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NASA's SMAP mission will monitor large swathes of Earth.

NASA's SMAP mission will monitor large swathes of Earth.


NASA's new soil moisture satellite could improve forecasts

Globally, soils hold a tiny fraction of Earth’s water. But that moisture is nevertheless a crucial quantity in water, carbon, and energy cycles: It determines how vulnerable regions are to drought and flood, how well plants grow and suck up atmospheric carbon, and how Earth heats up and cools off—a key driver for storms. Yet for the most part, soil moisture has been monitored by a sparse set of probes stuck in the ground. “The three biggest cycles in a climate model are being modeled with something that’s a complete fantasy,” says Dara Entekhabi, a hydrologist at the Massachusetts Institute of Technology in Cambridge.

That will change with NASA’s Soil Moisture Active Passive (SMAP) probe, a $916 million satellite due to launch on 29 January. A high priority among U.S. earth scientists, the mission will generate a global map of soil moisture every 2 to 3 days at a resolution of 10 kilometers, helping improve weather forecasts, flood forecasts, and drought monitoring. “This is an important factor that people have been chasing from the earliest days of optical remote sensing,” says Entekhabi, the science team leader for the mission.

The satellite looks like a contraption out of a Dr. Seuss book: a dish antenna 6 meters across spins like a lasso atop a long boom. Soon after reaching orbit, the boom will extend and the dish will unfurl. The dish, made of a gold-plated molybdenum webbing, serves as a reflector for both of the satellite’s instruments: a radar and a radiometer. The radar works actively, by sending a pulse bouncing off the dish to Earth and then listening for the reflection. The radar has higher resolution, but has a harder time penetrating foliage than the radiometer, which detects radiation emitted from the soil itself (with poorer resolution but fewer interference problems).

Taken together, SMAP’s two instruments will represent a major improvement over the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) satellite, which was launched in 2009 with only a radiometer. Already, data from SMOS are being folded into the weather forecasts of the Meteorological Service of Canada and the European Centre for Medium-Range Weather Forecasts, says Matthias Drusch, the mission scientist at the European Space Research and Technology Centre in Noordwijk, the Netherlands. “Traditionally, they didn’t pay any attention to soil moisture,” he says.

But the resolution of SMOS is a rather coarse 30 to 50 kilometers, Drusch says, and the SMOS team has also struggled to eliminate interference from military and civilian radars, especially in China. SMAP avoids that issue by listening in several narrow radio frequency bands. Some bands may be contaminated by interference, but it is unlikely that all of them will be blocked at any given moment.

The data from SMAP will help improve drought forecasts from the U.S. Department of Agriculture, in addition to flood guidance issued by the National Weather Service. But scientists also have questions to answer. For instance, how does soil moisture affect the mechanics of rain storms? Storms require a combination of moisture in the atmosphere and convection to drive the storm clouds aloft. Evaporation from moist soils can lead to a phenomenon called recycled precipitation, but there is also a negative feedback, because evaporation cools Earth and inhibits the heat that drives convection. Storms may require alternating patches of wet and dry soil—one to increase atmospheric moisture, the other to increase convective energy—at length scales of 10 kilometers, and SMAP will have the resolution to test this idea, Entekhabi says.

The satellite is slated to launch on Thursday atop a Delta II rocket at Vandenberg Air Force Base in California. The launch window opens at 6:20 a.m. PT.