Cassini uses roughly 1 kW. A power supply for an interplanetary spacecraft must provide a large percentage of its rated power over a lifetime measured in years or decades. As the term suggests, photovoltaic materials have the ability to convert light directly to electricity. Crystalline silicon and gallium arsenide are typical choices of materials for deep-space applications. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are available in less-expensive standard ingots which are produced mainly for consumption in the microelectronics industry. When exposed to direct sunlight at 1 AU, a current of about an ampere at 0.25 volt can be produced by a 6 cm diameter silicon cell.
Gallium arsenide is notably tougher and more efficient.
Spacecraft solar panels are constructed of these cells trimmed into appropriate shapes and cemented onto a substrate, sometimes with protective glass covers. Electrical connections are made in series-parallel to determine total output voltage. The resulting assemblies are called solar panels, PV panels, or solar arrays. The cement and the substrate must be thermally conductive, because in flight the cells absorb infrared energy and can reach high temperatures, though they are more efficient when kept to lower temperatures. Farther than about the orbit of Mars, the weaker sunlight available to power a spacecraft would require panels larger than practicable because of the increased launch mass and the difficulty in supporting, deploying, and articulating them.