Curiosity Rover Power

November 5, 2012

There's more electrical power on Mars than at my own home. This is discouraging, since the available Martian power is just a little more than a hundred watts, and I'm expected to be off the grid for at least another week. My lack of power is because of Hurricane Sandy, which hit my area of New Jersey especially hard.

The electrical power on Mars is concentrated at the Curiosity rover, a robotic science mission that's been on Mars since August 6, 2012. Curiosity has snapped a self-portrait of itself, as shown below. This photograph is actually a composite image created with 55 snaps of a camera, called the Mars Hand Lens Imager, attached to the rover's robotic arm.[1]

A self-portrait of the Curiosity Rover on Mars. The rover is at Bradbury Landing, a region of Gale Crater named after Ray Bradbury, a prolific author of science fiction who wrote The Martian Chronicles. The rover is powered by a radioisotope thermoelectric generator, as described in the text. (NASA/JPL/Caltech/Malin Space Science Systems image).

The Curiosity is powered by a simple nuclear power source called a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) which was designed to provide power for at least a Martian year (687 Earth days), but potentially up to fourteen Earth years.[2] This radioisotope thermoelectric generator functions by converting the heat of the radioactive decay of the plutonium isotope, plutonium-238, (²³⁸Pu) in plutonium dioxide. The initial decay reaction is straightforward, with production of uranium and an alpha particle; viz.,

²³⁸Pu -> ²³⁴U + ⁴He

The half-life of plutonium-238 is 87.7 years, so the power source remains fairly active for many years. The energy output of plutonium-238 by radioactive decay is 560 watts/kg. The emitted alpha particles transfer their energy to the mass of the isotope and its surroundings to create heat. The generator in the Curiosity rover contains 4.8 kg (10.6 lb.) of plutonium dioxide, which creates about 2,000 watts of heat. A schematic of the generator is shown below.

The Multi-Mission Radioisotope Thermoelectric Generator, as used by the Curiosity rover. The thermoelectric modules are made from junctions of lead telluride. (Modified US Department of Energy image).[3]

Conversion of this heat energy to electrical power is done through use of the Seebeck effect, which relies on the temperature differential between the hot plutonium and the colder Martian environment. As we know from the second law of thermodynamics, we need a temperature differential to do useful work. Junctions of n- and p-doped lead telluride perform this conversion, but the efficiency is low.[3-5] The 2,000 watt thermal power produces just a little more than a hundred watts of electrical power when the plutonium is fresh, and decreasing power thereafter.[2]

Curiosity is not the first time that such a plutonium power source has been used. As I wrote in a previous article (Radioactive Heat, July 26, 2011), the Cassini-Huygens spacecraft to Saturn contained 7.8 kilograms of plutonium-238 as the heat source. Since it's colder at Saturn than on the surface of Mars, this generator produced more electrical power per unit weight of plutonium than the Curiosity generator.

Similar thermoelectric conversion, using heat sources other than radioisotopes, has been proposed for energy-harvesting applications. These include power generation from automotive exhaust systems. You can experiment with this technology, since commercial modules are available.[6]

References:

1. High-Resolution Self-Portrait by Curiosity Rover Arm Camera, NASA/JPL, November 1, 2012.
2. Curiosity Rover, Mars Science Laboratory Web Site, NASA/JPL.
3. Space Radioisotope Power Systems, Multi-Mission Radioisotope Thermoelectric Generator, US Department of Energy.
4. Ralph Wyrick and Henry Levinstein, "Thermoelectric Voltage in Lead Telluride," Phys. Rev., vol. 78, no. 3 (May, 1950), pp. 304-305.
5. William E. Kortier, John J. Mueller and Philip E. Eggers, "Thermoelectric module," US Patent No. 4,211,889, July 8, 1980.
6. Thermoelectric Power Generation Products, Tellurex Corporation.