Solar cells have made space habitation and space research possible. The International Space Station gets its electrical power from massive arrays of solar cells. Similarly, the earlier Space Lab and Mir Space Stations also used solar power. The Space Shuttle does not use solar cells, but uses fuel cells, which produces drinking water as a by-product. Most satellites orbiting the earth use solar cells for power. Planetary probes go away from the earth and sun and the light intensity diminishes, so they do not rely on solar power but use other power sources.