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. The United States stopped making plutonium 238 in the late 1980s, when the Energy Department shut down the reactors at the Savannah River Plant, near Aiken, S.C., which it was using for production of various radioactive materials, including tritium for nuclear weapons. Since then, the United States has bought plutonium 238 from Russia, but that country no longer makes it, either.
A 2009 report by the National Academy of Sciences called for restarting production, but this has not been done, mostly for cost reasons.
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.
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