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By NASA
Research Astrophysicist and Roman’s Deputy Wide Field Instrument Scientist – Goddard Space Flight Center
From a young age, Ami Choi — now a research astrophysicist at NASA — was drawn to the vast and mysterious. By the fifth grade, she had narrowed her sights to two career paths: marine biology or astrophysics.
“I’ve always been interested in exploring big unknown realms, and things that aren’t quite tangible,” Choi said. That curiosity has served her all throughout her career.
In addition to conducting research, Ami Choi shares science with the public at various outreach events, including tours at NASA’s Goddard Space Flight Center in Greenbelt, Md. This photo captures one tour stop, outside the largest clean room at Goddard.Credit: NASA/Travis Wohlrab As a student at University Laboratory High School in Urbana, Illinois, Choi gravitated toward astrophysics and was fascinated by things like black holes. She studied physics as an undergraduate at the University of Chicago, though she says math and physics didn’t necessarily come easily to her.
“I wasn’t very good at it initially, but I really liked the challenge so I stuck with it,” Choi said.
Early opportunities to do research played a pivotal role in guiding her career. As an undergraduate, Choi worked on everything from interacting galaxies to the stuff in between stars in our galaxy, called the interstellar medium. She learned how to code, interpret data, and do spectroscopy, which involves splitting light from cosmic objects into a rainbow of colors to learn about things like their composition.
After college, Choi read an article about physicist Janet Conrad’s neutrino work at Fermilab and was so inspired by Conrad’s enthusiasm and inclusivity that she cold-emailed her to see if there were any positions available in her group.
On October 14, 2023, Ami took a break from a thermal vacuum shift to snap a selfie with a partial eclipse. She was visiting BAE, Inc. in Boulder, Co., where the primary instrument for NASA’s Nancy Grace Roman Space Telescope was undergoing testing. Credit: Courtesy of Ami Choi “That one email led to a year at Fermilab working on neutrino physics,” Choi said.
She went on to earn a doctorate at the University of California, Davis, where she studied weak gravitational lensing — the subtle warping of light by gravity — and used it to explore dark matter, dark energy, and the large-scale structure of the universe.
Her postdoctoral work took Choi first to the University of Edinburgh in Scotland, where she contributed to the Kilo-Degree Survey, and later to The Ohio State University, where she became deeply involved in DES (the Dark Energy Survey) and helped lay the groundwork for the Nancy Grace Roman Space Telescope — NASA’s next flagship astrophysics mission.
“One of my proudest moments came in 2021, when the DES released its third-year cosmology results,” Choi said. “It was a massive team effort conducted during a global pandemic, and I had helped lead as a co-convener of the weak lensing team.”
Choi regularly presents information about NASA’s Nancy Grace Roman Space Telescope to fellow scientists and the public. Here, she gives a Hyperwall talk at an AAS (American Astronomical Society) meeting.Credit: Courtesy of Ami Choi After a one-year stint at the California Institute of Technology in Pasadena, where Choi worked on SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer)—an observatory that’s surveying stars and galaxies—she became a research astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. She also serves as the deputy Wide Field Instrument scientist for Roman. Choi operates at the intersection of engineering, calibration, and cosmology, helping translate ground-based testing into flight-ready components that will help Roman reveal large swaths of the universe in high resolution.
“I’m very excited for Roman’s commissioning phase — the first 90 days when the spacecraft will begin transmitting data from orbit,” Choi said.
Choi, photographed here in Death Valley, finds joy in the natural world outside of work. She cycles, hikes, and tends a small vegetable garden with a friend from grad school. Credit: Insook Choi (used with permission) She’s especially drawn to so-called systematics, which are effects that can alter the signals scientists are trying to measure. “People sometimes think of systematics as nuisances, but they’re often telling us something deeply interesting about either the physics of something like a detector or the universe itself,” Choi said. “There’s always something more going on under the surface.”
While she’s eager to learn more about things like dark energy, Choi is also looking forward to seeing all the other ways our understanding of the universe grows. “It’s more than just an end goal,” she said. “It’s about everything we learn along the way. Every challenge we overcome, every detail we uncover, is an important discovery too.”
For those who hope to follow a similar path, Choi encourages staying curious, being persistent, and taking opportunities to get involved in research. And don’t let the tricky subjects scare you away! “You don’t have to be perfect at math or physics right away,” she said. “What matters most is a deep curiosity and the tenacity to keep pushing through.”
By Ashley Balzer
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Sep 09, 2025 EditorAshley BalzerLocationGoddard Space Flight Center Related Terms
Goddard Space Flight Center Nancy Grace Roman Space Telescope People of Goddard View the full article
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By NASA
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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Last Updated Aug 18, 2025 Related Terms
Earth Science View the full article
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By NASA
5 Min Read What Are Asteroids? (Ages 14-18)
What are asteroids?
