On May 4, 2002, at 2:55 am Pacific Time, a rocket carrying NASA’s Aqua satellite—the second flagship satellite of the agency’s Earth Observing System (EOS)—launched into space from Vandenberg Air Force Base, located in California. It was the first step of a momentous Earth-observation voyage that has continued for more than two decades—far beyond anyone’s expectations.
“It is remarkable,” said Aqua Project Scientist Dr. Claire Parkinson. “The design life was six years and although you expect to exceed it, 20 years has been great. If we knew we were going to have a spacecraft and instruments that would last 20 years, we might have put more fuel in it so that it would last even longer.”
The EOS was established to acquire a long-term record of Earth observations to enhance understanding of the total Earth system and the effects of natural and human-induced changes on the environment. Conceived in the 1980s and implemented in the 1990s and early 2000s, EOS includes three flagship satellite missions—Aqua, its predecessor Terra, and Aura—and several smaller missions, plus the EOS Project Science Office (EOSPSO) and a data system, NASA's Earth Observing System Data and Information System (EOSDIS).
Named after the Latin word for water, Aqua has spent the past twenty years in a sun-synchronous polar orbit 705 kilometers (438 miles) above the Earth’s surface, collecting and transmitting data about the Earth's water cycle, including evaporation from the oceans, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, ice and snow covering Earth’s terrestrial surfaces. At the same time, Aqua obtains measurements of Earth’s radiative energy fluxes, aerosols, terrestrial vegetation, ocean color (i.e., phytoplankton and dissolved organic matter in the oceans), and temperatures on the surface of land, the surface of the ocean, and throughout the atmosphere.
In addition to being one of three flagship EOS satellites, Aqua has been a cornerstone of the “A-Train,” a moniker given to the international constellation of satellites that closely follow one another (within seconds to minutes of each other) along the same orbital track, crossing the equator in an ascending (northbound) direction at about 1:30 PM local time. This allows near-simultaneous observations from a wide variety of instruments that are synergistically used to aid the scientific community in advancing a wide range of Earth-system science.
In January 2022, Aqua descended from the A-Train when, due to its limited remaining fuel supply, it transitioned from its tightly controlled orbit to a free-drift mode, wherein its equatorial crossing time is slowly drifting to later times.
The Aqua mission is a joint endeavor among the United States, Japan and Brazil. NASA provided the spacecraft bus and four of its six instruments: NASA's Goddard Space Flight Center Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Sounding Unit (AMSU), NASA's Jet Propulsion Laboratory Atmospheric Infrared Sounder (AIRS), and NASA's Langley Research Center Clouds and the Earth's Radiant Energy System (CERES). Japan's National Space Development Agency (now the Japan Aerospace Exploration Agency, or JAXA) provided the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), and the Instituto Nacional de Pesquisas Espaciais (Brazilian Institute for Space Research) provided the Humidity Sounder for Brazil (HSB). Aqua began broadcasting data from these instruments in the weeks following launch and, save for the AMSR-E and HSB and occasional disruptions, it hasn’t stopped since.
Parkinson attributes Aqua’s longevity to the engineering expertise of NASA and its industry partners, who produced such a “well-constructed” spacecraft and instruments. She also credits NASA’s Earth Science Mission Operations (ESMO) team, which has operated the systems that command and control the Aqua satellite, “with great skill and care,” during the past two decades.
She is not alone in this assessment.
“[Aqua’s longevity] is a testament to the high quality of engineering design and build that went into the instruments, the dedication of the ESMO team in quickly addressing anomalies, the expertise of the calibration scientists who ensured good characterization of the instruments so that consistent measurements can be made over many years, and NASA Headquarters which recognized Aqua’s success on multiple fronts and provided sustained funding for the mission,” said Deputy Aqua Project Scientist Dr. Lazaros Oreopoulos.
Yet, Aqua’s longevity is noteworthy for reasons beyond its mere stamina in the harsh conditions of space. Aqua’s ability to endure in orbit is significant because it was intended to be the first in a series of three nearly identical EOS afternoon satellites.
“Originally, [the first two flagship EOS satellites] weren’t called Aqua and Terra. They were called EOS AM, which became Terra, and EOS PM, which became Aqua, and there were to be two more versions of each satellite,” Parkinson said. “The plan was that, after 6 years, we’d launch the next one. However, the budget for the EOS program was cut and the size of the program was reduced. Well, both Aqua and Terra have lasted 20 years, so it turns out we’ve more than covered the 18-year time frame that was going to be covered by both three-satellite sequences.”
