Solar cells from the Landsat 5 satellite, 1984.

Solar cells convert sunlight directly into electricity. They are usually made from sheets of silicon to which a small quantity of boron has been added. One side is also coated with small quantities of phosphorus. They have a different atomic make-up to silicon, and when sunlight falls on the phosphorus-rich side, the phosphorus gives up electrons, which flow to the boron-rich region, constituting an electric current. Each cell only produces a small amount of electricity and so thousands, arrayed on a panel, are needed to power a large satellite. The Landsat satellites (originally Earth Resources Technology Satellites) can be used to provide information for map making, studies of land use, pollution monitoring and mineral prospecting.