SATELLITE communication has become reality, crossing over from science fiction to emerge as the lifeblood of our times. The information superhighway - known better as the Internet - live television discussions, those essential cricket matches from countries halfway across the globe and a lot more could not have been possible without these eyes in the skies'. Positioned high above Earth, satellites receive, transfer and broadcast information at amazing speeds. In short, take them away and we revert back to the Stone Age. However, the only problem with them is their short life span: most 'die out' after a decade or so. But now, scientists have come up with a novel propulsion system that gives satellites what they always missed: longevity. A motor that uses jets of ionised gas to keep spacecraft in their allotted place in the sky could soon boost satellite television and communications, they say.

Most of the satellites in orbit today last some 10-odd years, after which they have to be replaced. The new ion propulsion system which recently went into orbit on the European television satellite Astra 2A in September, has an astounding life span of 25 years. Launched from Kazakhstan's Baikonur Cosmodrome, the Astra 2A satellite brought Rupert Murdoch's 200-channel Sky Digital Television service to British viewers from October. Thanks to the new propulsion system, Astra 2A will be working well after 2020.

Every satellite needs some mechanism to keep it accurately positioned in space. This mechanism, usually a motor, has to be operated every day to compensate for the effects of solar and lunar gravity which continually threaten to pull the craft out of its allocated corner of the sky.

Until now, these station-keeping motors used chemical rockets, usually powered by a mixture of monomethyl hydrazine and nitrogen tetroxide. When this fuel runs out, the satellite wanders out of control. This not only makes reception of its signals unreliable, but also brings the risk that it might collide with one of its neighbours in space. So the last drops of fuel are used to 'dump' the satellite into a 'graveyard orbit' where it sits useless but safe before finally falling to Earth and burning up. Early satellites lasted only a few years. More recent models have a life of seven to 10 years.

California-based Hughes Space and Communications began looking for an alternative to chemical thrusters in the early 1960s. In the beginning it tried ionising caesium or mercury vapour, but this corroded the jet nozzles. Then in 1984, Hughes' engineers found that they could ionise xenon, an gas that is inert or non-reactive, to create as much Eye in the sky-, Astra 2A, ready for a longer life thrust as a chemical jet. And the new Astra satellite uses this xenon ion propulsion system (XIPS), to maintain an accurate orbit.

Years of ground and vacuum chamber testing by Hughes' scientists have finally paid off, resulting in the XIPS, a reliable engine that can safely power a spacecraft. In the XIPS motor, xenon gas is fed into a chamber where it is ionised. A series of grid electrodes, like those found in old radio valves, accelerate the gas into 3,000 parallel streams which leave the chamber through an outlet 13 cm in diameter, at a speed of 30 km per second. A neutraliser electrode mounted outside the jet squirts electrons at the positive ions to prevent them from flowing back towards the satellite casing, slowing the craft down.

The xenon fuel weighs only one-tenth as much as an equivalent amount of chemical fuel. Astra 2A has had enough xenon on board to keep it going for more some 15-odd years. Moreover, Astra asked Hughes to include a chemical rocket too - for use as a backup incase of an emergency. If the systems operated as planned, Astra's working life will be almost doubled.

"This has a mixed meaning for us," says Hal McDonnell from Hughes, whose job is to sell satellites to broadcasters. But if the new motor performs as planned, future satellites will need to be replaced less frequently.