Asteroids are rocky objects that orbit the Sun just like planets do. In fact, sometimes asteroids are called “minor planets.” These space rocks were left behind after our solar system formed about 4.6 billion years ago.
Asteroids are found in a wide range of sizes. For example, one small asteroid, 2015 TC25, has a diameter of about 6 feet – about the size of a small car – while the asteroid Vesta is nearly 330 miles in diameter, almost as wide as the U.S. state of Arizona. Some asteroids even have enough gravity to have one or two small moons of their own.
There are more than a million known asteroids. Many asteroids are given names. An organization called the International Astronomical Union is responsible for assigning names to objects like asteroids and comets.
This illustration depicts NASA’s Psyche spacecraft as it approaches the asteroid Psyche. Once it arrives in 2029, the spacecraft will orbit the metal-rich asteroid for 26 months while it conducts its science investigation.NASA/JPL-Caltech/ASU What’s the difference between asteroids, meteors, and comets?
Although all of these celestial bodies orbit the Sun, they are not the same. Unlike asteroids, which are rocky, comets are a mix of dust and ice. Meteors are small space rocks that get pulled close enough to enter Earth’s atmosphere, where they either burn up as a shooting star or land on the ground as a meteorite.
What are asteroids made of?
Different types of asteroids are composed of different mixes of materials. Most of them are made of chondrites, which are combinations of materials such as rocks and clay. These are called “C-type” asteroids. Some, called “S-type,” are made of stony materials, while “M-type” asteroids are composed of metallic elements.
NASA’s Dawn spacecraft captured this image of Vesta as it left the giant asteroid’s orbit in 2012. The framing camera was looking down at the north pole, which is in the middle of the image.NASA/JPL-Caltech/UCLA/MPS/DLR/IDA How did the asteroids form?
Asteroids formed around the same time and in the same way as the planets in our solar system. A massive, dense cloud of gas and dust collapsed into a spinning disk, and the gravity in the disk’s center pulled more and more material toward it. Over time, these pieces repeatedly collided with each other, sometimes resulting in smaller fragments and other times clumping together, resulting in much bigger objects.
Objects with a lot of mass – like planets – produced enough gravity to pull themselves into spheres, but many smaller objects didn’t. These ended up becoming comets, small moons, and, yes, asteroids. Although some asteroids have a spherical shape, most have irregular shapes – sometimes oblong, bumpy, or jagged.
The main asteroid belt lies between Mars and Jupiter, and Trojan asteroids both lead and follow Jupiter. Scientists now know that asteroids were the original “building blocks” of the inner planets. Those that remain are airless rocks that failed to adhere to one another to become larger bodies as the solar system was forming 4.6 billion years ago.Credits: NASA, ESA and J. Olmsted (STScI) Where are asteroids found?
Most of the asteroids we know about are located in an area called the main asteroid belt, which is found in the space between Mars and Jupiter. But asteroids are found in other parts of the solar system, too.
Trojan asteroids orbit the Sun on the same orbital path as a planet. They’re found at two specific points on the planetary orbit called Lagrange points. At these points, the gravitational pull of the planet and the Sun are in balance, making these points gravity-neutral and stable. Many planets have been found to have Trojan asteroids, including Earth.
An asteroid’s location can also be influenced by the gravity of planets it passes and end up pushed or pulled onto a path that brings it close to Earth. When asteroids or comets are on an orbital path that comes within 30 million miles of Earth’s orbit, we call them near-Earth objects.
Illustration of NASA’s DART spacecraft and the Italian Space Agency’s (ASI) LICIACube, with images of the asteroids Dimorphos and Didymos obtained by the DART spacecraft.Credit: NASA/Johns Hopkins APL/Joshua Diaz Could an asteroid come close enough to hit Earth?
Yes! Throughout history, asteroids or pieces of asteroids have collided with Earth, our Moon, and the other planets, too. The effects of some of these impacts are still visible. For example, Chicxulub Crater was created 65 million years ago when a massive asteroid struck Mexico’s Yucatan Peninsula. The resulting cloud of dust and gas released into Earth’s atmosphere blocked sunlight, leading to a mass extinction that included the dinosaurs. More recently, in 2013, people in Chelyabinsk, Russia, witnessed an asteroid almost as wide as a tennis court explode in the atmosphere above them. That event produced a powerful shockwave that caused injuries and damaged structures.
This is why NASA’s Planetary Defense Coordination Office keeps a watchful eye on near-Earth objects. The Planetary Defense team relies on telescopes and observatories on Earth and in space to detect and monitor objects like these that could stray too close to our planet.
The agency is working on planetary defense strategies to use if an asteroid is discovered to be heading our way. For example, NASA’s DART (Double Asteroid Redirection Test) mission in 2022 was a first-of-its-kind test: an uncrewed spacecraft with an autonomous targeting system intentionally flew into the asteroid Dimorphos, successfully changing its orbit.
