The power of curiosity

Principles over 150 years old power the Mars Curiosity rover.  Are you curious?    Photo: The RTG that powered Cassini before its installation (NASA)

NEW YORK, August 9, 2012 – It turns out that an old dog can be taught new tricks after all. The scientific principles powering the Mars Curiosity were discovered over 150 years ago, and have been used by NASA to power several of its most famous missions, including the Apollo moon missions, the Viking missions, Voyagers 1 and 2, Cassini and Galileo. This old dog now is also now powering the 2,000 pound Mars Curiosity rover before, during and now after its successful landing on Mars on August 6, 2012. 

The rover utilizes a power plant that is called a radioisotope thermoelectric generator (RTG).  With no moving parts, the RTG has proven to be a robust power plant. The RTG aboard the Pioneer 10 craft operated for 30 plus years, and it’s likely it is still operational, although the signal from the craft was lost nearly a decade ago. The RTG consists of two main components: a device to convert heat into electricity, and a heat source.

Thermocouple

Thermocouple example (Wikipedia)

The RTG harnesses a principle called the “Seebeck Effect,” named after German physicist Thomas Johann Seebeck, and this principle has been utilized in engineering since its inception, primarily in instrumentation. Seebeck observed that two dissimilar metals (having differing characteristics now known as thermoelectric sensitivities) joined together in a closed circuit, will generate a very small electric potential (voltage) when the junction on one end of the circuit of these dissimilar metals is heated.  Electrons at the heated junction become thermally excited compared to those at the cool end, and begin to migrate toward the cooler end of the circuit. Since electrons are negatively charged, this migration creates a buildup of negative charge on the cool end, and the absence of electrons at the hot end creates a positive charge.  This creates a difference in electrostatic charge in the circuit, allowing current to flow in the circuit. This device is called a thermocouple.

MMRTG Cross section

Cross-section of an RTG. (NASA)

In order for the thermocouple to work, a heat source is required that will not be extinguished by the cold and extreme conditions on the Martian surface. The rover uses the natural decay heat of plutonium dioxide, a radioisotope, encased in modules that provide approximately 2000 watts of thermal (heating) power. The thermal heat is converted into electrical power by the thermocouple and can generate about 100 to 120 watts of electrical power, amazingly enough to drive and operate the rover’s ancillary equipment such as cameras, lasers and communications. However, the rover cannot move along the surface, take pictures, samples, scans, and broadcast to earth simultaneously. Doing so would overload the circuitry, so care must be taken to stay within operating parameters of the power plant.

Assuming no major system failures occur, the Curiosity rover could theoretically remain in service much longer than the lifespan of the mission, 23 earth months or one Martian year, according to Larry Trager, general manager at Hamilton Sundstrand Rocketdyne, the developer of the Curiosity RTG.

At the time that Thomas Seebeck discovered the thermoelectric effect, there was no way he could have envisioned the principle becoming the primary power source for space vehicles in the future. Although his training was in medicine, Seebeck’s love and curiosity of physics produced an amazing contribution to science that is powering spacecraft on distant planets and beyond our solar system.


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Michael Jaeger

Mike Jaeger's column, Greater than Energy (">Energy") can be found under the Health and Science area of the Washington Times Communities and he has been writing this column since July of 2012.  He occasionally writes pieces on economics and politics as well.  He has expertise in energy, energy markets and energy production and holds a Bachelor of Science Degree in Applied Science and Technology in Nuclear Engineering Technology.

 

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