Death of a Pioneer

The tiny probe which boldly went where no man-made object had gone before is to be shut down in space.

In a few weeks, a rather sad ceremony will take place at one of Nasa's network of radio telescopes which tracks probes in outer space. After a few melancholy words, a scientist will tap a few keys on a keyboard instructing a computer to transmit a simple message across space.

Travelling at the speed of light from the radio telescope, the signal will flash past the moon in little more than a second. A few minutes later, it will have crossed the orbit of Mars, and, after almost four hours, it will have passed the region occupied by the most distant known planet. But its journey will be far from over. It will take nine hours for the signal to reach the tiny Pioneer 10 probe, the most distant man-made object ever sent into the cosmos.

Upon receiving the message, Pioneer 10 will do just one thing: switch itself off. A tiny voice in the immensity of space is about to fall silent forever.

Pioneer 10 was launched in 1972, with an estimated lifetime of three years. Its main mission was to reach Jupiter, the largest planet in our solar system. Before its launch, no spacecraft had travelled further than Mars. And beyond Mars, there lay what might have been an insuperable barrier to space flight: orbit upon orbit of tens of thousands of pieces of rocky debris known as the asteroid belt, which no spacecraft had yet braved.

But Pioneer 10 emerged unscathed from the asteroid belt and blazed the way to Jupiter. When it got there, it gave scientists their first close- up on the planet's kaleidoscopic display of multi-coloured clouds, detected its intense magnetic and radiation fields, and even found a thin ring of dust around the planet.

Leaving Jupiter intact was a real bonus for the scientists, as Pioneer 10's mission could now be extended to find the edge of the solar system. This was expected to be not far beyond Jupiter. The Sun gives off not just light but also a stream of particles called the solar wind. These carry with them a magnetic field, and together form a vast magnetic bubble around the Sun. This is the heliosphere, and at the time of the launch of Pioneer 10, its edge, the heliopause, was estimated to be just beyond the orbit of Jupiter.

The heliopause is an important place. Here, the Sun's influence ceases and galactic space begins. The solar wind and the material between the stars clash in a region of shock waves and turbulence. With instruments on board a spacecraft orbiting the Earth, we can look upstream to this region, in the direction of the Sun's travel through space, and see a faint glow. But week after week, month after month, Pioneer 10 did not reach the heliopause. It crossed the orbits of Saturn, of Uranus, of Neptune and tiny Pluto, yet seemed to be getting no closer to the edge of solar system space. But meanwhile, Pioneer was dying. Its tiny energy source, of radioactive plutonium-238, was fading. All but three of its 11 scientific instruments were shut down last year, and now it is down to just one - its cosmic ray detector.

Almost 9.7 billion miles away, the final goal of the most distant man- made object in the universe still remains beyond its reach. Every few days, one of the large telescopes that makes up Nasa's deep space tracking network turns towards the distant Pioneer 10 and picks up its signal - barely discernible against the background noise - from the radio transmitter, which has an output of just eight watts. It's too little for a night-light.

As it travels ever outwards at 27,800 miles per hour, its instruments are still searching for the outer boundary of the solar system; but the solar wind continues to waft past, even out in space's cold outer darkness. Faced with diminishing scientific return versus the cost of tracking it, Nasa has ordered all communications to cease on 31 March.

It is sad news, not just for the Pioneer scientists, but also for those scientists involved in scanning the sky for signals from any extraterrestrial intelligence. They have frequently used Pioneer's feeble voice to test their systems, which hope to pick up even fainter radio messages from distant civilisations. Soon they will lose their main diagnostic tool, making the search for life in space that little bit harder.

But - fate willing - the silent Pioneer 10 may still have one final task to perform as it begins its eternal drift amid the stars. On its side is a plaque inscribed with symbols, binary numbers and drawings telling of the creatures who made it. In a mere 100,000 years, it will drift past the nearest star to our solar system, a faint red star we call Proxima Centauri. From then on, who knows what other stars it will visit? It will still be somewhere among the stars long after our Sun and the Earth are gone, and possibly long after mankind is just a memory.

After Pioneer 10 is closed down by a signal from Earth, who knows if the next signal it receives will be from someone, or something, else? Who knows what they will make of this primitive space probe? Pioneer 10 could be what mankind is judged by

The writer is the BBC's science correspondent