Satellite quartet to track most precious Earth resource

LOS ANGELES (AP) -- A successful launch next month of a nearly $1 billion satellite would mark the fourth spacecraft NASA has sent into orbit recently to follow the global movement of life's most precious resource: water.

The satellite Aqua will follow the Jason 1 and a pair of twin spacecraft called Grace, launched in December and March, respectively.

Although each is different, the missions are designed to help piece together the puzzle of how water moves between the Earth's atmosphere, oceans and land.

"Each one of them is a critical element in this great hydrologic cycle, which really sustains life on Earth," said William Patzert, a NASA research oceanographer and scientist on the Jason 1 mission.

Scientists hope the three missions will lead to more accurate weather forecasts, better advance notice of El Ninos and a clearer understanding of how human activity affects the world at large.

Water moves through the world at varying paces before returning to the oceans that cover 70 percent of the planet. That cycle drives both climate and weather, affecting in turn life and its every activity.

Water lasts just days in clouds as a vapor but weeks as a liquid in the world's rivers. As ice, it can remain locked in the polar caps for tens of thousands of years.

Monitoring water's movement -- where, how quickly and in what phase it moves -- requires a global perspective, something scientists hope the flotilla of Earth-orbiting satellites can provide.

"The whole idea of the Earth is it's a closed system. So if you don't take global measurements, you can't model the whole thing," said Martin Mohan, who oversaw Aqua's development for Redondo Beach, California-based satellite builder TRW Inc.

Aqua, Latin for "water," and its six instruments will look almost exclusively at the hydrologic cycle, focusing primarily on the atmosphere.

Although the atmosphere holds just a sliver of all the world's water, that vapor is the most important greenhouse gas. As such, it also represents the biggest unknown in gauging the effect of global warming and its impact on the hydrologic cycle, said Aqua project scientist Claire Parkinson.

"The hope is we will be able to get an indication of whether or not the cycling through the system is speeding up, staying steady or slowing down," he said. "The hypothesis is it might be speeding up."

Plans call for Aqua data to be plugged into daily weather forecasts, a first for NASA. It will also help keep tabs on droughts, hurricanes before they make landfall, and other markers that, together, suggest climate change.

The $952 million mission is scheduled for a May 2 launch from Vandenberg Air Force Base in California.

Jason 1, a joint U.S.-French mission, uses radar to measure the topography -- or shape -- of an ocean's surface. That data gives an indication of circulation patterns that move water and heat around the globe, influencing both evaporation and precipitation.

Even subtle differences in those patterns can have an enormous impact on the climate of any region of the world, Patzert said.

Grace, a joint U.S.-German mission, is the most unusual of the bunch. Its twin satellites measure tiny variations in the Earth's gravity field, including those caused by the shifts in mass caused by large-scale movements of water.

When the satellites become operational this summer, the mission will be sensitive enough to detect changes in mass due to the siphoning off of the Ogallala Aquifer, located east of the Rocky Mountains, and the melting of polar glaciers.

Combining data from Grace and Jason 1 will allow scientists to determine which variations in the surface of the oceans are due to lumps in the planet's gravity field and which are due to events within the ocean. They also hope to use the two missions to study any rise in sea levels.

Data from Grace and Aqua will also sort out how water in the Earth is split between soil moisture and aquifers deeper below the surface, which should allow for better management of the resource.

The overlap between the different, yet complementary missions, is "exactly the right way to view Earth system science," said Michael Watkins, the Grace project scientist.

"It's assembling several pieces of the puzzle together to see what's going on," he said.