Using radioactive isotopes to power the Curiosity Mars Rover. The most recent rover sent to survey the surface of Mars, the Curiosity Rover, uses a nuclear battery to supply its energy needs for the life of its mission.
So, what is the Curiosity Rover?
In November 2011, NASA sent its 4th wheeled space rover to check out the surface of Mars – the Curiosity Rover. Its primary function is to search areas of the planet that may have once held liquid water, for evidence of past life. The baseline mission is expected to last approximately 2 years, but Curiosity’s generator will produce power for over 14 years. So like most space exploration programs, data will most likely be collected and sent back to Earth for many, many years.
What type of energy needs does the Curiosity Rover have?
The average temperature on Mars is in the -50oC range so to prevent the components of the Rover freezing it must be heated. Additionally, the CPU’s, extendable arms, wheels and other gadgets all require electrical power to carry out their function. So the power source on Curiosity needs to be able to supply both heat and electricity.
What was used to power the previous Mars Rovers?
The previous Mars Rovers were equipped with solar panels to provide their energy requirements. Mars has a very thin, dry atmosphere, so during the day the planet is bathed in sunlight. However, very dense dust storms that completely block out sunlight are frequent enough on Mars to provide 3 main problems: a physical risk to the integrity of the panels from the very fast moving dust particles, difficulties in restarting the computers and functions of the Rovers remotely from Earth after a storm, and layers of dust on the solar panels reducing their efficiency.
So, what is Curiosity's energy source?
Curiosity uses a Radioisotope Thermal Generator which uses a radioactive material as the heat source for a thermocouple to generate electrical current. First used in 1961, this technology has been used to power at least 26 NASA missions, including Earth orbiting satellites, some Moon missions, solar system probes and planetary landers. It is important to note that RTGs are not nuclear reactors, they have no moving parts and use neither fission nor fusion processes to produce energy.
What's a thermocouple?
When two dissimilar metals are joined and the joint is heated, an electric voltage is produced across the cold ends of the two metals. This figure below depicts a heat source on the left side, with p and n-type conducting materials that are joined on the hot end. The temperature gradient causes electrons and holes to conduct across to the heat sink on the right.
This conduction creates an electric potential between the top and bottom electrodes, with positive and negative ends on the heat sink side of the thermocouple.
 |
Where does the heat come from?
Plutonium-238 is a radioactive isotope that works well in an RTG, usually in the form of Plutonium-Dioxide. When Pu-238 decays it emits alpha particles, which are simply energised and completely ionised helium atoms, the alpha particles then lose their energy in the form of heat when interacting with other matter. This is a similar mechanism to how friction generates heat on a surface.
Since Pu-238 has a half-life of 87.7 years and alpha particles can be shielded by as little as a piece of paper, the fuel source lasts long enough to be useful and can be shielded easily, so there is little worry of the radiation damaging other components of the RTG.
On Curiosity's RTG, the heat produced by decaying Pu-238 is used for 2 purposes. Firstly, as the heat source for a thermocouple, along with fins exposed to the cold Martian atmosphere for the heat sinks, to create a voltage across the thermocouple.
Secondly, since a thermocouple is unable to convert all the heat energy generated from the plutonium decay, the “waste” energy is used to warm the components of the Rover so they do not freeze in the cold conditions.
It's Plutonium! Is it safe?
Plutonium always has 94 protons in its nucleus but there are several “isotopes” of Plutonium, i.e. same number of protons with different numbers of neutrons.
Plutonium-239, the isotope with 94 protons and 145 neutrons, is fissile - which means that if it absorbs a low energy neutron it has a possibility of splitting, or fissioning. It is this property that allows plutonium-239 to be used in nuclear reactors and weapons. It is Pu-239 that gets all the bad media coverage.
However, devices that use plutonium to produce power use the plutonium-238 isotope, which has 94 protons and 144 neutrons. It is not fissile, can't be used in atomic bombs or nuclear reactors and is pretty much only useful as a radioisotope heat source.
Summary and Stats of the Curiosity Rover
- Type of generator - MultiMission Radioisotope
- Thermoelectric Generator
- Built By: Boeing and Idaho National Laboratory.
- Amount of plutonium - 32 cubes of plutonium-238 dioxidefor total mass of 4.8 kg
- Thermal power – 2000 watts
- Thermocouple Metals – PbTe and TAGS (an alloy of Tellurium, Silver, Germanium, Antimony)
- Electrical power – 125 watts, providing approx 9 MJ (2.6 kilowatt hours) per day. Supplied by: Teledyne Energy Systems
Other power sources
- Solar panel giving 2.1 MJ (0.6 kilowatt hours) per day
- Lithium-ion batteries giving 42 amp-hours (to provide extra power to the rover when the demand exceeds the generator's limited output.)
Plutonium-238 in short supply
Globally, plutonium-238 is in very limited supply and future robotic missions to Mars, and inter-planetary missions beyond Mars are at risk as the United States and Russia no longer produce their own supplies.
NASA has stated that there is enough Pu-238 to fuel NASA missions to around 2022 and that if the United States restarted production of Pu-238, it would take 5 years to produce enough fuel for a single spacecraft mission.
In February 2013 the Oak Ridge High Flux Isotope Reactor reported the first production of a small amount of Pu-238, the first produced in the USA since the late 1980s, however no plans have yet been announced for the ongoing production of this important “nuclear battery” material.
Published: 01/05/2013