During that time, Aqua and its companion EOS satellites have given the Earth science and remote sensing communities a treasure trove of data that have been incorporated into weather prediction models, processed into an array of data products for use in a wide variety of scientific research, and used in near real-time (NRT) applications for monitoring and managing natural and anthropogenic hazards and disasters, such as storms, wildfires, and volcanic eruptions.
The Aqua Data Record
The Aqua data record represents one of the longest single-satellite climate data records ever compiled. Each time Aqua orbits the Earth its data are transmitted from the spacecraft through two processes: direct downlink and direct broadcast. The direct downlink transmits the data from an on-board solid-state recorder (SSR) to polar ground stations in Alaska and Svalbard, Norway. (Direct downlink is routinely done each orbit, although the SSR has the capacity to hold up to two and a half orbits of data.) When direct downlink is not taking place, the direct broadcast system is generally in operation and allows anyone with a direct broadcast receiver to receive the raw Aqua data.
From the polar ground stations, the downlinked data are transmitted to Goddard, where the data processing is done for the MODIS, AIRS, and AMSU-A data. CERES data are sent to Langley for processing and, while it was operational, AMSR-E data were sent to Japan’s Earth Observation Center (EOC) for initial processing, followed by further processing at Remote Sensing Systems and at NASA's Marshall Space Flight Center. After processing, Aqua instrument data are made available through several discipline specific EOSDIS Distributed Active Archive Centers (DAACs).
According to figures from the Earth Science Data and Information System (ESDIS) Project’s Metrics System, approximately 9.7 petabytes (PB) of Aqua data reside in the EOSDIS collection at the end of 2021, making up roughly 16.4 percent of the approximately 59 PB EOSDIS data collection. During the 2021 Fiscal Year (FY), which ran from October 1, 2020, to September 30, 2021, 8.6 PB of Aqua data were distributed. Since 2002, the year the first Aqua data were available, approximately 55.2 PB of Aqua data have been distributed to global data users. Distribution of data from the Aqua and Terra MODIS instruments remains the highest of any instrument data in the EOSDIS collection, and 14 PB of MODIS data (7.5 PB of Aqua and 6.5 PB of Terra) were distributed during FY 2021.
Statistics, of course, tell only part of the Aqua story. Aqua’s impressive data record is composed of observations from each of its instruments, and the data they provide—both individually and in concert—offer a more detailed picture of the contributions the mission has made to the remote sensing and Earth science communities over the past 20 years.
The Moderate Resolution Imaging Spectroradiometer
With 36 spectral channels, a 2,330 km by 10 km swath, and spatial resolution ranging from 250 to 1,000 meters (depending on the channel) the MODIS instrument is among the most celebrated of any satellite instrument in orbit. MODIS first went into orbit aboard Terra in 1999 and, in conjunction with the MODIS sensor on Aqua, the pair have reliably and consistently provided the Earth science and remote sensing communities with an array of global Earth system observations, including interactions between the land, ocean, and atmosphere, for more than two decades.
The MODIS instrument aboard Aqua has proven especially useful in the satellite’s mission to collect data about Earth's water cycle.
“Clouds are important elements of the water cycle and the MODIS instrument on Aqua has contributed a great deal to our understanding of the optical properties of clouds,” said MODIS Science Team Leader Dr. Michael King. “And with Aqua being part of the A-Train, there were also a lot of opportunities to compare MODIS cloud property measurements with those from CloudSat and CALIPSO to help validate them.”
The Earth science and remote sensing communities also benefited from having a pair of MODIS instruments in different orbits (i.e., morning and afternoon), which offered complementary observations of high-priority features of Earth’s atmospheric, oceanic, and terrestrial components.
“The second MODIS was very helpful in looking at morning and afternoon differences in the Earth system," said King. “For example, we observed that there were more clouds over land in the afternoon, which is partly why Terra was flown in an earlier orbit—to minimize cloud obscuration of the land during the morning. However, the reverse is true for the ocean, where there are more clouds in the morning and fewer in the afternoon. Having two MODIS instruments also enabled studies to investigate changes in other phenomena, such as aerosol concentrations (i.e., optical thickness), in the morning versus the afternoon.”
Having two MODIS instruments in orbit has also provided a greater chance of cloud-free observations, allowed a better assessment of fire regimes, and enabled the creation of the Bidirectional Reflectance Distribution Function (BRDF)-Adjusted Land Surface Reflectance dataset, which uses 16 days of 500-meter resolution MODIS data from both Terra and Aqua to measure surface albedo, or the percentage of radiant energy scattered up and away from the Earth’s surface.
