“Follow the water”: This theme has driven NASA’s Mars robotic exploration missions in the last 20 years, as part of the space agency’s efforts to understand the role of the life-enabling liquid in the physical processes that have shaped the Red Planet throughout its history. A similar understanding of the importance of water is the focus of another NASA mission, the Soil Moisture Active Passive, or SMAP, which will enable the global mapping of the moisture distribution and freeze-thaw state, not of the Martian soil but that of our home planet instead, in unprecedented detail allowing scientists to gain a better understanding of the ways that soil moisture affects Earth’s hydrological, energy, and carbon cycles.
Terrestrial ecosystems are greatly interconnected, forming the many different and totally interdependent parts of the Earth’s entire biosphere. All living organisms within them are directly affected by the many different elements of their environment, like water, air, and the various nutrients and minerals in the soil. One defining characteristic of the latter is its water content, better known as soil moisture, that is a key part of the Earth’s water cycle which in turn greatly affects the climate and overall environment of the many different ecosystems across the globe. Furthermore, soil moisture directly links our planet’s very complex water, energy, and carbon cycles together in a unique way. The soil at the top few inches of the Earth’s surface plays a very important role in the amount of water that evaporates back into the atmosphere, affects the growth rate of vegetation which in turn regulates the amount of atmospheric carbon consumption, and also plays an important factor in the overall absorption of the Sun’s energy that reaches the Earth’s surface and its reflection back into space that helps to regulate our planet’s surface temperature. In addition, soil moisture and the growth rate of vegetation are both greatly affected by the Earth’s annual water freeze-thaw cycles, which in turn have a direct global impact on many essential food production activities that sustain human life, like agriculture, farming, and crop yields during growing seasons.
Direct measurements of soil moisture distribution are presently obtained from networks of ground-based sensors, like the Soil Climate Analysis Network by the U.S. Department of Agriculture and NOAA’s Climate Reference Network, which unfortunately only allow for a very limited coverage of the Earth’s surface. Several space-based measurements have also been obtained by a few Earth-observing satellites as well, like the European Space Agency’s ERS-1, ERS-2, and the Soil Moisture and Ocean Salinity, or SMOS, missions, as well as the joint NASA/NOAA/Department of Defense WINDSAT. Nevertheless, these observations have either been a secondary science objective (WINDSAT, ERS-1 and 2), or have been of a relatively low sensitivity and spatial resolution of approximately 50 km (SMOS). The need for a dedicated soil moisture measurement mission which would provide global coverage and return higher-resolution data was eventually recognized in the first-ever Decadal Survey for Earth science that was released by the National Research Council in January 2007 and identified as one of the top priorities of NASA’s Earth Science Division for the next decade.
Answering to this call by the science community, NASA announced the SMAP mission a year later, in February 2008. In order to meet all of the mission’s requested science objectives that were laid out in the Decadal Survey in the most cost-effective manner, SMAP’s definition team implemented a science instrument architecture design that draw heavily on the heritage of a previous similar mission concept called Hydros, which had been cancelled in 2005 due to budgetary constrains. Based on this design, SMAP will utilize a single, large, 6-meter-wide, deployable gold mesh antenna that will be shared by the mission’s two science instruments: a synthetic aperture radar and a passive radiometer which will be operating in the L-band frequency range of the microwave spectrum between 1.20-1.41 and GHz, where the Earth’s atmosphere is highly transparent. Mounted on top of a moving platform that will be constantly spinning at a rate of 14 rpm, SMAP’s antenna will produce a conical-shaped scanning beam which will continuously image a large 1,000-km-wide swath of the surface as the satellite will be circling the Earth from its planned 685-km-high, high-inclination near-polar, Sun-synchronous orbit. This particular orbit orientation will allow SMAP to scan the top 5 centimeters of soil on the Earth’s surface in 2 to 3 days, providing scientists with a continuous global coverage of soil moisture distribution and freeze-thaw states throughout the satellite’s three-year primary mission.
The use of two different science instruments is necessitated by the fact that both have their respective advantages and drawbacks. For instance, the synthetic aperture radar will be able to achieve a high-resolution footprint of 1 to 3 kilometers but its measurements will be of a very low accuracy, whereas the passive radiometer will have a lower spatial resolution of approximately 40 km but its measurements will be of a much higher accuracy. By combining the different data sets that will be gathered by both of SMAP’s instruments, scientists will be able to create global soil moisture maps with a very high accuracy and an intermediate spatial resolution between 9 and 10 km—a sensitivity approximately 10 times greater than that of similar ground- and space-based sensors.
