B and B: Tropical Rainfall Measuring Mission will struggle on

Tropical Rainfall Measuring Mission will live on

Some good news on the NASA Earth science front: it looks like NASA is extending the Tropical Rainfall Measuring Mission.

TrmmI first learned about TRMM back in early May when I was researching a post on the future Global Precipitation Measurement (GPM) mission. GPM will be the new-and-improved version of TRMM, measuring rainfall all over the world at approximately 3-hour intervals. TRMM, more modestly, measures rainfall throughout the tropics, and only once a day or so.

The data so provided by TRMM, though, has turned out to be amazingly useful for both research on tropical weather and climate, and also, in an area particularly relevant to the U.S. at this time of year, the tracking of hurricanes.

Alas, the TRMM satellite was due to run so low on fuel by this summer, that NASA would have to terminate the mission and save up all the remaining fuel to do a controlled reentry. After all, ever since Skylab in 1979, NASA has had an aversion to headlines about large satellites plunging through the atmosphere, potentially raining havoc on unsuspecting innocents underneath.

TrmmdennisNo problem, though. TRMM, a joint American and Japanese project launched back in 1997, is a couple years past its expected lifetime, has been tremendously successful, and has an even-better successor lined up to take its place. Time to bid it adieu. It had a good life, but it's time has come.

Alas, due to budget decisions at NASA, the successor has been pushed back to a 2010 launch at the earliest
. Without TRMM, that would leave at least a 5-year gap in the climatological data record, and at least 5-years worth of dangerous tropical weather occurring without forecasters having the advantage of all this great precipitation data.

Did it make sense to purposefully dump a very useful satellite in virtually perfect working order into the ocean because of a very small risk of injuries during an uncontrolled reentry?

The answer is—it's hard to say. At least according to a report from a scientific panel commissioned by NASA to look into that question late last year:

Many organizations and individuals have invested in bringing TRMM data into the operational environment [weather and hurricane forecasting] because of the unique aspects of TRMM's orbit and sensor suite. This reflects their professional judgment of the value of doing so based on their experiences of improvements in such things as accuracy of center fixes for tropical cyclones and prediction of storm intensity. Nonetheless, the effect of TRMM data on operational applications has not been widely quantified because the data record is too short for meaningful statistical analysis and no one has done control experiments wherein the TRMM data are eliminated and the analysis is rerun. Further, the socioeconomic effects on end-users of improved forecasts have not been quantified.

An earlier review by the NASA Office of Safety and Mission Assurance in 2002 also couldn't come up with an answer: Is it riskier to dump TRMM or to keep it alive as long as possible?

In the case of a TRMM uncontrolled reentry, the casualty risk of 2/10,000 events appears to fall into an intermediate, or tolerability zone, where the risk may be tolerated in return for other (public safety) benefits.

A (defendable) quantitative estimate of the benefits derived from up to five extra years of TRMM data on improvement of storm analysis, forecasting, and public safety could not be developed. As a result NASA will need to rely on subjective estimates based on expert judgment.

So detailed analysis can't answer the question. What does the "expert judgment" say? A NASA workshop in 2001 came up with this:

Most, but not all, workshop participants subjectively estimated that the risk to human life of an uncontrolled reentry would be exceeded by the risk to human life of not having TRMM data for operational [forecasting] uses.

Not unanimous, but an answer! Despite that, earlier this year, NASA was leaning the other way. Back in April, the NASA associate administrator for science was prepared to terminate TRMM's mission and start planning for the controlled reentry. Given the uncertainties, it would have been hard to argue with that decision, even if it wouldn't have been ideal. TRMM had already been extended an extra year after a previous termination announcement.

But in June, NASA's new top gun, Mike Griffin, overruled that decision, and is "coordinating" (which I read as 'trying to figure out who's going to pay the relatively small amount of money for it') the extension of TRMM's mission until 2010 or 2011. At that point, TRMM will be completely out of fuel, unable to preserve its orbit against the drag effect of the thin atmosphere 400 km up, and thus will fall back to Earth of its own volition.

But by then, GPM, the next-generation TRMM, will be ready to take over.

Hopefully no one will be hurt when TRMM comes crashing back through the atmosphere in five or six years. And hopefully many fewer will be hurt due to the better hurricane tracking that TRMM will enable in the meantime.

B and B: Science, Earth


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