joint venture between NASA and the Indian Space Research Organization (ISRO). The NASA-ISRO Synthetic Aperture Radar<\/a> (NISAR) took to the skies on July 30, 2025, from the Satish Dhawan Space Centre\u00a0<\/a>on India\u2019s southeastern coast\u00a0aboard an ISRO Geosynchronous Satellite Launch Vehicle (GSLV) rocket 5. The mission was designed to observe and measure some of the planet’s most complex processes \u2013 see Figure 14. The launch was lauded in the Editor\u2019s Corner<\/a> published online on Sept. 10, 2025.<\/p>\n NISAR uses two different radar frequencies \u2013 L-band and S-band synthetic aperture radar (SAR). The dual system can penetrate clouds and forest canopies to allow researchers to measure changes on the planet\u2019s surface, down to a centimeter (~0.4 in). This level of detail allows the research community to investigate ecosystem disturbances, ice-sheet collapse, natural hazards, sea level rise, and groundwater issues. The satellite will also capture changes in forest and wetland ecosystems. It will expand on our understanding of deformation of the planetary crust that can help predict earthquakes, landslides, and volcanic activity. All of this data will help mitigate damage from a disaster and help communities prepare a disaster response. Some early results from the both NISAR radars are discussed in the Final Editor’s Corner<\/a> column, published online on Dec. 29, 2025.<\/p>\n Conclusion<\/p>\n Over the past 36 years, The Earth Observer has borne witness to some of the most monumental scientific achievements of NASA Earth Science and chronicled those stories for the community. While the format of the publication evolved considerably over the years, the satellite missions that have been the focus of this article are one of the primary \u201clenses\u201d that the newsletter has had to observe and reflect on the story of NASA Earth Science. These continuous global observations have revolutionized society\u2019s knowledge of our home planet and how humans might be altering it.<\/p>\n The staff of\u00a0The Earth Observer\u00a0have navigated many different modes of communication over the past three-and-a-half decades, but the commitment to delivering high-quality content has remained constant. It has been the highest honor of every member of our publication team \u2013 past and present \u2013 to work on this material. While the newsletter is coming to an end, it is hoped that the Archives page<\/a> continues to be a rich source of historic information about NASA\u2019s EOS and Earth science over the past three and a half decades.\u00a0<\/p>\n On behalf of the current Editorial Team, we, the authors of this reflection, wish to thank every person who has contributed to the success of this newsletter over the years \u2013 and to extend to all in the NASA Earth Science community best wishes for the year ahead and continued success in your remote observation endeavors.\u00a0<\/p>\n Stacy Kish
Visualization credit: NASA<\/p>\n
NASA\u2019s Goddard Space Flight Center\/EarthSpin
stacykishwrites@gmail.com<\/a><\/p>\n