NASA Weighs Power-Source Options for Mars Rover

NASA expects to decide by the end of the year whether to useconventional solar arrays or a nuclear battery to power the 2009 Mars ScienceLaboratory rover.

NASA clearly would prefer to use a so-called multi-missionradioisotope thermoelectric generator, or MMRTG, a device that converts heatfrom decaying Plutonium-238 into electricity. But federal environmentalregulations require the U.S. agency to give the general public a chance toweigh in before making a final decision on the rover, which will be roughly thesize of a compact car and equipped with 10 instruments.

NASA officials discussed the rover mission and itspower-source options Oct. 9 during a sparsely attended public meeting held at ahotel here. It was the last of two public meetings NASA is required to holdbefore making its decision on how to power the Mars Science Laboratory, whichis scheduled to launch in fall 2009 from Cape Canaveral Air Force Station, Fla.,aboard an Atlas 5 rocket. The rover is due to arrive at the red planet 10 to 12months later depending on the landing site selected.

Mark Dahl, NASA's Mars Science Laboratory program executive,said the agency could use conventional solar arrays for the duration of therover's nominal 687-day mission, which spans one martian year. But Dahl said asolar-powered rover would be more limited in where it could land. Because ofsunlight considerations, Dahl said, NASA would have to pick a landing spotwithin 15 degrees north or south of Mars' equator. A nuclear-powered rover, onthe other hand, could operate anywhere within 60 degrees north or south of theequator.

NASA intends to wait until one year prior to launch to make a final landing site selection, which Dahl said will allow the agency to use the latest information gathered by the newly operational Mars Reconnaissance Orbiter to identify a safe spot deemed likely to harbor the most clues to the planet's watery past.

In addition to giving NASA a wider choice of landing sites,a nuclear-powered rover could operate for far longer than a similar vehiclepowered by solar arrays. The MMRTG that NASA is designing with the U.S. Departmentof Energy for the Mars Science Laboratory, sized to generate 110-120 watts ofpower at the time of landing, could still be cranking out 90 watts of useableelectricity 14 years later, according to Dahl. Solar arrays, in contrast,degrade much faster and would not be expected to last more than a few yearsonce exposed to the dust and radiation on Mars.

While the vast majority of NASA spacecraft rely on solararrays for electrical power, the agency has launched 16 probes equipped withnuclear batteries to date, the most recent being the Pluto-bound New Horizonscraft that lifted off in January from Cape Canaveral.

Because using nuclear spacecraft batteries is not withoutrisk -- the United States has had three known mishaps in four decades, one ofwhich dispersed 3 kilograms of plutonium in the atmosphere -- NASA is requiredby law to evaluate the potential dangers posed by any such mission to humansand the environment, publish the results, and give the public chance tocomment.

The Mars Science Laboratory's MMRTG is designed to survivean accidental atmospheric re-entry intact. NASA calculates the odds of acatastrophic launch failure that could actually result in the release ofradioactive material at 0.4 percent, according to Dahl.

NASA published a draft environmental impact statementdetailing the risks of the mission Sept. 8, starting the clock on a 45-daypublic comment period that ends Oct. 23. Tina Norwood, a NASA environmentalscientist, said the agency expects to publish a final environmental impactstatement in November and issue a record of decision no sooner than 30 dayslater, but hopefully by the end of the year.

Norwood said at the Oct. 9 meeting that the agency so farhad received only one written comment in response to its draft environmentalimpact statement.

During the meeting, despite two hours reserved for publiccomment, no one rose to speak. Most of the two dozen people in attendance wereeither NASA and Department of Energy officials or part of a group of localuniversity students enrolled in a class on federal environmental regulationsand procedures.

NASA spokesman Dwayne Brown said the first public meetingheld Sept. 27 in Cocoa, Fla., also was sparsely attended.

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