SAN FRANCISCO — From California’s Central Valley to the Tibetan Plateau, scientists are relying on the twin Gravity Recovery and Climate Experiment (GRACE) satellites to monitor changing water supplies, detect signs of drought and measure the impacts of global climate change. Unfortunately, the GRACE satellites, developed jointly by NASA and the German Aerospace Center, DLR, are 8 years old and scientists fear a lengthy delay between the end of the mission and a successor capable of offering comparable data.

“It’s a huge problem,” said Jay Famiglietti, an Earth science professor at the University of California, Irvine, who has been involved in the $130 million GRACE mission since its inception. “The satellite will die and it may be a solid decade before a GRACE follow-on mission could fly.”

In December, Famiglietti and NASA scientists attending the American Geophysical Union conference here presented data obtained by GRACE showing a dramatic decrease in the ground water in California’s Sacramento and San Joaquin river valleys, an area that covers 154,000 square kilometers and supplies $17 billion a year in agricultural crops. In August, researchers from the Goddard Space Flight Center in Greenbelt, Md., used GRACE to document declines in ground water levels in Northern India, home to 114 million people. Similarly, researchers are relying on GRACE to track water movement in the Mississippi River Basin, the Ogallala Aquifer in the Midwestern United States, Alaska and Tibet.

All these initiatives benefit from the unique ability of GRACE to provide a global picture of water moving across land, oceans and the atmosphere. Unlike other remote sensing satellites that use the light spectrum to gather data, GRACE measures changes in the Earth’s gravity field. The twin spacecraft, launched from Russia’s Plesetsk Cosmodrome aboard a three-stage Rockot vehicle in March 2002, circle the globe traveling approximately 220 kilometers apart. When the first satellite passes over a mountainous area, where the gravitational pull is stronger, it is pulled toward the region, moving slightly away from the second satellite. As the trailing spacecraft passes over the same region, it reacts in the same manner. By calculating the distance between the spacecraft down to a single micrometer, one millionth of a meter, instruments can chart the varying levels of gravity on Earth, whether the change in density is the result of mountains or underground aquifers.

Before GRACE, hydrologists relied on ground-based gauges to measure water levels. The problem, scientists said, is that there are not enough ground-based instruments. “We lack a comprehensive hydrology observation system at the continental scale and at the global scale,” Famiglietti said.

GRACE offers that global scale as it maps variations in gravity from month to month. “Nothing can hide from GRACE,” said Michael Watkins, GRACE project scientist at the Jet Propulsion Laboratory in Pasadena, Calif. Vegetation does not bother it and clouds do not bother it, because you do not have to see gravity, he added.

Nevertheless, it has taken years for the scientific community to begin to fully realize the potential utility of the GRACE data to reveal water shortages. “This is a new technique, not something a typical graduate student would know how to use,” Watkins said.

When GRACE was conceived in the 1990s, a primary goal of mission planners was to develop a capability to monitor surface and subsurface ground water, according to Byron Tapley, GRACE principal investigator and director of the Center for Space Research at the University of Texas, Austin. “The full utilization of the signal required the attention of the respective hydrology or oceanography community,” Tapley wrote in an e-mail. “This interaction required that the GRACE project understand what was important to the hydrology community and that we interact with them to help explain the GRACE data product. This collaboration occurred over a period of years as the mission data products matured.”

After nearly eight years, GRACE has become an important feature of hydrology studies. “The degree to which a number of communities have embraced and utilized the measurements has been a rewarding surprise,” Tapley added.

Now, scientists worry that their studies, which often rely on multiyear time scales to chart changes in water resources, will face glaring data shortages when the GRACE mission ends. GRACE includes two satellites built by the Astrium division of Europe’s largest aerospace company EADS and an attitude control system manufactured by Space Systems/Loral of Palo Alto, Calif. Both were designed to last at least five years. After nearly eight years in orbit, the equipment is “aging pretty gracefully,” Watkins said. Nevertheless, it is aging.

Each spacecraft has lost one of its 20 batteries and spent approximately 50 percent of its gaseous nitrogen fuel. While NASA has approved funding for the extended mission until 2013, program officials are wary of possible electronic failures. “We worry that with the age of the satellites, there may be an instrument or satellite component failure earlier,” Tapley wrote.

Compounding that fear is the realization that no GRACE follow-on appears imminent. “The possibilities for continuing the measurement are being discussed by a number of countries, but at present there is no firm plan for a GRACE follow-on mission,” Tapley wrote.

The National Research Council called for a $450 million GRACE 2 mission to be launched between 2016 and 2020 in its Earth Science decadal survey. Budget constraints coupled with intense competition for NASA resources make even that timeframe seem optimistic, according to mission officials. “I wish I knew a way to accelerate it,” Famiglietti said. “All we can do is make our science known and hope that NASA and Congress make replacing GRACE a priority.”

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She...