NASA’s plan to drop a healthy environmental satellite into the ocean next year has provoked an outcry from scientists and touched off a flurry of last minute discussions between the U.S. and Japanese space agencies.

NASA informed the Japanese Aerospace Exploration Agency (JAXA) earlier this year that it intends to decommission the Tropical Rainfall Measuring Mission (TRMM) satellite in the weeks ahead and steer it into the ocean in 2005. News of the decision leaked to Japan’s Kyodo news service in early June and NASA’s public affairs division quickly confirmed that TRMM would indeed be coming down in 2005.

But now NASA and JAXA officials say they still are discussing the future of the joint satellite mission and that no final decisions have been made regarding when to end science operations and bring the spacecraft out of orbit. Both sides, however, say they cannot afford to pay the $3 million to $4 million a year it costs to keep the satellite flying.

“As regards the money, JAXA’s overall budget situation is in an extremely bad position and finding that sort of money would be a heavy burden for us,” a JAXA official told Space News.

NASA spokeswoman Gretchen Cook-Anderson called continued funding of TRMM operations “unfeasible” in light of the U.S. space agency’s other Earth science priorities.

TRMM was launched in 1997 on a planned 18-month mission to study rainfall in the Earth’s tropical regions. The $650 million spacecraft, designed to last at least three years, is now in its seventh year of operations.

NASA built the satellite and has paid the operating costs for the past six and a half years. Japan supplied TRMM’s sensitive radar instrument and paid to launch the satellite on a Japanese H-2 rocket.

The spacecraft remains healthy with all its major instruments and critical subsystems still in good working order. TRMM team members say the satellite could operate another two years and still have enough fuel left for a controlled de-orbit.

Scientific demand for TRMM data also remains strong, they said, and a number of weather forecasting agencies around the world are using TRMM data to improve hurricane and typhoon tracking.

Operational users of TRMM data include the U.S. National Weather Service, the U.S. Navy’s Joint Typhoon Warning Center and the European Centre for Medium-Range Weather Forecasts.

Louis Uccellini, the director of the National Centers for Environmental Prediction, Camp Springs, Md., wrote NASA in the summer of 2003 urging the agency to continue funding TRMM. Uccellini called the TRMM mission “a major crossover (research and application) success for NASA and for user agencies such as [National Oceanic and Atmospheric Administration] who continue to use the data in real-time to improve forecast models and support hurricane forecast operations.”

The JAXA official said word of TRMM’s imminent demise is not sitting well with Japanese researchers either. “The mission equipment is still alive,” the JAXA official said. “We hear that the user community finds it disappointing that the mission will end.”

TRMM team members said they cannot understand why NASA would de-orbit a useful satellite just to save the $3 million to $4 million a year it costs to operate.

“If we were on our last legs I could understand the decision,” a TRMM scientist said. “But the satellite’s systems are all in great shape. Not only are we doing great science, people around the world are using the data everyday in real time.”

Ghassem Asrar, NASA’s associate administrator for space science, told Space News the cost of continuing TRMM operations is only one factor.

Safety, he said, is another. “This is not only about money,” he said. “We have to balance scientific return against public safety.”

Asrar said it is essential that TRMM’s mission ends soon enough to permit a controlled de-orbit of the 3,500-kilogram satellite. If the satellite is left to re-enter on its own after exhausting its fuel, the risk to people on the ground, according to a NASA analysis, would exceed the U.S. government’s probability threshold of 1 in 10,000.

NASA concluded in a 2002 risk assessment that TRMM stood a 1 in 5,000 chance of hurting somebody on the ground if left to re-enter the atmosphere on its own. Nicholas Johnson, head of NASA’s Orbital Debris Program Office at Johnson Space Center in Houston, said bringing TRMM down over an uninhabited stretch of ocean would dramatically lower the odds that any piece of the satellite surviving re-entry would hurt somebody.

But the issue of public safety is more complicated than calculating that the odds a piece of TRMM may hit someone. NASA’s top safety official said TRMM’s contribution to storm tracking should be factored into any de-orbit decision.

Bryan O’Connor, NASA’s associate administrator for safety and mission assurance, told Asrar that the benefits of extending the TRMM mission to the point where a controlled de-orbit is no longer possible could outweigh the risks of an uncontrolled re-entry.

“[T]hese 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,” O’Connor wrote in a Sept. 4, 2002 memo to Asrar.

A TRMM team member said even if NASA insists on a controlled de-orbit, there is no compelling safety argument for turning the satellite off this year.

“We could get another two years of science data and still do a controlled re-entry,” the team member said. “That’s why the fight has to be about money.”

Officials at NASA headquarters, including Asrar, say TRMM has made important contributions to science and weather forecasting, but that it is time to move on.

Asrar said NASA wants to get on with development of a proposed TRMM follow-on mission, a constellation of satellites that would measure precipitation on a global scale.

The Global Precipitation Measurement mission would rely heavily on international collaboration. NASA plans to partner with Japan to build the constellation’s central satellite, with other nations fleshing out the constellation with less sophisticated satellites equipped with passive radiometers. NASA and JAXA had hoped to launch the central satellite in 2008 but are now shooting for 2010.

But TRMM team members said NASA’s interest in the Global Precipitation Measurement mission should boost TRMM’s fortunes, not hasten its demise.

“From a scientific and product development standpoint, TRMM is not standing in the way of GPM development, it is the pathway to GPM development,” a team member said.

Still, the word around NASA’s Goddard Space Flight Center is that TRMM’s days are numbered. TRMM team members said they expect the order to arrive any day to turn off the satellite’s four working instruments and start preparing the craft for an ocean disposal. Those preparations would include repositioning the satellite’s solar arrays to increase atmospheric drag, a change that would reduce the amount of onboard power available and end science operations.

NASA headquarters officials told the TRMM team June 29 to continue science operations for now but to scrap a station-keeping operation planned for July 1. The canceled maneuver was designed to boost TRMM’s altitude to keep it within its proper orbit. Once satellites are pegged for a targeted de-orbit, routine orbit boosts typically cease. The satellite’s orbit is allowed to naturally degrade for a period before commands are given to fire onboard thrusters and re-enter the atmosphere.