Having successfully completed its Critical Design Review in July 2012, the SMAP mission moved toward construction at the Jet Propulsion Laboratory’s Spacecraft Assembly Facility in Pasadena, Calif. Originally scheduled for launch on Nov. 5, 2014, from Vandenberg Air Force Base, Calif., the spacecraft underwent some further testing of several critical hardware components, which pushed back the launch date to early 2015. This additional testing and evaluation process was concluded earlier this month, when SMAP was delivered on Oct. 15 to Vandenberg’s Astrotech payload processing facility for final preparations prior to its launch on top of a United Launch Alliance Delta II rocket, which is currently scheduled for Jan. 29, 2015. “Water is vital for all life on Earth, and the water present in soil is a small but critically important part of Earth’s water cycle,” says Kent Kellogg, SMAP’s project manager at the Jet Propulsion Laboratory. “The delivery of NASA’s SMAP spacecraft to Vandenberg Air Force Base marks a final step to bring these unique and valuable measurements to the global science community.”
The data that SMAP will gather throughout its mission will help to address five primary science objectives, as outlined by the mission’s science definition team:
- Understand processes that link the terrestrial water, energy and carbon cycles
- Estimate global water and energy fluxes at the land surface
- Quantify net carbon flux in boreal landscapes
- Enhance weather and climate forecast skill
- Develop improved flood prediction and drought monitoring capabilities
Beyond their value to basic science research, as the last one of the mission’s science objectives indicates, SMAP’s data will greatly enhance flood and drought forecasts and will allow scientists to better assess the water availability in the soil on a global scale, which directly affects plant productivity and potential crop yield. “Scientists see tremendous potential in SMAP,” says Forrest Melton, a senior research scientist in the Ecological Forecasting Lab at NASA Ames Research Center in Moffett Field, Calif. “It is not going to provide field-level information, but it will give very useful new regional observations of soil moisture conditions, which will be important for drought monitoring and a wide range of applications related to agriculture. Having the ability provided by SMAP to continuously map soil moisture conditions over large areas will be a major advance.” One other application area that will potentially greatly benefit from SMAP’s measurements is that of human health and security. “Improved seasonal soil moisture forecasts using SMAP data will directly benefit famine early warning systems, particularly in sub-Saharan Africa and South Asia, where hunger remains a major human health factor and the population harvests its food from rain-fed agriculture in highly monsoonal (seasonal) conditions,” reads the mission’s Handbook which was released by JPL earlier this year. “In the temperate and extra-tropical latitudes, freeze/thaw measurements from SMAP will benefit environmental risk models and early warning systems related to the potential expansion of many disease vectors that are constrained by the timing and duration of seasonal frozen temperatures. SMAP will also benefit the emerging field of landscape epidemiology (aimed at identifying and mapping vector habitats for human diseases such as malaria) where direct observations of soil moisture and freeze/thaw status can provide valuable information on vector population dynamics.”
SMAP’s launch early next year will be the last one in a series of five NASA Earth science mission launches that will have occurred within a 12-month period, including the launch of the Global Precipitation Measurement (GPM) Core Observatory on Feb. 27, the Orbiting Carbon Observatory (OCO)-2 on July 2, the ISS-RapidScat on Sept. 21, and the upcoming launch of the Cloud-Aerosol Transport System, or CATS, which is currently scheduled for Dec. 9. These missions join a network of a total of 17 missions that are already operating or are under development by NASA as part of the agency’s Earth Systematic Missions Program. “The nature of Earth science is such that it takes many different kinds of measurements, done in many different ways in order to start getting a comprehensive picture of our planet and how it works,” said Sam Thurman, Deputy Project Manager for SMAP, during a public lecture at JPL. “SMAP is really one part of a larger suite of missions and programs that we hope will lead to a better understanding of our planet and some applications that can make our life better as well.”
Video Credit: Alaska Satellite Facility
Below are more pictures from SMAP’s delivery to Vandenberg Air Force Base and from the assembly of ULA’s Delta II launch vehicle: