Space invader

An electric propulsion engine developed in the UK is to be used to send a mission to map and study Mercury. The European Space Agency’s BepiColombo mission, which will take three and a half years, will cover the 92.7 million kilometres to Mercury using just 350kg of propellant, travelling at the equivalent of around 658,000mpg.

The mission, which is scheduled for launch in 2009 or 2011, has only been made possible by the development of electric propulsion, said Angela Brown, senior space engineer at Qinetiq, which is working on the gridded ion engines to be used for BepiColombo. The system allows spacecraft to travel to distant planets with a much lower mass of propulsion, she said. ‘This is crucial for the Mercury mission – you could not carry it out with chemical propulsion, the mass would be too great. The only way to do so is with electric propulsion.’

Electric propulsion converts sunlight into electricity via solar panels, and uses it to electrically charge inert xenon atoms, which accelerate from the spacecraft at high velocity, driving the spacecraft forwards. Ion engines expel gas at much higher velocities than chemical propulsion systems.

There are two types of electric propulsion system. hall-effect thrusters, developed by companies such as Snecma, which accelerate the xenon ions out of the engine using an electron cloud, and gridded ion engines. These use a series of biased grids to accelerate the ions electrostatically. Qinetiq’s engine was chosen for the trip to Mercury as gridded ion systems are more efficient than Half effect types, making them more suitable for missions where journey time is not a major concern.

The company will begin testing the engines in its new ion propulsion chamber in January to ensure they can withstand the difficult conditions they are likely to experience when they reach the planet. ‘Mercury is so different – there is nine times the amount of sunlight and radiation experienced on earth – so the whole system will have to cope with a much harsher environment. Esa is already satisfied with the basic technology. What we don’t know is how it will cope with effectively being put into a furnace,’ said Brown.

The team will heat the primary engine in the vacuum chamber to produce similar conditions to those on Mercury. The researchers will then start up a second engine and move the two closer together to ensure they can operate close to each other, said Brown. ‘The design for the spacecraft uses a triangle of three engines, with two operating at any one time, so we need to be sure this won’t have an impact.’