HENNING DALHOFF / SCIENCE PHOTO LIBRARY HENNING DALHOFF / SCIENCE PHOTO LIBRARY
Computer illustration of the xenon ion thruster used to propel the Dawn spacecraft. The propellant xenon (green spheres) is bombarded with electrons (yellow spheres). This causes the xenon atoms to lose electrons, resulting in positively-charged xenon ions (blue spheres). These ions are expelled from the engine, creating thrust that propels it through space. Although the ions are fired in great numbers out of the thruster at more than 110,000 kilometres per hour, their mass is so low that the engine produces a gentle thrust of only 90 millinewtons. However, it provides more thrust for weight of fuel than chemical rockets and so is well suited to long space journeys. Dawn was launched in 2007 to explore the protoplanet Vesta and dwarf planet Ceres, which are located in the asteroid belt.
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