Jason Dworkin, OSIRIS-REx mission project scientist, holds up a vial containing part of the sample from asteroid Bennu in 2023.Credit: NASA/James Tralie How does NASA study asteroids?
NASA detects and tracks asteroids using telescopes on the ground and in space, radar observations, and computer modeling. The agency also has launched several robotic explorers to learn more about asteroids. Some missions study asteroids from above, such as the Psyche mission, launched in 2023 to study the asteroid Psyche beginning in 2029. Other missions have actually made physical contact with asteroids. For example, the DART mission mentioned above impacted an asteroid to change its orbit, and the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security – Regolith Explorer) spacecraft collected a sample of material from the surface of asteroid Bennu and delivered the sample to Earth in 2023 for scientists to study.
Career Corner
Want a career where you get to study asteroids? Here are some jobs at NASA that do just that:
Astronomer: These scientists observe and study planets, stars, and galaxies. Astronomers make discoveries that help us understand how the universe works and how it is changing. This job requires a strong educational background in science, math, and computer science. Geologist: Asteroids are made of different types of rock, clay, or metallic materials. Geologists study the properties and composition of these materials to learn about the processes that have shaped Earth and other celestial bodies, like planets, moons, and asteroids. More About Asteroids
Asteroid Facts
Gallery: What’s That Space Rock?
Center for Near Earth Object Studies
Planetary Defense at NASA
Asteroid Watch: Keeping an Eye on Near-Earth Objects
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By USH
In recent months, Earth has been experiencing a string of bizarre and unsettling phenomena. Massive power outages have struck Spain and Portugal, with similar blackouts occurring across the globe. Aircraft have inexplicably crashed or fallen from the sky. Lights - streetlamps, billboards, car headlights, even indoor lighting are flickering erratically, and the problem persists.
Power failures have disrupted air traffic control centers. Strange, unexplained noises have been heard coming from the sky. In parts of the U.S., blue rain has reportedly fallen. The Schumann Resonance, Earth’s natural electromagnetic frequency, has spiked dramatically. Most disturbing of all, now birds have been seen suddenly dropping dead, either mid-flight or while perched on power lines.
It feels as if the planet is enveloped in a powerful, unseen force, an invisible energy field swarming the Earth, disrupting both man-made and natural systems. But where is it coming from?
One theory suggests that we may be experiencing the delayed impact of a massive astronomical event that occurred thousands of years ago, such as a supernova, the cataclysmic explosion of a dying star. These cosmic blasts release enormous amounts of electromagnetic radiation, including gamma rays and X-rays, which can travel across space for thousands or even millions of years before reaching other celestial bodies, like Earth.
Interestingly, some scientists have speculated that a gamma-ray burst from a distant supernova might have triggered the Ordovician mass extinction around 440 million years ago. If such radiation can wipe out entire ecosystems, could a similar event be silently influencing the strange phenomena we're seeing today?
It might sound improbable, but what if Earth is now being bathed in residual energy from a long-past cosmic event, energy that is only just now arriving and interacting with our atmosphere and technology?
And if that's true… could these strange occurrences be the early signs of something even more serious to come?
Additional: MrMBB333, a well-known YouTuber, is also closely following these remarkable events. He shares daily live footage from around the world and often questions what is truly happening. In his latest video below he shares the mystery of the birds dropping dead while perched on power lines.
You can watch his videos on his YouTube channel: https://www.youtube.com/user/MrMBB333/videosView the full article
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By NASA
5 min read
How NASA Science Data Defends Earth from Asteroids
Artist’s impression of NASA’s DART mission, which collided with the asteroid Dimorphos in 2022 to test planetary defense techniques. Open science data practices help researchers identify asteroids that pose a hazard to Earth, opening the possibility for deflection should an impact threat be identified. NASA/Johns Hopkins APL/Steve Gribben The asteroid 2024 YR4 made headlines in February with the news that it had a chance of hitting Earth on Dec. 22, 2032, as determined by an analysis from NASA’s Center for Near Earth Object Studies (CNEOS) at the agency’s Jet Propulsion Laboratory in Southern California. The probability of collision peaked at over 3% on Feb. 18 — the highest ever recorded for an object of its size. This sparked concerns about the damage the asteroid might do should it hit Earth.
New data collected in the following days lowered the probability to well under 1%, and 2024 YR4 is no longer considered a potential Earth impactor. However, the event underscored the importance of surveying asteroid populations to reveal possible threats to Earth. Sharing scientific data widely allows scientists to determine the risk posed by the near-Earth asteroid population and increases the chances of identifying future asteroid impact hazards in NASA science data.
