Once a feature of science fiction stories, the ion drive is to power a new generation of commercial communications satellite for the internet age.
The drive, five times as efficient as chemical rockets, is to be used for the first time to power a commercial satellite by Inmarsat, the satellite network company, for its new generation of broadband communications satellites.
‘This [efficiency] means the lifespan of the satellite is increased, which can expand mission duration and also means that new types of missions can be undertaken,’ said Paul Masters, Inmarsat’s electronic systems engineering manager.
The force produced by the ion drive, or plasma thruster, is small, no more than the force of a piece of paper lying on someone’s hand. Masters said: ‘This is just millinewtons as opposed to newtons or tens of newtons normally generated by chemical rockets, but in space it doesn’t matter because the satellites are in a micro gravity environment.’
As long as the drive can provide its thrust for a long period of time then a spacecraft can accelerate to very high speeds. NASA used an ion drive for its Deep Space One probe, which was sent out to encounter asteroid 1992KD in 1998. It accelerated the probe for 200 days until the spacecraft reached a speed of 7,000mph.
In the case of the Inmarsat satellites the drive will be used to make changes to their orbit.The fuel it uses is the inert gas xenon. A tank of this is stored as a liquid at 300bar on the satellite. It is released at the lower pressure of 2bar, whereupon it becomes a gas.
The flow of gas has its electrons stripped away to make the atoms positively charged. They are then electromagnetically accelerated and directed to create a flow of ions in one direction to push the satellite forward. The exhaust emits a blue glow rather like the engines of the USS Enterprise-D, from Star Trek: The Next Generation.
Astrium, the joint venture between the European Aeronautic Defence and Space Company and BAE Systems, is building the series of satellites for Inmarsat. However, the ion drive has been supplied by the Russian firm Fakel via the French engine company Snecma.
The Russians have produced many different sizes of plasma thruster, which have so far all been used on government spacecraft rather than commercial satellites.
Fakel’s range includes units smaller than conventional chemical engines, another advantage of the ion drive: the weight saving on a satellite with an ion drive can be over one tonne on a 4.5 tonne spacecraft.
The size of the ion drive can also widen the mission remit, as it allows more electronics to be built into the satellite.