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By NASA
6 min read
NASA’s IMAP Mission to Study Boundaries of Our Home in Space
Summary
NASA’s new Interstellar Mapping and Acceleration Probe, or IMAP, will launch no earlier than Tuesday, Sept. 23 to study the heliosphere, a giant shield created by the Sun. The mission will chart the heliosphere’s boundaries to help us better understand the protection it offers life on Earth and how it changes with the Sun’s activity. The IMAP mission will also provide near real-time measurements of the solar wind, data that can be used to improve models predicting the impacts of space weather ranging from power-line disruptions to loss of satellites, to the health of voyaging astronauts. Space is a dangerous place — one that NASA continues to explore for the benefit of all. It’s filled with radiation and high-energy particles that can damage DNA and circuit boards alike. Yet life endures in our solar system in part because of the heliosphere, a giant bubble created by the Sun that extends far beyond Neptune’s orbit.
With NASA’s new Interstellar Mapping and Acceleration Probe, or IMAP, launching no earlier than Tuesday, Sept. 23, humanity is set to get a better look at the heliosphere than ever before. The mission will chart the boundaries of the heliosphere to help us better understand the protection it offers and how it changes with the Sun’s activity. The IMAP mission will also provide near real-time measurements of space weather conditions essential for the Artemis campaign and deep space travel.
“With IMAP, we’ll push forward the boundaries of knowledge and understanding of our place not only in the solar system, but our place in the galaxy as a whole,” said Patrick Koehn, IMAP program scientist at NASA Headquarters in Washington. “As humanity expands and explores beyond Earth, missions like IMAP will add new pieces of the space weather puzzle that fills the space between Parker Solar Probe at the Sun and the Voyagers beyond the heliopause.”
Download this video from NASA’s Scientific Visualization Studio.
Domain of Sun
The heliosphere is created by the constant outflow of material and magnetic fields from the Sun called the solar wind. As the solar system moves through the Milky Way, the solar wind’s interaction with interstellar material carves out the bubble of the heliosphere. Studying the heliosphere helps scientists understand our home in space and how it came to be habitable.
As a modern-day celestial cartographer, IMAP will map the boundary of our heliosphere and study how the heliosphere interacts with the local galactic neighborhood beyond. It will chart the vast range of particles, dust, ultraviolet light, and magnetic fields in interplanetary space, to investigate the energization of charged particles from the Sun and their interaction with interstellar space.
The IMAP mission builds on NASA’s Voyager and IBEX (Interstellar Boundary Explorer) missions. In 2012 and 2018, the twin Voyager spacecraft became the first human-made objects to cross the heliosphere’s boundary and send back measurements from interstellar space. It gave scientists a snapshot of what the boundary looked like and where it was in two specific locations. While IBEX has been mapping the heliosphere, it has left many questions unanswered. With 30 times higher resolution and faster imaging, IMAP will help fill in the unknowns about the heliosphere.
Energetic neutral atoms: atomic messengers from our heliosphere’s edge
Of IMAP’s 10 instruments, three will investigate the boundaries of the heliosphere by collecting energetic neutral atoms, or ENAs. Many ENAs originate as positively charged particles released by the Sun but after racing across the solar system, these particles run into particles in interstellar space. In this collision, some of those positively charged particles become neutral, and an energetic neutral atom is born. The interaction also redirects the new ENAs, and some ricochet back toward the Sun.
Charged particles are forced to follow magnetic field lines, but ENAs travel in a straight line, unaffected by the twists, turns, and turbulences in the magnetic fields that permeate space and shape the boundary of the heliosphere. This means scientists can track where these atomic messengers came from and study distant regions of space from afar. The IMAP mission will use the ENAs it collects near Earth to trace back their origins and construct maps of the boundaries of the heliosphere, which would otherwise be invisible from such a distance.
“With its comprehensive state-of-the-art suite of instruments, IMAP will advance our understanding of two fundamental questions of how particles are energized and transported throughout the heliosphere and how the heliosphere itself interacts with our galaxy,” said Shri Kanekal, IMAP mission scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
The IMAP mission will study the heliosphere, our home in space. NASA/Princeton University/Patrick McPike Space weather: monitoring solar wind
The IMAP mission will also support near real-time observations of the solar wind and energetic solar particles, which can produce hazardous conditions in the space environment near Earth. From its location at Lagrange Point 1, about 1 million miles from Earth toward the Sun, IMAP will provide around a half hour’s warning of dangerous particles headed toward our planet. The mission’s data will help with the development of models that can predict the impacts of space weather ranging from power-line disruptions to loss of satellites.
“The IMAP mission will provide very important information for deep space travel, where astronauts will be directly exposed to the dangers of the solar wind,” said David McComas, IMAP principal investigator at Princeton University.
Cosmic dust: hints of the galaxy beyond
In addition to measuring ENAs and solar wind particles, IMAP will also make direct measurements of interstellar dust — clumps of particles originating outside of the solar system that are smaller than a grain of sand. This space dust is largely composed of rocky or carbon-rich grains leftover from the aftermath of supernova explosions.
The specific elemental composition of this space dust is a postmark for where it comes from in the galaxy. Studying cosmic dust can provide insight into the compositions of stars from far outside our solar system. It will also help scientists significantly advance what we know about these basic cosmic building materials and provide information on what the material between stars is made of.
David McComas leads the mission with an international team of 27 partner institutions. APL is managing the development phase and building the spacecraft, and it will operate the mission. IMAP is the fifth mission in NASA’s Solar Terrestrial Probes Program portfolio. The Explorers and Heliophysics Projects Division at NASA Goddard manages the STP Program for the agency’s Heliophysics Division of NASA’s Science Mission Directorate. NASA’s Launch Services Program, based at NASA’s Kennedy Space Center in Florida, manages the launch service for the mission.