“The planetary defense community realizes the value of making data products available to everyone,” said James “Gerbs” Bauer, the principal investigator for NASA’s Planetary Data System Small Bodies Node at the University of Maryland in College Park, Maryland.
How Scientists Spot Asteroids That Could Hit Earth
Professional scientists and citizen scientists worldwide play a role in tracking asteroids. The Minor Planet Center, which is housed at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, collects and verifies vast numbers of asteroid and comet position observations submitted from around the globe. NASA’s Small Bodies Node distributes the data from the Minor Planet Center for anyone who wants to access and use it.
A near-Earth object (NEO) is an asteroid or comet whose orbit brings it within 120 million miles of the Sun, which means it can circulate through Earth’s orbital neighborhood. If a newly discovered object looks like it might be an NEO, information about the object appears on the Minor Planet Center’s NEO Confirmation Page. Members of the planetary science community, whether or not they are professional scientists, are encouraged to follow up on these objects to discover where they’re heading.
The asteroid 2024 YR4 as viewed on January 27, 2025. The image was taken by the Magdalena Ridge 2.4m telescope, one of the largest telescopes in NASA’s Planetary Defense network. Asteroid position information from observations such as this one are shared through the Minor Planet Center and NASA’s Small Bodies Node to help scientists pinpoint the chances of asteroids colliding with Earth. NASA/Magdalena Ridge 2.4m telescope/New Mexico Institute of Technology/Ryan When an asteroid’s trajectory looks concerning, CNEOS alerts NASA’s Planetary Defense Coordination Office at NASA Headquarters in Washington, which manages NASA’s ongoing effort to protect Earth from dangerous asteroids. NASA’s Planetary Defense Coordination Office also coordinates the International Asteroid Warning Network (IAWN), which is the worldwide collaboration of asteroid observers and modelers.
Orbit analysis centers such as CNEOS perform finer calculations to nail down the probability of an asteroid colliding with Earth. The open nature of the data allows the community to collaborate and compare, ensuring the most accurate determinations possible.
How NASA Discovered Risks of Asteroid 2024 YR4
The asteroid 2024 YR4 was initially discovered by the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey, which aims to discover potentially hazardous asteroids. Scientists studied additional data about the asteroid from different observatories funded by NASA and from other telescopes across the IAWN.
At first, 2024 YR4 had a broad uncertainty in its future trajectory that passed over Earth. As the planetary defense community collected more observations, the range of possibilities for the asteroid’s future position on Dec. 22, 2032 clustered over Earth, raising the apparent chances of collision. However, with the addition of even more data points, the cluster of possibilities eventually moved off Earth.
This visualization from NASA’s Center for Near Earth Object Studies shows the evolution of the risk corridor for asteroid 2024 YR4, using data from observations made up to Feb. 23, 2025. Each yellow dot represents the asteroid’s possible location on Dec. 22, 2032. As the range of possible locations narrowed, the dots at first converged on Earth, before skewing away harmlessly. NASA/JPL/CNEOS Having multiple streams of data available for analysis helps scientists quickly learn more about NEOs. This sometimes involves using data from observatories that are mainly used for astrophysics or heliophysics surveys, rather than for tracking asteroids.
“The planetary defense community both benefits from and is beneficial to the larger planetary and astronomy related ecosystem,” said Bauer, who is also a research professor in the Department of Astronomy at the University of Maryland. “Much of the NEO survey data can also be used for searching astrophysical transients like supernova events. Likewise, astrophysical sky surveys produce data of interest to the planetary defense community.”
How Does NASA Stop Asteroids From Hitting Earth?
In 2022, NASA’s DART (Double Asteroid Redirection Test) mission successfully impacted with the asteroid Dimorphos, shortening the time it takes to orbit around its companion asteroid Didymos by 33 minutes. Didymos had no chance of hitting Earth, but the DART mission’s success means that NASA has a tested technique to consider when addressing a future asteroid potential impact threat.
Artist’s impression of NASA’s upcoming NEO Surveyor mission, which will search for potentially hazardous near-Earth objects. The mission will follow open data practices to improve the chances of identifying dangerous asteroids. NASA/JPL-Caltech To increase the chances of discovering asteroid threats to Earth well in advance, NASA is working on a new space-based observatory, NEO Surveyor, which will be the first spacecraft specifically designed to look for asteroids and comets that pose a hazard to Earth. The mission is expected to launch in the fall of 2027, and the data it collects will be available to everyone through NASA archives.
“Many of the NEOs that pose a risk to Earth remain to be found,” Bauer said. “An asteroid impact has a very low likelihood at any given time, but consequences could be high, and open science is an important component to being vigilant.”
For more information about NASA’s approach to sharing science data, visit:
https://science.nasa.gov/open-science.
By Lauren Leese
Web Content Strategist for the Office of the Chief Science Data Officer
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Last Updated Apr 10, 2025 Related Terms
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