NASA expects to decide by the end of the year whether to use conventional solar arrays or a nuclear battery to power the 2009 Mars Science Laboratory rover.

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

NASA officials discussed the rover mission and its power-source options Oct. 9 during a sparsely attended public meeting held at a hotel here. It was the last of two public meetings NASA is required to hold before making its decision on how to power the Mars Science Laboratory, which is 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 12 months 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 the rover's nominal 687-day mission, which spans one martian year. But Dahl said a solar-powered rover would be more limited in where it could land. Because of sunlight considerations, Dahl said, NASA would have to pick a landing spot within 15 degrees north or south of Mars' equator. A nuclear-powered rover, on the other hand, could operate anywhere within 60 degrees north or south of the equator.

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 vehicle powered by solar arrays. The MMRTG that NASA is designing with the U.S. Department of Energy for the Mars Science Laboratory, sized to generate 110-120 watts of power at the time of landing, could still be cranking out 90 watts of useable electricity 14 years later, according to Dahl. Solar arrays, in contrast, degrade much faster and would not be expected to last more than a few years once exposed to the dust and radiation on Mars.

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

Because using nuclear spacecraft batteries is not without risk -- the United States has had three known mishaps in four decades, one of which dispersed 3 kilograms of plutonium in the atmosphere -- NASA is required by law to evaluate the potential dangers posed by any such mission to humans and the environment, publish the results, and give the public chance to comment.

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

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

Norwood said at the Oct. 9 meeting that the agency so far had received only one written comment in response to its draft environmental impact statement.

During the meeting, despite two hours reserved for public comment, no one rose to speak. Most of the two dozen people in attendance were either NASA and Department of Energy officials or part of a group of local university students enrolled in a class on federal environmental regulations and procedures.

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