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When it arrives at its destination in July 2015, NASA's New Horizons will be the first spacecraft to encounter the solar system's most distant dwarf planet, Pluto. This is the latest chapter in over 45 years of planetary exploration, which has involved fly-bys of eight planets and numerous moons, orbiting missions to Venus, Mars, Jupiter and Saturn, and even a landing on Titan, Saturn's mysterious, orange, cloud-covered moon. As non-experts, we can only marvel at some of the technology that made these missions possible.

However, we can appreciate something of the computer systems on board the spacecraft that have taught us so much about our neighbours in space and taken man to the Moon. As we'll soon discover, these space-borne computers bear only a passing resemblance to the PC sitting on your desk, due to the hostility of the space environment to computers and electronic equipment.

Taking to the skies

According to one astronaut, lift-off in the Space Shuttle feels like "driving down a railroad track in a car with no shock absorbers". The deafening sound generated by the rocket engines vibrates the entire Shuttle and everything in it. Not only that, but the launch vehicle is also subjected to intense vibrations caused by engine ignitions and shutdowns, as well as the sudden shocks that occur when the solid fuel boosters are jettisoned, the stages are separated and the payload is ejected into orbit. Spacecraft also experience extreme temperature variations during takeoff and landing. The combination of high levels of vibration and

Hazards in space are not restricted to forces that we can see, feel and intuitively understand. Space also poses an invisible threat to electronic equipment. On Earth, we're protected from radiation by the atmosphere. In space, there's no such protection. Spacecraft, astronauts and computers are constantly bombarded by high-energy particles from the Sun and from outer space. Low-level radiation can cause premature ageing of electronic circuits, while particularly high-energy particles can result in single-event upset (SEU) failures. In the best possible scenario, these events cause a glitch. In the worst-case scenario, they can cause catastrophic hardware failure.

The life expectancy of a domestic or work PC is generally thought to be three to five years. However, computers on board spacecraft may be called upon to last for longer - sometimes much longer. Take the extreme case of the twin Voyager spacecraft, which were launched in August and September 1977. Their primary mission was to undertake a grand tour of the gas giants, the huge planets that reside beyond the asteroid belt. They took 18 months, three years, eight-and-a-half years and 12 years to reach Jupiter, Saturn, Uranus and Neptune respectively, which demonstrates the sort of timeframe in which space computers need to operate reliably. That's just the start. Voyagers 1 and 2 are now involved in an inter-stellar mission, exploring the outer limits of the solar system and possibly beyond. This extended period of their mission is expected to last until 2020, when the spacecraft will be 43 years old.

Spacecraft, especially unmanned probes en route to the planets, have to be self-contained. Unfortunately, power on board a spacecraft isn't exactly in abundance. Any power source - be it solar panels, fuel cells or a nuclear reactor - adds to the size and weight of the spacecraft, hence the difficulty of getting it airborne. So it goes without saying that computers must be very frugal in their power requirements.