NASA readies SMAP for launch to monitor soil moisture

CBS News

A NASA satellite is poised for launch Thursday on a three-year $916 million mission to measure the amount of water in the top 2 inches of soil around the world to help scientists understand its effects on weather, its role in climate change and to better predict and respond to droughts, floods and other environmental disasters.

Using a rotating 19.7-foot-wide mesh antenna to map out a 620-mile-wide swath as it orbits Earth's poles, the Soil Moisture Active Passive, or SMAP, satellite will use a powerful radiometer to "see" the moisture, liquid and frozen, in the top few inches of soil and a radar to improve the resolution, or scale, of the measurements.

The result will be a global map of soil moisture updated every two to three days, allowing near-real-time analysis of weather and sudden environmental changes around the world. It also will shed light on long-range changes to the planet's climate, helping scientists better understand the the role of ground moisture in the water, energy and carbon cycles that are critical to life.

An artist's concept showing NASA's SMAP satellite and the large offset antenna it will use to measure soil moisture in broad swaths that will be combined to make global maps every two to three days. (Credit: NASA)

"The fraction of water that's in soil is actually tiny, it's much less than 1 percent," said Dara Entekhabi, leader of the SMAP science team at the Massachusetts Institute of Technology. "About 97 percent of the water in the globe is locked up in the oceans and the rest is in the cryosphere, the ice.

"But that small percentage that's in the soil is rather important and very active, because it's what's interacting with the terrestrial biosphere, with the vegetation, it's what determines how much runoff occurs due to incident precipitation, how much fresh water there is in the rivers and lakes. It's a tiny amount, but a very important amount."

Case in point: the ongoing California drought. SMAP data will help scientists get a better understanding of the processes that contribute to such phenomena on a global scale.

"The measurements that SMAP makes will be direct measurements of the indicator of agricultural drought, which is the deficit in soil moisture," Entekhabi said. "So it will produce a high resolution ... map of the drought. But droughts are initiated, forced and maintained by much larger-scale processes, things such as the interaction between the oceans and the atmosphere over land and over continental regions, land and the atmosphere.

"So it's not just mapping the local California region, but seeing how the continent as a whole reinforce and feed back onto the climate system in order to make these things last beyond (short-duration) weather scales."

Perched atop a United Launch Alliance Delta 2 rocket, the 2,000-pound SMAP satellite, built at NASA's Jet Propulsion Laboratory, is scheduled for liftoff Thursday from Space Launch Complex 2 at Vandenberg Air Force Base, Calif., at 9:20 a.m. EST (GMT-5, 6:20 a.m. local time), the opening of a short three-minute launch window.

Forecasters predicted an 80 percent chance of good weather. If launch is delayed for any reason, the forecast improves to 90 percent "go" on Friday.

The Delta 2 will take off on a southerly trajectory, releasing SMAP into a 426-mile-high 98.5-minute orbit around Earth's poles. After the spacecraft's solar panels unfold, engineers will spend about two weeks checking out the satellite's systems and preparing to unfold the large dish antenna needed by the radar and radiometer instruments.

The fully deployed antenna dwarfs the SMAP satellite and engineers plan a very careful, step-by-step deployment. First, starting 16 days after launch, the antenna's boom assembly will be commanded to unfold.

"You'll notice the spacecraft looks somewhat like the tail wagging the dog with this very large antenna deployed with a very small spacecraft," said Kent Kellogg, SMAP project manager at JPLy. "So we want to make sure the spacecraft is behaving with this new mass distribution."

Then, starting about 20 days after launch, the satellite's antenna will be commanded to unfurl.

"It starts at 12 inches in diameter," Kellogg said. "This is passive release of the strain energy, that'll bloom the antenna out to about seven feet in diameter. Then we'll do the powered deployment, which will deploy the antenna out to its final 20-feet-in-diameter size. That process takes about 30 minutes to complete from start to finish."

To collect the desired soil moisture data, the antenna, which is tilted with respect to the satellite's body, must rotate, sweeping out a circular swath directly below the spacecraft. Starting about 50 days after launch, engineers will spin the antenna up to 4.5 rpm.

"As we begin to spin up, the spacecraft actually counter-rotates in the opposite direction," Kellogg said. "This is by design, and is a feature of the fact that we're trying to spin up a very large structure with a relatively small spacecraft. Once the antenna has reached a stable spin rate, the spacecraft attitude system will regain sun pointing very quickly and we continue on."

After confirming the stability of the satellite with its now-spinning antenna, the spin rate will be gradually increased to 14.6 rpm. The entire process will take two months to complete.

"With all the testing and work that's gone on behind us, we have a lot of confidence that this mission will meet both its technical and scientific objectives and will enjoy a long and productive life in space," Kellogg said.

SMAP will join 18 other operational NASA satellite and ground-based programs designed to study Earth's environment in unprecedented detail.

SMAP will focus "on the water that lives and moves through the soil," said Christine Bonniksen, NASA's program executive at agency headquarters in Washington. "With the launch of this project, decision makers will be better able to understand the water cycle and how soil moisture fits into that."

SMAP's radiometer will act somewhat like a camera, Entekhabi said, using L-band microwaves to "see" in daylight and darkness, penetrating clouds, vegetation and the top few inches of soil to measure water volume. The satellite's L-band radar, while less sensitive, will provide higher resolution.

SMAP is unique in that "it has returns in two very distinct areas," Entekhabi said. "One of them is in fundamental understanding of how the environment works, It's addressing some fundamental Earth science question. The second is in the arena of applications. SMAP provides data that affect our everyday lives in terms of dealing with some really serious natural hazards."

In terms of Earth science, he said soil moisture links the planet's water, energy and carbon cycles. "If it wasn't for the soil moisture variable, these three processes over land would vary independently, but they don't. They work in concert like gears in a clock, they are linked together through the soil moisture variable."

Understanding the details of those interactions will help scientists improve short- and long-term forecasting.

"As water evaporates from soil water to vapor in the atmosphere, it feeds the water cycle," Entekhabi said. "It takes energy to vaporize water, and water vaporizing cools the surface and maintains the temperature much like humans have evolved sweating to regulate body temperature. The same thing happens with Earth system. And as plants transpire and pick up biomass through absorbing carbon dioxide from the atmosphere and releasing water vapor, they are engaged in the water and energy cycles as well.

"So these three cycles are intimately linked through the water variable. Through measurements SMAP can make, we can test and improve models that we use for atmospheric weather prediction and climate change projections."