By Mara Johnson-Groh
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Sep 17, 2025 Related Terms
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By NASA
6 min read
Preparations for Next Moonwalk Simulations Underway (and Underwater)
This artist’s concept shows a brown dwarf — an object larger than a planet but not massive enough to kickstart fusion in its core like a star. Brown dwarfs are hot when they form and may glow like this one, but over time they get closer in temperature to gas giant planets like Jupiter. NOIRLab/NSF/AURA/R. Proctor An unusual cosmic object is helping scientists better understand the chemistry hidden deep in Jupiter and Saturn’s atmospheres — and potentially those of exoplanets.
Why has silicon, one of the most common elements in the universe, gone largely undetected in the atmospheres of Jupiter, Saturn, and gas planets like them orbiting other stars? A new study using observations from NASA’s James Webb Space Telescope sheds light on this question by focusing on a peculiar object that astronomers discovered by chance in 2020 and called “The Accident.”
The results were published on Sept. 4 in the journal Nature.
As shown in this graphic, brown dwarfs can be far more massive than even large gas planets like Jupiter and Saturn. However, they tend to lack the mass that kickstarts nuclear fusion in the cores of stars, causing them to shine. NASA/JPL-Caltech The Accident is a brown dwarf, a ball of gas that’s not quite a planet and not quite a star. Even among its already hard-to-classify peers, The Accident has a perplexing mix of physical features, some of which have been previously seen in only young brown dwarfs and others seen only in ancient ones. Because of those features, it slipped past typical detection methods before being discovered five years ago by a citizen scientist participating in Backyard Worlds: Planet 9. The program lets people around the globe look for new discoveries in data from NASA’s now-retired NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer), which was managed by NASA’s Jet Propulsion Laboratory in Southern California.
The brown dwarf nicknamed “The Accident” can be seen moving in the bottom left corner of this video, which shows data from NASA’s now-retired NEOWISE (Near-Earth Object Wide-Field Infrared Survey Explorer), launched in 2009 with the moniker WISE. NASA/JPL-Caltech/Dan Caselden The Accident is so faint and odd that researchers needed NASA’s most powerful space observatory, Webb, to study its atmosphere. Among several surprises, they found evidence of a molecule they couldn’t initially identify. It turned out to be a simple silicon molecule called silane (SiH4). Researchers have long expected — but been unable — to find silane not only in our solar system’s gas giants, but also in the thousands of atmospheres belonging to brown dwarfs and to the gas giants orbiting other stars. The Accident is the first such object where this molecule has been identified.
Scientists are fairly confident that silicon exists in Jupiter and Saturn’s atmospheres but that it is hidden. Bound to oxygen, silicon forms oxides such as quartz that can seed clouds on hot gas giants, bearing a resemblance to dust storms on Earth. On cooler gas giants like Jupiter and Saturn, these types of clouds would sink far beneath lighter layers of water vapor and ammonia clouds, until any silicon-containing molecules are deep in the atmosphere, invisible even to the spacecraft that have studied those two planets up close.
Some researchers have also posited that lighter molecules of silicon, like silane, should be found higher up in these atmospheric layers, left behind like traces of flour on a baker’s table. That such molecules haven’t appeared anywhere except in a single, peculiar brown dwarf suggests something about the chemistry occurring in these environments.
“Sometimes it’s the extreme objects that help us understand what’s happening in the average ones,” said Faherty, a researcher at the American Museum of Natural History in New York City, and lead author on the new study.
Happy accident
Located about 50 light-years from Earth, The Accident likely formed 10 billion to 12 billion years ago, making it one of the oldest brown dwarfs ever discovered. The universe is about 14 billion years old, and at the time that The Accident developed, the cosmos contained mostly hydrogen and helium, with trace amounts of other elements, including silicon. Over eons, elements like carbon, nitrogen, and oxygen forged in the cores of stars, so planets and stars that formed more recently possess more of those elements.
Webb’s observations of The Accident confirm that silane can form in brown dwarf and planetary atmospheres. The fact that silane seems to be missing in other brown dwarfs and gas giant planets suggests that when oxygen is available, it bonds with silicon at such a high rate and so easily, virtually no silicon is left over to bond with hydrogen and form silane.
So why is silane in The Accident? The study authors surmise it is because far less oxygen was present in the universe when the ancient brown dwarf formed, resulting in less oxygen in its atmosphere to gobble up all the silicon. The available silicon would have bonded with hydrogen instead, resulting in silane.
“We weren’t looking to solve a mystery about Jupiter and Saturn with these observations,” said JPL’s Peter Eisenhardt, project scientist for the WISE (Wide-field Infrared Survey Explorer) mission, which was later repurposed as NEOWISE. “A brown dwarf is a ball of gas like a star, but without an internal fusion reactor, it gets cooler and cooler, with an atmosphere like that of gas giant planets. We wanted to see why this brown dwarf is so odd, but we weren’t expecting silane. The universe continues to surprise us.”
Brown dwarfs are often easier to study than gas giant exoplanets because the light from a faraway planet is typically drowned out by the star it orbits, while brown dwarfs generally fly solo. And the lessons learned from these objects extend to all kinds of planets, including ones outside our solar system that might feature potential signs of habitability.
“To be clear, we’re not finding life on brown dwarfs,” said Faherty. “But at a high level, by studying all of this variety and complexity in planetary atmospheres, we’re setting up the scientists who are one day going to have to do this kind of chemical analysis for rocky, potentially Earth-like planets. It might not specifically involve silicon, but they’re going to get data that is complicated and confusing and doesn’t fit their models, just like we are. They’ll have to parse all those complexities if they want to answer those big questions.”
More about WISE, Webb
A division of Caltech, JPL managed and operated WISE for NASA’s Science Mission Directorate. The mission was selected competitively under NASA’s Explorers Program managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. The NEOWISE mission was a project of JPL and the University of Arizona in Tucson, supported by NASA’s Planetary Defense Coordination Office.
