The story of Radioisotope thermoelectric generator

Have you ever thought how the spaceships, space probes and the satellites get the electricity? At a glance, it is like by batteries or by a solar panel. But do you think that all the spaceships are getting proper sunlight throughout the journey? No, As an example, space probes like Voyager and Cassini are not getting sunlight for a long time more often. Mostly in deep space nothing is boundless.

You may think that nuclear reactors are used for this. Even it can be done, scientists are not considered about that very much. Because it is not easy to keep nuclear reactors inside a spaceship.

So how it really happens?

As the best solution for this, RTGs (radioisotope thermoelectric generator) are used. The generators that we see every day are worked with the power of diesel or petrol. But this generator is worked by the power of a Radioactive isotope. These generators work on the kinetic energy of the alpha particles emitted from radioactive isotope by generating the heat. So it is easy to use thermocouple arrays and get the electricity.

The main advantage in this method is, these RTGs can be used without manpower. And in another hand, can generate power between 50W to 100W with no issues. But it doesn’t mean that spaceships are not using solar panels or battery packs. The Curiosity rover on the mars is also using an RGT like this. The Insight Mars lander which was landed on November 26, 2018, is also using an improved RTG called Multi-Mission Radioisotope Thermoelectric Generator (MMRTG).

The RTG was introduced to the world in 1954 by famous scientists, Ken Jordan and John Birden. These RTGs were used practically on Galileo, pioneer, Voyager, Cassini spaceships. Other than that, Soviets used the same method for the lighthouses in areas such as arctic since people couldn’t reach those areas.

Let me explain, how it really works. Mostly the alpha particles are used for this, you may think why it is being used since it has the least ability to go through a metal plate. But that is what has to have happened here. These particles have a positive charge of 2+. This action causes the alpha particles to quickly damage the energy. When it doesn’t go through an aluminum plate with a thickness of 0.1mm, it is more likely to stop at a very low distance. In this way, the metal is heated because the alpha particles do not go through the plate. That is what needed by the thermocouple to be functioned. The other thing is that the radioactive isotope must have a balanced half-life, which is not more or less. Due to that reason, plutonium 238 is used for this process. Plutonium 238 has a half-life of 87.7 years.

When the spaceships are getting away from the sun, electronic items can be damaged due to the cold weather. As an option for that RHU (radioisotope heater units) are placed inside the spaceships to balance the temperature. These heater units are also operated by plutonium238.

The only disadvantage of RTGs is the relatively low performance (7%). And another important fact is the efficiency depends on the metal used for the Thermal couple. Silicone germanium semiconductor is used make Thermal couple this RTG. If a metal is used without a semiconductor in the thermal couple, the efficiency of RTG is reduced by 3%.

And there is a very special occasion where these RTGs are used, in the medical field. When the pulses are given by a pacemaker, a very small RTG is placed in that equipment. Since it is not practical to give energy to this equipment by a battery, RTG is the most practical solution to get a lifetime electric energy. In another hand alpha particles are not getting out of RTG it is safe to the body cells, mmmm the only thing to be concerned is plutonium.

If you have ever watched the Martian movie, Marc is keeping RTG in the back to avoid the cold !

article :- Kavindu Sri Supekshala Hettiarachchi

resources:-internet,NASA newsletter,matter and radiation book by prof.S.R.D Rosa