Further, several low latency NRT MODIS data products are available through NASA’s Land, Atmosphere Near Real-time Capability for EOS (LANCE), typically within three hours of observation. Although NRT products undergo less processing than the MODIS data products used in scientific research, their near-immediate availability make them valuable tools for monitoring on-going events like wildfires, flooding, volcanic eruptions, and other hazards.
Given the multi-disciplinary nature of MODIS data, they are archived at and distributed from different EOSDIS DAACs:
- MODIS land products are available from NASA’s Land Processes DAAC (LP DAAC),
- MODIS atmosphere products are available from NASA’s Level-1 and Atmosphere Archive and Distribution System DAAC (LAADS DAAC),
- MODIS snow and ice products are available from NASA's National Snow and Ice Data Center DAAC (NSIDC DAAC),
- MODIS ocean color products are available from NASA's Ocean Biology Distributed Active Archive Center (OB.DAAC), and
- MODIS sea surface temperature data are available from NASA’s Physical Oceanography DAAC (PO.DAAC)
Clouds and the Earth's Radiant Energy System
The CERES instrument was designed to help scientists better understand the Earth’s radiation budget – the balance (or imbalance) between Earth’s incoming and outgoing energy. As Aqua orbits Earth, CERES measures the energy radiated and reflected from the Earth at the top of the atmosphere, and the CERES Science Team combines the CERES measurements with other satellite data to calculate the radiated and reflected energy within the atmosphere and at the surface. Together, these data sets are used to better our understanding of the energy flows within the climate system.
For CERES Principal Investigator Dr. Norman Loeb, obtaining basic measurements of the amount of energy coming in and going out is critical to understanding both the Earth’s climate and climate change.
“From these measurements, we are able to assess the heat budget of the planet, meaning, how much energy is absorbed and how much is emitted,” he said. “It’s important because, over time, if more energy is absorbed than emitted, the Earth will heat up, more ice and snow will melt over land, which will eventually find its way to the ocean and raise sea level, and heat the ocean, which will also cause sea level rise.”
CERES data are also used in conjunction with data from other instruments aboard Aqua, particularly MODIS and AIRS, to investigate how atmospheric parameters and surface characteristics might impact Earth’s radiation budget.
“We use MODIS quite a bit, which enables us to look at properties in the atmosphere and at the surface that are driving the changes in radiation at the top of the atmosphere. So, we use the two synergistically, to provide a lot more information than one alone can provide,” said Loeb. “We make use of AIRS as well to help us constrain the upper tropospheric humidity, which we need to make calculations of the surface radiative fluxes. AIRS provides us with a way of making those calculations more accurate.”
Currently, there are six CERES instruments in orbit—one on the Joint Polar Satellite System’s NOAA-20 satellite, one on the joint NASA/NOAA Suomi National Polar-orbiting Partnership satellite (Suomi NPP), two on Terra, and two on Aqua—and the last four have been operational for 20 or more years.
Having that long of a data record has been “incredible,” said Loeb.
“With the combination of Terra and Aqua both lasting this long we’ve learned a lot about the changes that are going on and we have had surprises,” Loeb said. “We’ve learned that there’s more energy coming in than going out and that this imbalance has actually doubled over the Aqua period. It’s a truly incredible thought given the implications I mentioned earlier. We wouldn’t have learned this had the missions lasted only as long as their six-year expected lifetimes.”
CERES data are available through NASA’s Atmospheric Science Data Center (ASDC), which archives and distributes EOSDIS data related to Earth’s radiation budget, clouds, aerosols, and tropospheric composition.
Advanced Microwave Scanning Radiometer for the Earth Observing System
Provided by JAXA, AMSR-E was designed to measure a variety of processes over land and ocean using natural emissions of microwave radiation from the Earth. Over land it measured precipitation, soil moisture, vegetation cover, and snow cover. Over the ocean, its measurements included sea surface temperature (SST), surface wind speed, total atmospheric water vapor, cloud water content, precipitation, and sea ice.
“Since AMSR-E measured at microwave frequencies, the instrument had the advantage of being able to ‘see’ many processes through cloud cover, somewhat like a radar,” said Dr. Roy Spencer, U.S. AMSR-E Team Leader. “These measurements complemented the MODIS, AIRS/AMSU, and CERES instruments for the monitoring of the global hydrologic cycle.”
For example, AMSR-E’s measurements of precipitation over the land and ocean have provided scientists with better estimates of Earth’s precipitation rates, as well as insights into the scattering effects of large ice particles, which later melt to form raindrops. Both these measurements have been used to improve cloud and weather modelling. Further, AMSR-E data enhanced the scientific community’s ability to monitor SST fluctuations, which are known to have a profound impact on weather patterns across the globe, and its measurements of atmospheric water vapor over the ocean provided insights into how this compound cycles through the atmosphere.
