nuclear-news

Nuclear power’s not doing too well on Earth, but then, there’s always Mars

The Competition Between Solar and Nuclear Energy Moves to Mars, NYT By MATTHEW L. WALD, 23 Nov 11 NASA’s last two Mars rovers, Spirit and Opportunity, launched in 2003
and carried solar panels, but the new Mars rover that the space agency is scheduled to launch on Saturday morning from Cape Canaveral uses nuclear power, a fact that did not escape the partisans of nuclear power here on Earth.

But it turns out that both nuclear power and solar power in space have problems.

The solar panels on Spirit and Opportunity are gallium-arsenide, a
chemistry that is used on Earth but is a niche product, where very
high production per square inch is required. Per unit energy produced,
if rooftop space is not an issue, silicon-based cells are far less
expensive.

Solar power worked acceptably on Mars, but with some difficulties.
Dust storms sometimes blotted out 90 percent of the sun’s light, and
dust collected on the panels themselves; project scientists referred
to “cleaning events” when some kind of wind current reduced the dust
coating on the cells.

Spirit, which is no longer operating, and Opportunity, which has
defied all expectations and is still in service after seven and a half
Earth years, weighed about 400 pounds each, and carried lithium-ion
batteries so they could store the output of their cells, which was, at
maximum, 140 watts. On earth, that amount of power is almost enough to
run a desktop computer and monitor, but on Mars it was enough to run
the radios, cameras and various instruments, and propel the vehicles.

Curiosity, the Mars rover that is scheduled for launch on Saturday,
weighs about 2,000 pounds. “This is a much larger vehicle, the size of
a small car,’’ said Stephen G. Johnson, the director of the space
nuclear systems and technologies division at the Idaho National
Laboratory, which prepared the nuclear power pack for the new probe.
And it carries a laser that will be used to zap rocks, so that other
instruments can analyze what they are made of. So its energy
requirements are far higher.

“It’s really only possible with plutonium 238 to do what it’s
intending to do,’’ said Dr. John M. Logsdon, a space expert at George
Washington University.

Curiosity is supposed to run for one Martian year, or two Earth years.

Curiosity carries a nuclear power pack that can generate 110 watts,
continuously. But this is not like a nuclear reactor on Earth, which
splits uranium atoms to make heat, uses the heat to boil water and
uses the steam to drive.

Space nuclear power packs are not reactors at all; they do not split
atoms. They carry plutonium 238, a manufactured isotope with a
half-life of just 88 years, which means that its radioactive decay is
so fast that it glows red-hot. The heat is converted directly to
electricity. The radioactive emissions are alpha particles, which are
easily blocked, and the material cannot be used in a bomb, although
the technology needed to purify the plutonium 238 can also be used to
recover the type of plutonium that is weapons-usable. And the material
is toxic.

The problem is that in the United States, all things nuclear are
getting old. ….
One alternative is to develop a better way to convert heat into
electricity in space. The National Academy report said that the method
NASA uses now is only about 6 percent efficient. A Stirling Engine
system could produce five times as much electricity from each unit of
heat, reducing the need for plutonium, but it has many moving parts
and has not been adapted to space use.

But the response so far has been to use solar cells whenever possible.
Steven W. Squyres, a professor of astronomy at Cornell who is the
chief scientist for the Opportunity and Spirit rovers, said: “You
always use solar when you can; it’s simpler, cheaper, just easier to
do. You only use nuclear when you have to.’’

This year, NASA launched another space ship, Juno, on a mission to
Jupiter. Because of the shortage of plutonium, it uses solar cells,
although the neighborhood is not conducive to that technology. At the
orbit of Jupiter, five times as far from the sun as the Earth is, the
sun’s intensity is 96 percent lower.

Restarting production of plutonium 238 and finding a more efficient
way to use it are urgent priorities, he said.

http://green.blogs.nytimes.com/2011/11/23/the-competition-between-solar-and-nuclear-energy-moves-to-mars/

November 24, 2011 - Posted by Christina Macpherson | technology, USA