Feature Sailing through space   An ion engine     Until now, nearly all space missions have used rocket motors that feed on chemical fuel. Unfortunately, these are not very efficient. For example, over half the launch weight of ESA's Rosetta spacecraft was made up of fuel.   ESA is now studying ways to cut the amount of fuel that is carried. One idea is an ion engine, which uses an electric 'gun' to fire gas into space.     Although the engine's forward push is very slight, it gradually builds up speed. Eventually, it enables a spacecraft to travel very quickly.         Artist's impression of SMART-1   An ion engine was successfully tested by SMART-1 on its way to the Moon. The engine produced 10 times more thrust than an ordinary rocket motor for every kilo of fuel it used. ESA is also studying spacecraft that use solar sails instead of engines. Sunlight hitting a large, mirror-like sail will be able to push a spacecraft toward other planets. After many months, such a spacecraft could reach a speed of 360 000 km/h. Another way to save fuel and weight is to slow down by flying through a planet's upper atmosphere. ESA is planning to build a spacecraft that will enter orbit around Mars using this 'aerocapture' method.       Last update: 2 December 2004 Exploration  •  ExoMars (http://www.esa.int/esaKIDSen/SEM2UIWJD1E_LifeinSpace_0.html)  •  Future Mars exploration (http://www.esa.int/esaKIDSen/SEM3VIWJD1E_LifeinSpace_0.html)  •  New transport (http://www.esa.int/esaKIDSen/SEMCSKWJD1E_LifeinSpace_0.html)  •  More than 50 years of space! (http://www.esa.int/esaKIDSen/SEMH9E7H07F_LifeinSpace_0.html)