Stock Photo - EARTH The Moon -- 2006 -- European Space Agency artist's impression of the SMART-1 spacecraft - the first ESA spacecraft to use the new and more efficient three-layered triple-junction gallium-arsenide (GaAs) solar cells. They have 21% efficiency compared to the 6-12% efficiency of those commercially available previously, which were made from materials similar to the solar cells used in pocket calculators. Soon GaA-based solar cells, which are the subject of further development by ESA and European industry, could also be a very powerful source of energy for other applications on Earth as well as in space. These high-powered solar cells ""ion engine"" which fires the ion particles which sent this spacecraft on it's three year journey to the Moon -- Picture by Lightroom Photos / ESA.