For more information about WISE, go to:
https://www.nasa.gov/mission_pages/WISE/main/index.html
The James Webb Space Telescope is the world’s premier space science observatory, and an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
To learn more about Webb, visit:
https://science.nasa.gov/webb
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Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
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Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.
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Last Updated Sep 09, 2025 Related Terms
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6 Min Read Upcoming Launch to Boost NASA’s Study of Sun’s Influence Across Space
Soon, there will be three new ways to study the Sun’s influence across the solar system with the launch of a trio of NASA and National Oceanic and Atmospheric Administration (NOAA) spacecraft. Expected to launch no earlier than Tuesday, Sept. 23, the missions include NASA’s IMAP (Interstellar Mapping and Acceleration Probe), NASA’s Carruthers Geocorona Observatory, and NOAA’s SWFO-L1 (Space Weather Follow On-Lagrange 1) spacecraft.
The three missions will launch together aboard a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center in Florida. From there, the spacecraft will travel together to their destination at the first Earth-Sun Lagrange point (L1), around one million miles from Earth toward the Sun.
The missions will each focus on different effects of the solar wind — the continuous stream of particles emitted by the Sun — and space weather — the changing conditions in space driven by the Sun — from their origins at the Sun to their farthest reaches billions of miles away at the edge of our solar system. Research and observations from the missions will help us better understand the Sun’s influence on Earth’s habitability, map our home in space, and protect satellites and voyaging astronauts and airline crews from space weather impacts.
The IMAP and Carruthers missions add to NASA’s heliophysics fleet of spacecraft. Together, NASA’s heliophysics missions study a vast, interconnected system from the Sun to the space surrounding Earth and other planets to the farthest limits of the Sun’s constantly flowing streams of solar wind. The SWFO-L1 mission, funded and operated by NOAA, will be the agency’s first satellite designed specifically for and fully dedicated to continuous, operational space weather observations.
Mapping our home in space: IMAP
The IMAP mission will study the heliosphere, our home in space.
NASA/Princeton University/Patrick McPike As a modern-day celestial cartographer, IMAP will investigate two of the most important overarching issues in heliophysics: the interaction of the solar wind at its boundary with interstellar space and the energization of charged particles from the Sun.
The IMAP mission will principally study the boundary of our heliosphere — a huge bubble created by the solar wind that encapsulates our solar system — and study how the heliosphere interacts with the local galactic neighborhood beyond. The heliosphere protects the solar system from dangerous high-energy particles called galactic cosmic rays. Mapping the heliosphere’s boundaries helps scientists understand our home in space and how it came to be habitable.
“IMAP will revolutionize our understanding of the outer heliosphere,” said David McComas, IMAP mission principal investigator at Princeton University in New Jersey. “It will give us a very fine picture of what’s going on out there by making measurements that are 30 times more sensitive and at higher resolution than ever before.”
The IMAP mission will also explore and chart the vast range of particles in interplanetary space. The spacecraft will provide near real-time observations of the solar wind and energetic particles, which can produce hazardous conditions not only in the space environment near Earth, but also on the ground. The mission’s data will help model and improve prediction capabilities of the impacts of space weather ranging from power-line disruptions to loss of satellites.
Imaging Earth’s exosphere: Carruthers Geocorona Observatory
An illustration shows the Carruthers Geocorona Observatory spacecraft. NASA/BAE Systems Space & Mission Systems The Carruthers Geocorona Observatory, a small satellite, will launch with IMAP as a rideshare. The mission was named after Dr. George Carruthers, creator of the Moon-based telescope that captured the first images of Earth’s exosphere, the outermost layer of our planet’s atmosphere.
The Carruthers mission will build upon Dr. Carruthers’ legacy by charting changes in Earth’s exosphere. The mission’s vantage point at L1 offers a complete view of the exosphere not visible from the Moon’s relatively close distance to Earth. From there, it will address fundamental questions about the nature of the region, such as its shape, size, density, and how it changes over time.
The exosphere plays an important role in Earth’s response to space weather, which can impact our technology, from satellites in orbit to communications signals in the upper atmosphere or power lines on the ground. During space weather storms, the exosphere mediates the energy absorption and release throughout the near-Earth space environment, influencing strength of space weather disturbances. Carruthers will help us better understand the fundamental physics of our exosphere and improve our ability to predict the impacts of the Sun’s activity.
“We’ll be able to create movies of how this atmospheric layer responds when a solar storm hits, and watch it change with the seasons over time,” said Lara Waldrop, the principal investigator for the Carruthers Geocorona Observatory at the University of Illinois at Urbana-Champaign.
New space weather station: SWFO-L1
SWFO-L1 will provide real-time observations of the Sun’s corona and solar wind to help forecast the resulting space weather.
NOAA/BAE Systems Space & Mission Systems Distinct from NASA’s research satellites, SWFO-L1 will be an operational satellite, designed to observe solar activity and the solar wind in real time to provide critical data in NOAA’s mission to protect the nation from environmental hazards. SWFO-L1 will serve as an early-warning beacon for potentially damaging space weather events that could impact our technology on Earth. SWFO-L1 will observe the Sun’s outer atmosphere for large eruptions, called coronal mass ejections, and measure the solar wind upstream from Earth with a state-of-the-art suite of instruments and processing system.
This mission is the first of a new generation of NOAA space weather observatories dedicated to 24/7 operations, working to avoid gaps in continuity.
“SWFO-L1 will be an amazing deep-space mission for NOAA,” said Dimitrios Vassiliadis, SWFO program scientist at NOAA. “Thanks to its advantageous location at L1, it will continuously monitor the solar atmosphere while measuring the solar wind and its interplanetary magnetic fields well before it impacts Earth — and transmit these data in record time.”