AMSR-E suffered a major anomaly in October 2011, and although it was able to transmit reduced-quality data for several more years, it was ultimately powered off in March 2016. Nevertheless, it lasted long enough to make significant contributions to the Earth science and remote sensing communities and paved the way for the development of new instruments.
“AMSR-E was the most advanced instrument in its class during the nine years it operated, building upon a long history of NASA and Department of Defense microwave radiometers that began flying in the early 1970s,” said Spencer. “A follow-on instrument called AMSR2 on the GCOM-W1 mission, designed and built by Japan, carried on the AMSR-E measurements from 2012 onward.”
AMSR-E data are archived and distributed by NSIDC DAAC.
Atmospheric Infrared Sounder, Advanced Microwave Sounding Unit, and Humidity Sounder for Brazil
Before Aqua, NOAA relied on satellite data from legacy sensors and land-launched weather balloons to update weather forecasts. The reliance on balloons left large portions of the world’s surface (e.g., areas of open ocean) uncovered. To improve weather forecasting, more frequent and detailed information about the atmosphere was necessary.
The AIRS instrument, working in conjunction with AMSU and Humidity Sounder for Brazil (HSB), was designed to meet this need and, together, the three instruments comprised the most advanced and accurate atmospheric sounding system ever deployed in space.
Every 2.67 seconds, AIRS probes a column of the air—from the top of the atmosphere to the Earth’s surface—collecting measurements of humidity, temperature, cloud properties, and greenhouse gases with its 2,378 infrared spectral channels and four visible/near-infrared channels. Each of these channels is associated with particular atmospheric properties, or combinations of them, and with particular heights or levels in the atmosphere, and because it has so many, AIRS has greatly improved the accuracy and vertical resolution of atmospheric profiles. The result of these improved soundings has been more reliable climate prediction, and improved weather forecasts.
“AIRS revolutionized weather prediction by providing, for the first time, a three-dimensional picture of the atmosphere,” said Dr. Joao Teixeira, AIRS Science Team Leader. “Now there are a few infrared sounders in orbit, but AIRS still is one of the key sensors and, for the first few years, it was the only one.”
Part of what made AIRS so ground-breaking is its ability to provide high-resolution observations of both temperature and water vapor.
“Temperature is a very fundamental variable in atmospheric physics and for climate,” said Teixeira. “Water vapor also plays a very large role, because it is responsible for clouds, which are condensed water vapor in the atmosphere, and water vapor essentially controls how many clouds there are and how much precipitation there is. Water vapor also interacts with the radiation emitted by the planet, and that’s why we can detect it with this instrument. It also plays a role in how much the atmosphere is mixing vertically and the processes that promote it.”
AIRS cannot “see” through clouds, but AMSU and HSB—both microwave sensors—can and therefore they play an important auxiliary role on Aqua. AMSU is particularly useful for obtaining temperature profiles in the atmosphere. HSB was designed to measure the amount of water vapor in the atmosphere, but it suffered a catastrophic failure in early 2003. Nevertheless, AIRS and AMSU have continued to provide atmospheric temperature and water vapor measurements that are much more accurate than previous space-based measurements.
Beyond enhancing weather forecasts, AIRS measurements of carbon dioxide and other greenhouse gases have also helped climatologists better understand climate variation trends and how increased concentrations of atmospheric greenhouse gases are impacting the climate system.
In fact, today AIRS and AMSU play a more important role in climate science than in weather science, Teixeira said.
“We know that many of the key aspects of climate change have changed dramatically. Now we have instruments that last 20 years and are stable, meaning they are able to look at the atmosphere and the planet the same way year after year, and we can estimate how the degradation of certain components affects the measurements,” he said. “We can also study the different channels of radiation that we measure, and we know that many of them are not degrading at all. So, the signal you’re measuring is the signal of climate change.”
The ability to capture these signals of climate change is what makes Aqua and its instruments unique, and solidifies their legacy in the history of satellite remote sensing, said Teixeira.
“You have these instruments at the same time you have the most dramatic change in climate, and we can measure it, but this was not what people necessarily planned in the early 1980s,” he said. “They knew climate what changing, but climate change was not the main preoccupation. They wanted to study the Earth system. It turns out we launched these instruments exactly at the time this is all happening.”
AIRS and AMSU-A data are archived and distributed through NASA’s Goddard Earth Sciences Data and Information Services Center (GES DISC).
