WASHINGTON — The U.S. government is expected to decide by summer whether to steer a healthy environmental satellite into the ocean or allow the spacecraft to stay in orbit until it runs out of fuel and re-enters on its own.

The satellite in question is the Tropical Rainfall Measuring Mission (TRMM), a joint U.S.-Japanese radar satellite launched in November 1997 on an 18-month mission to study precipitation in the Earth’s wettest regions. The $650 million spacecraft, designed to last at least three years, is now in its seventh year of operations.

In that time, data from TRMM have proven valuable to researchers trying to understand tropical climate patterns and to meteorologists who use the live feed from the satellite to monitor tropical cyclones and other severe weather. Many of these users are now reluctant to let go of a satellite that the National Academy of Sciences recently concluded has at least several more years of useful life ahead of it.

NASA plans to shut down the satellite later this year and dump it safely into the ocean unless the National Oceanic and Atmospheric Administration (NOAA) agrees to take over the mission and accept the added risk of letting the 3,500-kilogram spacecraft re-enter on its own around the end of the decade.

The battle over the future of TRMM has been going on for more than a year. Although there has been some national press attention, the TRMM saga has been overshadowed by the battle over the future of a much more expensive and much better known NASA spacecraft, the Hubble Space Telescope.

Last summer, after NASA announced that it planned to turn off the TRMM satellite in a matter of weeks in preparation for steering it into the ocean in 2005, TRMM users rallied enough political support to win a temporary reprieve. NASA agreed to continue operating TRMM through the end of 2004 and called upon the National Academy of Sciences to study the merits of a further extension.

NASA approved a second extension in early January — this t ime through spring — after the National Academy of Sciences issued an interim report saying that TRMM was worth keeping in service at least until such a time that NASA must act to ensure a controlled re-entry.

NASA estimates that TRMM needs to have at least 138 kilograms of propellant left in its tank to perform a deorbit burn meant to guide the spacecraft safely into an uninhabited stretch of ocean. According to NASA’s latest projections, TRMM will reach that fuel threshold as soon as September.

In the meantime, NASA and the National Oceanic and Atmospheric Administration (NOAA) are discussing the possibility of operating TRMM beyond September, a decision that would entail waiving the U.S. government’s own safety guidelines for determining whether a satellite should be allowed to re-enter the atmosphere on its own.

NASA concluded in a 2002 risk assessment that the odds of somebody on the ground being injured by any piece of TRMM surviving an uncontrolled reentry would be 1 in 5,000, or twice as risky as the government deems acceptable.

Proponents of extending TRMM beyond September argue that the public safety benefits of improved storm tracking outweigh the public safety risks of permitting a satellite like TRMM to plunge to Earth at an unknown time and place. Back in 2002 NASA’s top safety official, Bryan O’Connor, reached a similar conclusion, writing in a memo accompanying the TRMM re-entry risk report that the scientific and public safety benefits of extending the mission beyond the point where a controlled deorbit is no longer possible could outweigh the added risks of an uncontrolled re-entry.

However, NASA’s deputy associate administrator for science, Ghassem Asrar, told Space News in a recent interview that NASA is not the right agency to make the case for waiving re-entry safety guidelines in the name of improved storm tracking. He said it would be more appropriate for NOAA, which operates the National Weather Service and other entities that issue warnings about weather-related hazards, to decide whether the potential benefits outweigh the potential risks.

“If an operational agency that has a mandate for public safety, like NOAA steps forward and says we can justify this … and they request such a waiver, we would be supportive of that,” Asrar said. “But as an R&D agency, NASA cannot make that case.”

Asrar said NASA has been talking to NOAA since late last year about handing over responsibility for TRMM. He said a NASA-NOAA team remains hard at work on this issue and expects to reach a decision about the future of the satellite before NASA’s latest mission extension expires in June.

NOAA spokesman John Leslie declined to say whether NOAA has any interest in taking over TRMM. He acknowledged that NASA and NOAA have been discussing the matter and that a decision would be made sometime this year.

There does not appear to be much hope among the TRMM community that NOAA will step forward and save the satellite.

“Although there have been some rumblings from NOAA they have changed their tune recently about how much they like TRMM data because I think they are afraid of having to pay for it,” said one TRMM user who has been following the back and forth between NASA and NOAA.

NASA spends about $4 million a year operating TRMM. Although NASA would save some money by not operating TRMM until it runs out of fuel around the end of the decade, the savings would not be immediate.

The National Academy of Sciences found through interviews with NASA personnel that even if TRMM science operations were suspended this year, NASA would have to spend $13 million to monitor the satellite through 2007 in preparation for a controlled deorbit. After suspending science operations, the spacecraft’s solar arrays would be repositioned to increase atmospheric drag, causing the spacecraft to drift down to a lower altitude over a period of about a year before its thrusters were fired to send the spacecraft plummeting into the ocean.

NASA currently spends about $16 million per year on precipitation research, a figure that does not include the $54.2 million NASA has spent over the last four years on the preliminary development of the Global Precipitation Measuring Mission, a TRMM follow-on mission NASA hopes to launch around 2010.

Comments: bberger@space.com