With SWFO-L1’s enhanced performance, unobstructed views, and minimal delay between observations and data return, NOAA’s Space Weather Prediction Center forecasters will give operators improved lead time required to take precautionary actions that protect vital infrastructure, economic interests, and national security on Earth and in space.
By Mara Johnson-Groh
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Sep 04, 2025 Related Terms
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Patricia White is a contracting officer at NASA’s Stennis Space Center, where she contributes to NASA’s Artemis program that will send astronauts to the Moon to prepare for future human exploration of Mars. NASA/Danny Nowlin When NASA’s Artemis II mission launches in 2026, it will inspire the world through discovery in a new Golden Age of innovation and exploration.
It will be another inspiring NASA moment Patricia White can add to her growing list.
White supports the Artemis program to send astronauts to the Moon to prepare for future human exploration of Mars as a contracting officer at NASA’s Stennis Space Center near Bay St. Louis, Mississippi.
White takes special pride in the test operations contract she helped draft. The contract provides support to the Fred Haise Test Stand, which tests the RS-25 engines that will help power NASA’s SLS (Space Launch System) rocket on Artemis missions.
“I was awestruck the first time I witnessed an engine test,” White said. “I remember how small I felt in comparison to this big and fascinating world, and I wondered what that engine would see that I would never be able to see.”
Four RS-25 engines tested at NASA Stennis will help launch Artemis II with four astronauts to venture around the Moon. As the first crewed Artemis mission, it will represent another milestone for the nation’s human space exploration effort.
From Interstate Signs to NASA Career
White describes NASA Stennis as a hidden gem. Growing up in nearby Slidell, Louisiana, she had driven by the interstate signs pointing toward NASA Stennis her entire life.
When she heard about a job opportunity at the center, she immediately applied. Initially hired as a contractor with only a high school diploma in February 2008, White found her motivation among NASA’s ranks.
“I work with very inspiring people, and it only took one person to say, ‘You should go to college’ to give me the courage to go so late in life,” she said.
Hard But Worth It
White began college classes in her 40s and finished at 50. She balanced a marriage, full-time job, academic studies, and household responsibilities. When she started her educational journey, her children were either toddlers or newborns. They were growing up as she stayed in school for nine years while meeting life’s challenges.
“It was hard, but it was so worth it,” she said. “I love my job and what I do, and even though it is crazy busy, I look forward to working at NASA every single day.”
She joined NASA officially in 2013, going from contractor to civil servant.
Setting an Example
White’s proudest work moment came when she brought home the NASA Early Career Achievement award and medal. It served as a tangible symbol of her success she could share with her family.
“It was a long road from being hired as an intern, and we all made extraordinary sacrifices,” she said. “I wanted to share it with them and set a good example for my children.”
As Artemis II prepares to carry humans back to lunar orbit for the first time in over 50 years, White takes pride knowing her work helps power humanity’s return to deep space exploration. Her work is proof that sometimes the most important journeys begin right in one’s own backyard.
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Explore This Section Earth Earth Observer Editor’s Corner Feature Articles Meeting Summaries News Science in the News Calendars In Memoriam Announcements More Archives Conference Schedules Style Guide 21 min read
A Decade of Global Water Cycle Monitoring: NASA Soil Moisture Active Passive Mission
Introduction
The NASA Soil Moisture Active Passive (SMAP) mission, launched in 2015, has over 10 years of global L-band radiometry observations. The low frequency [1.4 GHz frequency or 21 cm (8 in) wavelength] measurements provide information on the state of land surfaces in all weather conditions – regardless of solar illumination. A principal objective of the SMAP mission is to provide estimates of surface soil moisture and its frozen or thawed status. Over the land surface, soil moisture links the water, energy, and carbon cycles. These three cycles are the main drivers of regional climate and regulate the functioning of ecosystems.
The achievement of 10 years in orbit is a fitting time to reflect on what SMAP has accomplished. After briefly discussing the innovative measurement approach and the instrument payload (e.g., a radiometer and a regrettably short-lived L-band radar), a significant section of this article is devoted to describing the mission’s major scientific achievements and how the data from SMAP have been used to serve society (e.g., applied sciences) – including SMAP’s pathfinding role as Early Adopters. This content is followed by a discussion of how SMAP has dealt with issues related to radio frequency interference in the L-Band region, a discussion of the SMAP data products suite, future plans for the SMAP active–passive algorithm, and a possible follow-on L-band global radiometry mission being developed by the European Union’s Copernicus Programme that would allow for data continuity beyond SMAP. This summary for The Earth Observer is excerpted from a longer and more comprehensive paper that, as of this article’s posting, is being prepared for publication in the Proceedings of the Institute of Electrical and Electronics Engineers (IEEE).
SMAP Measurement Approach and Instruments
The SMAP primary and operating instrument is the L-band radiometer, which collects precise surface brightness temperature data. The radiometer includes advanced radio frequency interference (RFI) detection and mitigation hardware and software. The radiometer measures vertical and horizontal polarization observations along with the third and fourth Stokes parameters (T3 and T4) of the microwave radiation upwelling from the Earth. The reflector boom and assembly, which includes a 6 m (20 ft) deployable light mesh reflector, is spun at 14.6 revolutions-per-minute, which creates a 1000 km (621 mi) swath as the SMAP satellite makes its Sun-synchronous orbit of the Earth – see Figure 1. This approach allows coverage of the entire globe in two to three days with an eight-day exact repeat. The radiometer instrument is calibrated monthly by pointing it to the deep sky.
