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Nuclear Powered Martian Science!

Early Monday morning, shortly after 1:30 AM, a six-wheeled vehicle about the size of a small SUV landed on the surface of Mars. Twice as long and five times as heavy as the Spirit and Opportunity rovers which arrived in 2004, the new arrival, nicknamed “Curiosity,” is the most sophisticated piece of scientific apparatus ever sent to another planet. NASA has invested $2.5 billion in this new mobile laboratory in order to determine whether Mars could ever have supported microbial life.

To accomplish this objective, Curiosity is equipped with an impressive array of geochemical instrumentation. The rover's SAM (Sample Analysis at Mars) instrument suite contains a gas chromatograph and a mass spectrometer, enabling it to identify a wide range of organic compounds in the planet's soil and atmosphere. An environmental monitoring station will take daily measurements of atmospheric conditions and ultraviolet radiation levels. Curiosity also contains an instrument known as ChemCam, which uses a laser to vaporize rocks or soil up to 23 feet (7 meters) away and uses a spectrometer to quickly determine the elemental composition of its target.

In short, Curiosity is a high-tech, laser-toting, all-terrain science machine! However, operating all of these instruments as well as its communications equipment, robotic arm, and wheels will require a lot of energy. Unlike Spirit and Opportunity, which relied on photovoltaic panels to convert sunlight into electricity, Curiosity will instead utilize an alternative power source: plutonium. Specifically, the heat generated from the radioactive decay of a 10.6 lb (4.8 kg) chunk of plutonium-238, as opposed to the plutonium-239 found in nuclear weapons, will be used to produce the 110 watts of electrical power needed to keep Curiosity running for at least two Earth (or one Martian) years.

Yes, in the words of Marty McFly, this sucker is nuclear, but plutonium is only one piece of Curiosity's power system. Arguably the key scientific breakthrough enabling the new rover to carry out its mission may be traced to a materials research group working at RCA's David Sarnoff Research Center in Princeton.

After the launch of Sputink in 1957, the federal government reached out to electronics firms, including RCA, to jump-start the nation's space program. One question of particular interest was how to ensure a spacecraft would have a reliable, long-term power supply. RCA metallurgist Fred Rosi believed the answer was a phenomenon known as the Seebeck effect. In 1821, German scientist Thomas Johann Seebeck demonstrated that two dissimilar metals placed in a closed circuit would generate an electric current if the metals were kept at different temperatures. Rosi believed a similar “thermoelectric” setup might address NASA's power generation problem and set to work exploring various combinations of materials.

During the course of his work, Rosi moved beyond metals to examine the thermoelectric properties of semiconductors. As his colleague, George Cody, explained to me during a recent lunchtime conversation, however, Rosi ran into difficulties measuring the thermal conductivity of these materials at high temperatures. In 1959, when Cody arrived at RCA, he and fellow physicist Ben Abeles picked up where Rosi left off, eventually identifying an alloy of silicon and germanium ideally suited for thermoelectric power generation.

In this 1963 photograph, Ben Abeles demonstrates how the silicon-germanium alloys he and George Cody developed could convert heat directly into electricity, which in this case was used to power a small fan. (Alexander Magoun, David Sarnoff Research Center)

Cody and Abeles patented their findings and by the early 1960s, radioisotope thermoelectric generators (RTGs) combining a nuclear power source, with silicon-germanium thermocouples were being used in satellites and manned space capsules.

The Pioneer, Voyager, and Cassini missions all relied upon RTGs, which provide a steady supply of power, regardless of extreme environmental conditions or access to sunlight. Indeed, without RCA's semiconductor alloys, Pioneer and Voyager might not have left the solar system, Cassini would not have photographed Saturn, and Curiosity—the latest addition to NASA’s long list of robotic explorers—would never have the chance to seek out signs of life on the surface of the red planet.

Benjamin Gross is a research fellow at CHF’s Center for Contemporary History and Policy.

Posted In: Fellows | Technology

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