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.