Figure 1. An artist’s rendering of the SMAP Observatory showing both the radiometer and radar. Figure credit: NASA/Jet Propulsion Laboratory/California Institute of Technology The original SMAP instrument design included a companion L-band radar, which operated from April through early July 2015, acquiring observations of co- and cross-polarized radar backscatter at a spatial resolution of about 1 km (0.6 mi) with a temporal revisit of about three days over land. This data collection revealed the dependence of L-band radar signals on soil moisture, vegetation water content, and freeze thaw state. The radar transmitter failed on July 7, 2015. Shortly thereafter, the radar receiver channels were repurposed to record the reflected signals from the Global Navigation Satellite System (GNSS) constellation in August 2015, making SMAP the first full-polarimetric GNSS reflectometer in space for the investigation of land surface and cryosphere.
Scientific Achievements from a Decade of SMAP Data
A decade of SMAP soil moisture observations have led to a plethora of scientific achievements. The data have been used to quantify the linkages of the three main metabolic cycles (e.g., carbon, water, and energy) on land. They have also been used to improve drought assessments and flood prediction as well as the accuracy of numerical weather prediction (NWP) models. They are also used to measure liquid water and thickness of ice sheets, and sea surface salinity. The subsections that follow describe how SMAP data are being put to use in myriad ways that benefit society.
Quantifying Processes that Link the Terrestrial Water, Energy, and Carbon Cycles
The primary SMAP science goal is to develop observational benchmarks of how the water, energy, and carbon cycles link together over land. Soil moisture is the variable state of the land branch of the water cycle. It links the water cycle to the energy cycle through limiting latent heat flux – the change in energy as heat exchanges when water undergoes a phase change, such as evapotranspiration at the land–atmosphere interface. Soil moisture also links the water and carbon cycles, which is evident through plant photosynthesis. SMAP global observations of soil moisture fields, in conjunction with remote sensing of elements of the energy and carbon cycles, can reveal how these three cycles are linked in the real world as a benchmark for weather and Earth system models.
Photosynthesis is down-regulated by both the deficit in water availability and the lack of an adequate amount of photosynthetically active radiation. Global maps reveal how soil moisture and light regulate photosynthesis – see Figure 2. These benchmark observational results can be used to assess how Earth system models link to the three main metabolic cycles of the climate system.
Figure 2. Observed regulation of photosynthesis by water availability [left] and light availability [right]. Blue denotes greater limitation. Photosynthesis rates for both maps determined using solar-induced fluorescence (SIF) measurements (mW/m2 nm sr) from the Tropospheric Ozone Monitoring Instrument (TROPOMI) on the European Union’s Copernicus Sentinel-5P mission. Water availability was determined using soil moisture (SM) measurements from the Soil Moisture Active Passive (SMAP) mission. Light availability was determined using measurements of photosynthetically active radiation (PAR) from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua platforms. The resulting maps show the model slope (mW/m2/nm/sr) of the estimated SIF-SM relationship in the water-limited regime [left] and the model slope (10-3/nm/sr) of estimated SIF-PAR relationship in the light-limited regime [right]. Figure credit: Jonard et al (2022) in Biogeosciences Development of Improved Flood Prediction and Drought Monitoring Capability
SMAP products have also been widely used in applied sciences and natural hazard decision-support systems. SMAP’s observation-based soil moisture estimates offer transformative information for managing water-related natural hazards, such as monitoring agricultural drought – defined as a persistent deficit in soil moisture – and flood volumes – defined as the landscape’s water absorption capacity during precipitation events. The SMAP project produces a parallel, near-real-time data stream that is accessed by a number of federal and state agencies in decision-support systems related to drought monitoring, food security, and landscape inundation and trafficability.
Enhancing Weather and Climate Forecasting Skill
SMAP’s enhancement of numerical weather prediction, model skill, and reduction of climate model projection uncertainties is based on the premise of the contribution of solar energy to weather and climate dynamics. Soil moisture has a strong influence on how available solar energy is partitioned into components (e.g., sensible heat flux versus latent heat flux) over land. The influence propagates through the atmospheric boundary layer and ultimately influences the evolution of weather.
To give an example, land surface processes can affect the evolution of the U.S. Great Plains low-level jets (GPLLJs). These jets drive mesoscale convective weather systems. Previous studies have shown that GPLLJs are sensitive to regional soil moisture gradients. Assimilation of SMAP soil moisture data improves forecasts of weakly synoptically forced or uncoupled GPLLJs compared to forecasts of cyclone-induced coupled GPLLJs. For example, the NASA Unified Weather Research and Forecasting Model, with 75 GPLLJs at 9 km (5.6 mi) resolution both with and without SMAP soil moisture data assimilation [SMAP data assimilation (DA) and no-DA respectively], shows how the windspeed mean absolute difference between SMAP DA and no-DA increase approximately linearly over the course of the simulation with maximum differences at 850 hPa (or mb) for the jet entrance and core – see Figure 3.
Figure 3. The impact of adding soil moisture data [SMAP data assimilation (DA) minus no-DA] to a model simulation from theNASA Unified Weather Research and Forecasting Model (NU-WRF)) of the Great Plains Low Level Jet (GPLLJ). The results show the mean over 75 independent GPLLJ events. The plots correspond to wind speed difference with height (y-axis) and time (hours on x-axis). The panels are for jet entrance [left], jet core [middle] and jet exit [right]. Soil moisture data assimilation enhances the intensity of the simulated GPLLJ. The stippling corresponds to 99% statistical confidence. Figure credit: Ferguson (2020) in Monthly Weather Review Measuring Liquid Water Content and Thickness of Ice Sheets
The mass loss of Greenland and Antarctica ice sheets contributes to sea-level rise – which is one of the most impactful and immediate damaging consequences of climate change. The melt rates over the last few years have raised alarm across the globe and impact countries with coastal communities. The cryosphere community has raised a call-to-action to use every observing system and model available to monitor the patterns and rates of land ice melt.
