NASA is allowing a highly successful satellite to fall out of Earth's orbit by refusing to fund it for as little as $28 million, dismaying the scientists and forecasters who use its unique abilities to study climate change and track hurricanes.

NASA officials said engineers did not order a planned firing of its rockets in early July to hold the Tropical Rainfall Measuring Mission satellite in orbit 241 miles above Earth. Without periodic assists from its thrusters, atmospheric drag will send the satellite's remains to a watery grave in six to nine months.

Engineers said the satellite, a joint venture with the Japanese Aerospace Exploration Agency, is working perfectly and could still be saved, but NASA officials said neither the Japanese nor other U.S. agencies were willing to contribute to the estimated $28 million to $36 million needed to keep the mission operating for as long as two more years.

The satellite is a unique space platform whose instruments have proved invaluable not only to researchers studying global change, but also to meteorologists who use its one-of-a-kind "rain radar" to probe deep into cloud cover to determine whether the makings of a cyclone lurk there.

In 2002, a NASA study determined that the potential lifesaving value of the satellite was great enough to justify keeping it aloft until it ran out of fuel and tumbled unguided back to Earth, possibly killing or injuring someone.

The decision instead to use a "controlled de-orbit" for the satellite, known by its initials TRMM, was announced quietly July 13 in an internal NASA memo, and came at a time when NASA's Earth observation budget is shrinking as the agency begins to focus on President Bush's plan for human exploration of the moon and Mars.

NASA officials said the agency decided to de-orbit TRMM because the money saved could be put to better use on a next-generation satellite scheduled for launch in 2011.

The Bush administration is already facing harsh public criticism for its decision to cancel space shuttle servicing of the Hubble Space Telescope, and congressional critics of the Bush initiative have publicly warned NASA not to rob Hubble or other valuable programs -- especially in earth science -- to fund the new undertaking.

But congressional sources said lawmakers do not necessarily see TRMM's problem as the harbinger of cuts to come: "TRMM would be a problem regardless, because it's an unanticipated expense," and not part of the NASA budget, said a knowledgeable Republican congressional staffer who declined to be identified. "NASA may even deserve credit for being willing to ante up" some of the money.

Or not. NASA has said little about TRMM's demise, but researchers all over the country and in Japan are questioning the decision. Data from TRMM on rainfall and storms are used by climate scientists and meteorologists all over the world.

"Unlike a lot of missions, it's worked great from the beginning -- something of a miracle in satellite meteorology, and we're still on the rising part of the curve," University of Washington atmospheric scientist and TRMM team member Robert Houze said in a telephone interview. "It seems almost unfathomable to me that you would not let it live out its full lifetime."

House Science Committee chief of staff David J. Goldston acknowledged in a telephone interview that his office had gotten "probably in the last week an inch-high pile of letters from researchers around the United States saying we're missing this great opportunity. We don't have a position yet, but we are looking into it."

TRMM was launched in Japan on Thanksgiving Day 1997 into an orbit that girdles the globe ranging from 35 degrees north of the equator -- the latitude of North Carolina -- to 35 degrees south (Santiago, Chile).

TRMM measures and analyzes rainfall, using microwave, infrared and lightning sensors supplied by the United States, and the Japanese-built rain radar. Together they provide the most detailed information on rainfall patterns ever created, from the part of the world that influences global climate more than any other.

"Having all these instruments on the same satellite can provide best estimates of rainfall over oceans and land, can measure the impacts of the El Nino ocean temperature changes and document the release of heat when water changes to rainfall," said Goddard Space Flight Center's Robert Adler, the NASA project scientist in charge of TRMM.

TRMM was supposed to last three years, but instead it lived long enough to become a victim of its own success. "It has been superb on all counts," said Ghassem Asrar, NASA's associate administrator for earth science. "The nominal life was 18 months, the goal was three years, and we just kept extending."

As the years crept by, scientists found they could use TRMM to improve the baseline accuracy of computer climate models and weather forecasts, or to give their local research a global context. And the longer TRMM operated, the more comprehensive the data became, because climate patterns take years to develop.

"But the biggest surprise, which I never anticipated, is this whole ability of the satellite to observe hurricanes in a way that no satellite can," Houze said. TRMM's radar can peer inside tropical storms to watch them evolve.

"A lot of times you'll just see a ball of white cloud, but TRMM can go to the core, see the eye wall start to develop: Is it intensifying? Is it getting better defined? Is it falling apart?" National Hurricane Center Director Max Mayfield said in a telephone interview from his Miami office. "It's been absolutely critical. Ask any of our hurricane forecasters."

Asrar said that engineers had planned a controlled de-orbit for TRMM "from the beginning," using onboard thrusters to steer the satellite into the ocean far from population centers. He said the satellite was sturdy enough for large pieces of it to survive reentry temperatures and potentially injure or kill people in the debris path.

As time passed, however, what seemed like a routine maneuver to end a successful mission loomed as the limiting factor for the mission itself. Operating systems were working perfectly, instruments were not wearing out, and the data were impeccable. The only thing being used up was the fuel needed to maintain orbit.

In 2002, Asrar asked Bryan O'Connor, NASA associate administrator for safety and mission assurance, to conduct a "disposal risk review." Did the benefits of using all the fuel to keep TRMM in orbit an additional five years outweigh the hazards of allowing the spacecraft to fall back to Earth without guidance?

In his reply on Sept. 4, 2002, O'Connor said the probability of a TRMM debris casualty would be one in every 5,000 reentries, twice as dangerous as NASA's standard of one in 10,000. NASA allows about six uncontrolled reentries a year.

Despite the heightened danger, O'Connor concluded that "these risks appear to be reasonable when subjectively weighed against the potential public safety benefits of improved storm analysis and forecasting capabilities that appear to be realized by extending the TRMM mission."

But uncontrolled reentry was never seriously considered, Asrar said, and the O'Connor analysis was used to reaffirm what Asrar described as NASA's original view: "What if the one in 5,000 becomes a reality?" Asrar said. "Can anybody stand up and say it was worthwhile?" He said he asked for the O'Connor report simply to show that "we had done due diligence" in evaluating TRMM's potential hazard.

In 2003, engineers bought some more time by boosting TRMM from its initial orbit 210 miles above Earth to its current height. At the higher orbit, the satellite needed station-keeping burns only once a month, instead of once every three days.

But by mid-2004, NASA had to make another choice: bring TRMM down in a controlled de-orbit this year for a landing next year or deplete the fuel for one or two more years, then let the satellite "drift down" unguided for a couple of years, using the last of the fuel to control the final reentry.

NASA officials who declined to be quoted by name questioned whether Asrar ever seriously considered prolonging the mission further, and disputed his view that it would cost $28 million to $36 million to keep TRMM running for up to two more years.

Asrar explained that extending TRMM would require payment not only for the two data years, but also for the two or more "drift-down" years, when the satellite would not be sending reliable data, but would still have to be watched. Spending more money now would mean postponing the 2011 launch of a new satellite designed to improve on TRMM's performance, he said.

Asrar said it was "absolutely incorrect" that NASA decided to begin the de-orbit now to save money for the Bush initiative, noting that "we started looking at this issue two years ago," long before the moon-Mars plan arose.

Instead, he said, NASA asked Japan or another U.S. agency to partner another extension. Finding no takers, NASA ordered the de-orbit to begin. "This has been my decision all along," Asrar said. "I can take full credit or blame for it."