The following article is from the London Times, Friday 19 May, 1995:

Scientists are listening this week for the failing heartbeat of a spacecraft carrying a message from the human race into infinity.

Four billion miles away, Pioneer 11 is about to lapse into silence more than 22 years after it was launched from Cape Canaveral in Florida. When it does, any future contact it makes with intelligent beings will leave them pondering over a plaque bearing the image of a naked man and woman, the man's palm raised in greeting.

The craft was designed to explore the outer planets, Jupiter and Saturn, then sail into interstellar space. Aware that it might one day be intercepted by aliens, Nasa placed the "visiting card" aboard. There is also a diagram of the hydrogen atom, a locator map identifying the position of the Sun and a diagram of the solar system. From this, it was believed, strangers could work out where Pioneer 11 had come from.

Pioneer 11, whose odyssey began in 1973, is now so distant that its fading signals - a mere four billionths of a trillionth of a watt - take more than six hours to reach Earth. Nasa officials are to conduct one final test that could extinguish those signals for ever.

Next week engineers will contact Pioneer 11 to see if it can operate at a reduced voltage. There is a risk that this may shut the spacecraft down but it is worth taking because, according to Fred Wirth, the project manager, its days are numbered: by August it is expected to have too little power to operate its scientific experiments.

"For all intents and purposes, after August the mission of the spacecraft is over because what good is a spacecraft if it doesn't return science data? You can't justify spending money on no data," Mr Wirth said. "That's why we're using Pioneer 11 at its last moments to see how it functions on reduced power."

Pioneer 11's sister ship, Pioneer 10, is even further away. Both use the decay of plutonium to provide their power. The two spacecraft were virtual twins but for reasons that are not understood Pioneer 10's power supply has lasted better. Already the most remote object ever made by man, it is 5.8 billion miles away and is expected to go on sending data for another three or four years.

James Van Allen, a physicist at the University of Iowa and discoverer of the radiation belts around the Earth that bear his name, has experiments on both craft that continue to work well.

He said: "The new tests planned this month may result in a loss of tracking. I don't discount that totally." However, he hopes that Pioneer 11 will survive a little longer. "I still have the strong impression that the funeral is a little bit premature."

The longevity of the two spacecraft, built by TRW Inc in California, has astonished engineers. Pioneer 10 was launched in March 1972, Pioneer 11 in April 1973. Both visited Jupiter and Pioneer 11 flew on to Saturn, making the first observations at close quarters in 1975 and its closest approach in 1979. It discovered Saturn's eleventh moon, the fact that the planet has a magnetic field a thousand times stronger than Earth's, and two new rings.

In 1990, it passed beyond the orbits of the planets, continuing to send back data about cosmic radiation. The precise paths followed by the two spacecraft have since been used to calculate whether there is a tenth planet in the solar system. The results suggest that if there is, it approaches the Sun only every 700 to 1,000 years.

Whatever happens, there will be no stopping either spacecraft, which will continue into space for ever. "I feel sad to see the old bird fold up," Dr Van Allen said. "But it's has a glorious history."