Surface melt affects the ice cap mass loss in many ways: the direct melt outflow from the ablation zone of the Greenland ice sheet, the structural change of the percolation zone of the Greenland ice sheet, changes in the melt water retention and outflow boundaries, changes in the structure of the Antarctic ice shelves, and destabilization of the buttressing of the glacier outflow through various processes (e.g., hydrofracturing and calving). The long-term climate and mass balance models rely on accurate representation of snow, firn, and ice processes to project the future sea level.
The SMAP L-band radiometer has relatively long wavelength [21 cm (8 in)] observations compared to other Earth-observing instruments. It enables the measurement of liquid water content (LWC) in the ice sheets and shelves as it receives the radiation from the deep layers of the snow/firn/ice column. Relatively high LWC values absorb the emission only partially, making the measurement sensitive to different liquid water amounts (LWA) in the entire column. Figure 4 shows the cumulative LWA for 2015–2023 based on SMAP measurements.
Figure 4. Total annual sum of SMAP daily liquid water amount (LWA) for 2015–2023. The black solid line on each map represents grid edges, and the grey color mask inside the ice sheet indicates melt detections by decreasing brightness temperature. Figure Credit: Andreas Colliander [Finnish Meteorological Institute]. The SMAP L-band radiometer has also been used to derive the thickness of thin sea ice [Soil Moisture and Ocean Salinity (SMOS) mission have been recalibrated to SMAP, using the same fixed incidence angle. The data show strong agreement and demonstrate clear benefits of a combined dataset. The L-band thin ice thickness retrievals provide a useful complement to higher-resolution profiles of thicker ice obtained from satellite altimeters (e.g. ESA’s CryoSat-2 and NASA’s Ice, Clouds and land Elevation Satellite–2 missions).
Extending and Expanding the Aquarius Sea Surface Salinity Record
The joint NASA/Argentinian Aquarius/Satélite de Aplicaciones Científicas (SAC)-D (Aquarius), which operated from 2011–2015, used an L-band radiometer and an L-band scatterometer to make unprecedented monthly maps of global sea surface salinity at 150-km (93-mi) resolution. The SMAP L-band radiometer has not only extended the sea surface salinity record in the post-Aquarius period, it has also increased the spatial resolution and temporal frequency of these measurements because of its larger reflector and wider swath. The increased resolution and revisit allow new and unprecedented perspectives into mixing and freshwater events, coastal plume tracking, and other more local oceanic features.
Providing New Perspectives on Global Ecology and Plant Water Stress
The L-band vegetation optical depth (VOD) – which is related to water content in vegetation – has been retrieved simultaneously with soil moisture using SMAP’s dual-polarized brightness temperatures and is being used to better understand global ecology. Water in above-ground vegetative tissue attenuates and thus depolarizes surface microwave emission, and VOD quantifies this effect. SMAP can provide global observations of VOD in all weather conditions with a two to three day temporal frequency. Changes in VOD indicate either plant rehydration or growth. Ecologists benefit from this new ecosystem observational data, which augments optical and near-infrared vegetation indices [e.g., leaf area index (LAI)] and has a higher temporal frequency that is not affected by clouds and does not saturate as rapidly for dense vegetation.
Examples of how the data have been used include deciphering the conditions when vegetation uptakes soil water only for rehydration (i.e., VOD increase with no LAI change) compared to plant growth (i.e., increase in both VOD and LAI). The applications of VOD are increasing and the ecology community views this product as a valuable additional perspective on soil–plant water relations.
At the moment, this measurement has no ground-based equivalent. Therefore, field experiments with airborne instruments and ground sampling teams are needed to firmly establish the product as a new observational capability for global ecology.
Applied Science Collaboration: SMAP Observations Serving Society
The SMAP project has worked with the NASA Earth Science Division Applied Sciences Program (now known as Earth Science to Action) and the natural hazards monitoring and forecasting communities for pre- and post-launch implementation of SMAP products in their operations. In some operational applications, for which long-term data continuity is a requirement, the SMAP data are still used for assessment of current conditions, as well as research and development.
The Original Early Adopters
Prior to its launch, the SMAP mission established a program to explore and facilitate applied and operational uses of SMAP mission data products in decision-making activities for societal benefit. To help accomplish these objectives, SMAP was the first NASA mission to create a formal Applications Program and an Early Adopter (EA) program, which eventually became a requirement for all future NASA Earth Science directed satellite missions. SMAP’s EA program increases the awareness of mission products, broadens the user community, increases collaboration with potential users, improves knowledge of SMAP data product capabilities, and expedites the distribution and uses of mission products after launch.
SMAP Data in Action
Several project accomplishments have been achieved primarily through an active continuous engagement with EAs and operational agencies working towards national interests. SMAP soil moisture data have been used by the U.S. Department of Agriculture (USDA) for domestic and international crop yield applications. For example the USDA’s National Agricultural Statistics Service (NASS) conducts a weekly survey of crop progress, crop condition, and soil moisture condition for U.S. cropland. NASS surveys and publishes state-level soil moisture conditions in the NASS Crop Progress Report.
The traditional field soil moisture survey is a large-scale, labor-intensive data collection effort that relies heavily on responses from farmers, agricultural extension agents and/or other domain experts for field observations. One weakness of these observations is that they are based on subjective assessments rather than quantitative measures and can lead to spatial inconsistency based on the human responses from the respective counties. Moreover, the NASS Crop Progress Reports do not provide specific geolocation information for the assessed soil moisture conditions – which are extremely useful metadata to provide to data users. NASS implemented the use of SMAP observations in their weekly reports during the growing period (March–November). SMAP maps estimated root-zone soil moisture for the week of November 14–20, 2022, over NASS Pacific (California and Nevada) and Delta (Arkansas, Mississippi and Louisiana) regional domains—see Figure 5.