Aqua’s Future and Legacy
Although there’s no doubt the Aqua mission has informed our understanding of the Earth system and how it’s changing, how much longer it will keep providing data is unclear. Due to limited fuel reserves, Aqua completed all mission maneuvers related to maintaining a 1:30 p.m. equatorial crossing time and 705 kilometer orbit altitude in December 2021. Since then, it has begun drifting to later equatorial crossing times and, by February 2023, the satellite is expected to reach, and possibly exceed, an equatorial crossing time of 1:45.
Nevertheless, Aqua will continue to transmit extremely valuable data as its equatorial crossing time drifts. In fact, Parkinson said that “If funded, Aqua will likely be able to collect good quality science data until August of 2026.”
“We are optimistic that we will be able to collect science quality data for the next few years, despite the progressive drift of Aqua’s orbit to crossings at later afternoon times, until spacecraft hardware limitations (i.e., power and fuel) will force mission termination, circa mid-2026,” said Oreopoulos. “While the drift is disruptive to the consistency of the climate record of certain geophysical observables with considerable diurnal variation, many scientists have expressed excitement at the prospect of observations under different geometries and at different times of the day where some phenomena are more intense.”
Among those scientists is Joao Teixeira, who said Aqua’s departure from its traditional orbit will provide an opportunity for AIRS to collect valuable new data.
“The value of these observations by a hyperspectral sounder such as AIRS has never been explored and will present a unique opportunity to assess the impact on forecast quality of having infrared sounders at these local times,” Teixeira said. “Discussions are currently underway between the AIRS project and some key weather centers to prepare for these impact studies, which will be essential for the design of new infrared sounders to inform decisions related to sampling of the diurnal cycle.”
The same is true for the CERES science team, said Loeb.
“For CERES specifically and for future Earth radiation satellites, we certainly would like to place the instrument in a different mode than it’s currently in,” he said. “We haven’t had data from as full a range of solar zenith angles because we’ve been fixed in mean local time. Having the orbit drift allows us to extend our angular information to other solar Earth-viewing geometries.”
In addition, Loeb noted that, when compared to other satellites with a 1:30 p.m. equatorial crossing, Aqua’s drifting may provide some insight into how mean-local-time drifts can be corrected in long-term data records.
“There is a long record of satellite instruments that have flown for 40-plus years and they have had mean-local-time drifts too,” said Loeb. “It’s been a struggle for the community to try to come up with climate data records when the orbits drift. So, having Aqua drift and Suomi NPP and NOAA-20 in a fixed local time provides a way of better understanding how these mean-local-time drifts can be corrected, and this information can then be used to improve the long, 40-plus year records.”
Yet, regardless of when the instruments aboard Aqua stop collecting data, the legacy of the spacecraft will continue to live on both within NASA and in the larger Earth observation community.
For example, the conjoined measurement strategies of the A-Train constellation are, according to Oreopoulos, leaving a lasting influence on the design of future missions such as the Atmospheric Observing System of the Earth System Observatory.
“Aqua has been for many years the cornerstone of the A-Train, which also included the CloudSat and Cloud-Aerosol LIDAR and Infrared Pathfinder Satellite Observation (CALIPSO) satellites, and in later years, GCOM-W, among others. The simultaneous views of cloud, precipitation, and aerosol fields with instruments aboard these satellites of different capabilities and sensitivities enabled us to obtain more holistic perspectives of their structures and interactions,” he said. “It also allowed us to cross-validate their measurements, better understand their limitations, and the degree to which consistency in the retrieved properties of these fields can be achieved.”
Aqua’s legacy will live on in the annals of weather prediction as well.
“Over the past 20 years, every decision that anyone on the planet made based on weather prediction has a little bit of AIRS in it,” said Teixeira. “Most NASA missions produce data, but it’s not out there immediately. It takes a while. MODIS and AIRS data are out there immediately. This was a requirement from the weather prediction community. If the data couldn’t be available quickly then they couldn’t be used for weather prediction.”
Ultimately, though, Aqua’s legacy will be evident in the myriad data products that members of the remote sensing communities will continue to use in their research on Earth’s atmosphere, cryosphere, lands, and oceans long after 2026.
“In contrast to some missions, where there’s a single or central goal, the Aqua mission collects data on all sorts of different variables, and so it has provided a wide range of datasets that can be used by a large number of scientists and others,” said Parkinson. “More than 20,000 scientific papers have incorporated Aqua data, and the data have been used in weather forecasting and numerous other practical applications, making Aqua a mission of significance for both the global remote-sensing science community and society in general.”