WASHINGTON — One year after NASA postponed until 2012 the launch of the key spacecraft in a proposed constellation of rain-measuring satellites, the U.S. space agency is now talking about launching the core satellite in 2010.

To make the earlier launch date without pumping additional money into the Global Precipitation Measurement (GPM) mission, NASA would abandon plans to develop a state of the art satellite platform made of lightweight composites and instead buy a proven satellite design from one of three U.S. spacecraft builders.

GPM is a joint mission with the Japanese Aerospace Exploration Agency (JAXA) and other international agencies to develop a constellation of satellites that will measure precipitation, a key factor in climate studies, on a global scale. The constellation is being designed to provide data with more frequency and greater accuracy than any system or satellite launched to date.

NASA and JAXA have been collaborating for several years on the development of a core satellite carrying a Japanese-built precipitation radar and a U.S.-built precipitation microwave radiometer. European and other nations plan to complete the constellation by launching various radiometer-equipped satellites.

NASA and JAXA originally planned to launch the core satellite in 2007. But GPM was dealt a setback in 2001 when U.S. President George W. Bush directed NASA to delay any new Earth science missions pending the outcome of a government-wide review he ordered of federally-funded climate change research efforts.

By the time that review wrapped up in 2003, GPM had fallen behind schedule. The following year, in the wake of Bush’s unveiling of a new vision for space exploration, NASA delayed GPM an additional two years to 2012.

This year’s budget request, due to be released Feb. 7, could hold better news for GPM. Ghassem Asrar, NASA’s deputy associate administrator for science, said the agency wants to launch the core GPM satellite by the end of the decade and believes it can accomplish this affordably by buying the satellite from the agency’s Rapid Spacecraft Acquisition catalog of flight-proven satellite platforms.

“We have set a goal for ourselves to launch the mission by the end of the decade, around 2010. That would be roughly two years earlier than what was put into the budget last year,” Asrar said in an interview. “As part of achieving this goal, we are looking at all ways of getting the mission done. One is to buy the spacecraft through the catalog.”

NASA has been looking at satellite designs from the three American vendors included in the spacecraft catalog: Ball Aerospace, Orbital Sciences Corp., and General Dynamics C4 Systems (which acquired small satellite builder Spectrum Astro in July 2004).

Asrar said NASA is confident it can find a spacecraft in the catalog that meets its requirements and expects to select a vendor by this summer. Industry officials said they expect NASA to narrow the field from three vendors down to two in the weeks ahead.

If NASA goes with one of the spacecraft designs in the catalogue, it would scrap plans to have the Goddard Space Flight Center in Greenbelt, Md. build the satellite in house.

Goddard has been designing a satellite made almost entirely of lightweight composite materials. Although there are satellites in orbit today with composite components, Asrar said GPM would have been NASA’s first fully composite spacecraft.

NASA and JAXA plan to launch GPM along with another satellite on a Japanese H-2A rocket. Although any comparable satellite bought through the Rapid catalog would be heavier than one made almost entirely of composite materials, Asrar said all of the satellites NASA is considering for the mission could still be dual manifested on an H-2A without giving up any capability.

NASA and JAXA are expected to finalize GPM’s target launch date by year’s end. Before then, NASA plans to select a company by the end of February to build the satellite’s microwave radiometer and choose one of the spacecraft designs from the Rapid catalog.

“Once the spacecraft and instrument are selected, we will be in a dialogue with Japan to finalize the launch manifest,” Asrar said. “We are hoping by fall this year to nail down the launch year.”

Comments: bberger@space.com