Figure 5. SMAP-based soil moisture estimates for California, Nevada, Arkansas, Mississippi, and Louisiana, used by the U.S. Department of Agriculture’s (USDA) National Agricultural Statistics Service (NASS) in their weekly report covering November 14–20, 2022. These data are available for selected states at the NASS website linked in the text. Figure Credit: NASS SMAP Radio Frequency Interference Detection and Mitigation
Although SMAP operates within the protected frequency allocation of 1400–1427 MHz, the radiometer has been impacted by radio frequency interference over the mission lifetime. Unauthorized in-band transmitters as well as out-of-band emissions from transmitters operating adjacent to the allocated spectrum have been observed in SMAP measurements since its launch. The previously launched SMOS and Aquarius radiometers provide evidence of global RFI at L-band. Consequently, SMAP was designed to incorporate a novel onboard digital detector on the back end to enable detection and filtering of RFI. The radiometer produces science data in time and frequency, enabling the use of multiple RFI detection methods in the ground processing software.
On-orbit data demonstrate that the RFI detection and filtering performs well and improves the quality of SMAP brightness temperature measurements. The algorithms are most effective at filtering RFI that is sparse in time and frequency, with minimal impact on the noise equivalent delta temperature (NEDT) – a measure of the radiometer sensitivity. Some areas of the globe remain problematic as RFI that is very high level and persistent results in high percentages of data loss due to removal of contaminated data. A global map of RFI detection rate for January 2025 shows a large contrast between Eastern and Western Hemispheres and between Northern and Southern Hemispheres – see Figure 6. Regions of isolated RFI and severe RFI correspond to populated areas. A detection rate of 100% means all pixels are flagged and removed, resulting in data loss. Analysis of spectral information reveal many sources are likely terrestrial radar systems; however, many wideband, high-level sources and low-level, non-radar sources also persist. Over areas of geopolitical conflict, the time-frequency data show interference covering the entire radiometer receiver bandwidth.
Figure 6. Percentage of pixels on a 0.25° grid for January 2025 that have been flagged for removal by the Soil Moisture Active Passive radio frequency interference detection algorithms. Figure Credit: Priscilla N. Mohammed [GSFC] The RFI challenge is further addressed through official spectrum management channels and formal reports that include the geolocated coordinates of sources, interference levels, frequency of occurrence during the observed period, and spectral information – all of which aid field agents as they work to identify potential offenders. Reports are submitted to the NASA Spectrum office and then forwarded to the country of interest through the Satellite Interference Reporting and Resolution System.
SMAP Science Data Products
The current suite of SMAP science data products is available in the Table. The principal data products are grouped in four levels designated as L1–4. The L1 products are instrument L-band brightness temperature in Kelvin and include all four Stokes parameters (i.e., horizonal and vertical polarization as well as third and fourth Stokes). Both 6:00 AM equatorial crossing (descending) and 6:00 PM equatorial crossing (ascending data) are contained in the products. The user has access to quality flags of the conditions under which measurements are available for each project. The L1B products are time-ordered and include fore and aft measurements. L1C products are on the Equal-Area Scalable Earth V2 (EASE2) grid with polar and global projections. L2 data products are geophysical retrievals (i.e., soil moisture, VOD, and binary freeze/thaw classification on a fixed Earth grid). The L2 half-orbit products are available to the public within a day of acquisition. L3 products are daily composites and include all half-orbits for that day.
The SMAP project also produces L4 data that are the result of data assimilation. The L4 products take advantage of other environmental observations, such as precipitation, air temperature and humidity, radiative fluxes at the land surface, and ancillary land use and soil texture information, to produce estimates of surface [nominally 0–5 cm (0–2 in)] and subsurface (e.g., root-zone up to a meter) soil moisture. The data assimilation system is a merger of model and measurements and hence resolves the diurnal cycle of land surface conditions. The data assimilation system also provides estimates of surface fluxes of carbon, energy, and water, such as evaporation, runoff, gross primary productivity (GPP), and respiration. The difference between GPP and respiration is the net ecosystem exchange, which is the net source/sink of the carbon cycle over land.
The SMAP suite of products also include near-real-time (NRT) brightness temperature and soil moisture products for use in operational weather forecast applications. The NRT product targets delivery to users within three hours of measurement acquisition. The NRT uses predicted SMAP antenna pointing (instead of telemetry) and model predicted ancillary data (soil temperature) in order to support operational centers that require more than three hours of data products for updating weather forecast models. To date SMAP has met its required and target (for NRT) latency requirements.
Two other data projects merge synergistically with other (colocated) satellite measurements. The SPL2SMAP_S merges SMAP L-band radio brightness measurements with C-band synthetic aperture radar (SAR) measurements from the ESA Copernicus Sentinel-1 mission. The SAR data have high resolution and allow the generation of 1 and 3 km (0.62 and 1.8 mi) merged surface soil moisture estimates. The high resolution soil moisture information, however, is only available when there is coincident SMAP and Sentinel-1 measurements. The refresh rate of this product is limited and can be as long as 12 days.
The merged SMOS–SMAP passive L-band radiometry data allows the generation of global, near daily surface soil moisture estimates, which are required to resolve fast hydrologic processes, such as gravity drainage and recharge flux. These parameters are only partially resolved with the SMAP, with a two to three day data refresh rate. This product interpolates the multi-angular SMOS data to the SMAP 40º incident angle and uses all SMAP algorithms, including correction of waterbody impact on SMAP brightness temperature, and ancillary data for geophysical inversions to soil moisture and VOD, ensuring consistency. The combined SMAP–SMOS data product may not be available daily across locations, such as Japan, parts of China, and the Middle East, where RFI affects data collection.
Table. Soil Moisture Active Passive suite of science products are available through the National Snow and Ice Data Center, one of NASA’s Distributed Active Archive Centers.
