Take a second and look around you. I’m willing to bet that you have at least half a dozen objects in your immediate vicinity use electricity in some way to power its systems. Maybe you’re using a battery or you’re plugged directly into an outlet, but either way, this energy came to you via coal/wind/solar/geothermal or a host of other methods we use to generate power here on earth. Well, what about our spacecrafts? How do they get the power they need? It depends on where they are in the solar system.
For spacecraft operating inside the inner solar system (including earth orbit), the traditional power source is solar. Solar panels (also called photovoltaics) convert light energy into usable electrical energy. They accomplish this feat because each solar panel is made up of individual solar cells. Each cell has a semiconductor, which in turn is wired to a circuit. As light hits the semiconductors, the light in converted into electricity and voilà, we have power. Because solar panels require light to work they are usually designed to be independent so they face the sun regardless of the direction the spacecraft is moving or pointing. As a general rule, these spacecraft are also equipped with small batteries in case it is eclipsed from the sun. These batteries are then recharged by the solar array when this eclipse is over.
These solar panels give spacecraft like the International Space Station and the Hubble Space Telescope that iconic look. But, what about spacecraft like Cassini, Galileo, and Juno? These crafts don’t have solar panels. That’s because, as you move further away from the sun (into the outer solar system and beyond), solar power starts getting too weak to use. Instead, these spacecraft use power from a Radioisotope Thermoelectric Generator (RTG). This fancily named device generates electricity obtained from the radioactive decay of isotopes. RTGs convert this decay into electricity by an array of thermocouples. A thermocouple is a device which converts thermal energy into electrical energy by the conductivity of two metals. When these metals are at different temperatures and connected to each other in a closed loop, electrical current flows between them. Energy which is then harnessed by the spacecraft.
So if ever you were wondering how these amazing vessels get the power to do what they do, well, now you know.