Product Type Product description Resolution (Gridding) Granule Extent SPL1BTB Geolocated, calibrated brightness temperature in time order 36 km Half Orbit SPL1CTB_E Backus-Gilbert interpolated, calibrated brightness temperature in time order (9 km) Half Orbit SPL1CTB Geolocated, calibrated brightness temperature on Equal-Area Scalable Earth V2 (EASE2) grid 36 km Half Orbit SPL1CTB_E Backus-Gilbert interpolated, calibrated brightness temperature on EASE2 grid (9 km) Half Orbit SPL2SMP Radiometer soil moisture and vegetation optical depth 36 km Half Orbit SPL2SMP_E Radiometer soil moisture and vegetation optical depth based on SPL1CTB (9 km) Half Orbit SPL2SMAP_S SMAP radiometer/Copernicus Sentinel-1 soil moisture 3 km Sentinel-1 SPL3SMP Daily global composite radiometer soil moisture and vegetation optical depth based on SPL1CTB 36 km Daily–Global SPL3SMP_E Daily global composite radiometer soil moisture and vegetation optical depth based on SPL1CTB_E (9 km) Daily–Global SPL3FTP Daily composite freeze/thaw state based on SPL1CTB 36 km Daily–Global SPL3FTP_E Daily composite freeze/thaw state based on SPL1CTB_E (9 km) Daily–Global SPL4SMAU Surface and Root Zone soil moisture 9 km 3 hours – Global SPL4CMDL Carbon Net Ecosystem Exchange 9 km Daily–Global SPL1BTB_NRT Near Real Time Geolocated, calibrated brightness temperature in time order 36 km Half Orbit SPL2SMP_NRT Near Real Time Radiometer soil moisture 36 km Half Orbit L2/L3 SMOS SM SMOS soil moisture and VOD based on SMAP algorithms (9 km) Half Orbit/Daily Global Future Directions for the SMAP Active–Passive Algorithm
Although the SMAP radar failed not long after launch, the data that were collected have been used to advance the development of the SMAP Active–Passive (AP) algorithm, which will be applied to the combined SMAP radiometer data and radar data from the NASA–Indian Space Research Organisation (ISRO) Synthetic Aperture Radar [NISAR] mission, a recently-launched L-Band Synthetic Aperture mission to produce global soil moisture at a spatial resolution of 1 km (0.62 mi) or better. The high resolution product can advance applications of SMAP data (e.g., agricultural productivity, wildfire, and landslide monitoring).
Data Continuity Beyond SMAP
A forthcoming mission meets some – but not all – of the SMAP measurement requirements and desired enhancements. The European Union’s Copernicus Program Copernicus Imaging Microwave Radiometer (CIMR) mission is a proposed multichannel microwave radiometry observatory that includes L-band and four other microwave channels sharing a large mesh reflector. The mesh reflector is similar to the one that is used on SMAP, but larger. The successful SMAP demonstration of rotating large deployable mesh antennas for Earth observations has been useful to the CIMR design.
In terms of RFI detection capability, CIMR will also use an approach that is similar to SMAP. With regard to instrument thermal noise (NEDT) and data latency, CIMR meets or comes close to the next-mission desired characteristics and equals or exceeds SMAP in most of the attributes. The native L-band resolution of CIMR is ~60 km (37 mi); however, the measurements are coincident and higher-resolution measurements in this configuration allow reconstruction of L-band radiometry at higher resolution than CIMR’s L-band. It may be possible to combine the L- and C-bands and achieve a reconstructed ~15 km (9 mi) L-band product based on the coincident and overlapping measurements. A refresh rate of one day is possible with the wide-swath characteristic of CIMR.
CIMR is currently in development; the first version, CIMR-1A, is expected to launch within this decade and the second version, CIMR-1B, in the mid 2030s. Since the Copernicus program supports operational activities (e.g., numerical weather prediction), the program includes plans for follow-on CIMR observatories so that the data record will be maintained without gaps in the future.
Conclusions
The SMAP mission was launched in 2015 and has produced over 10 years of science data. Because of its unique instrument and operating characteristics, the global low-frequency microwave radiometry with the SMAP observatory has resulted in surface soil moisture, vegetation optical depth, and freeze/thaw state estimates that outperform past and current products. The data have been widely used in the Earth system science community and also applied to natural hazards applications.
The Earth system science and application communities are actively using the decade-long, high-quality global L-band radiometry. The intensity and range of SMAP science data usage is evident in the number of peer-reviewed journal publications that contain SMAP or Soil Moisture Active Passive in their title or abstract and use SMAP data in the study (i.e., search: www.webofscience.com data-base). The authors acknowledge that many publications escape this particular query approach. Currently the bibliography includes over 1700 entries and over 20,000 citations spanning several elements of Earth system science, including hydrologic science and regional and global water cycle, oceanic and atmospheric sciences, cryosphere science, global ecology as well as microwave remote sensing technologies.
To Learn More About SMAP
A more comprehensive bibliography of studies published based on SMAP data products, a set of one-page SMAP science and applications highlights in standardized format, and SMAP project documents including assessment reports are all available online via the links provided.
Acknowledgements
The authors wish to acknowledge the contributions of the SMAP Science Team, the SMAP Algorithm Development Team, and the SMAP Project Office engineers and staff. All of these teams contribute to the ongoing SMAP science product generation and uses reported in this article.
Dara Entekhabi
Massachusetts Institute of Technology
darae@mit.edu
Simon Yueh
Jet Propulsion Laboratory/California Institute of Technology
simon.h.yueh@jpl.nasa.gov
Rajat Bindlish
NASA Goddard Space Flight Center
rajat.bindlish@nasa.gov
Mark Garcia
Jet Propulsion Laboratory/California Institute of Technology
mark.d.garcia@jpl.nasa.gov
Jared Entin
NASA Headquarters
jared.k.entin@nasa.gov
Craig Ferguson
NASA Headquarters
craig.r.ferguson@nasa.gov
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