AFTER 22 years of service, NASA is about to abandon one of its longest-serving space probes, leaving it to carry on its journey to the stars entirely alone. When Pioneer 11 set off on 5 April 1973, Donny Osmond was number one in the charts and Sunderland on course to win the FA Cup. It is now farther from the Sun than Pluto, the outermost planet. 鈥淚t鈥檒l be a very sad day for all of us,鈥� says Manfred Wirth, manager of the Pioneer programme. 鈥淏ut nobody thought the probe would survive even this long.鈥�
Pioneer 11 was launched a year after its twin, Pioneer 10. The two 500-kilogram spacecraft demonstrated that it was possible to fly through the asteroid belt unscathed and that the intense radiation belts around Jupiter need not be fatal. While Pioneer 10 headed out of the Solar System after making its rendezvous with Jupiter in 1973, Pioneer 11 flew on to Saturn and in 1979 became the first probe to visit the planet.
鈥淧ioneer 11鈥檚 problems began in 1991,鈥� says Wirth. 鈥淚t became impossible to manoeuvre the spacecraft to point its antenna at the Earth precisely.鈥� Since then, contact with the probe has been lost periodically as the Earth moves in its orbit around the Sun.
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Pioneer 11 has also lost most of its power supply. Its plutonium-powered thermoelectric generator can barely deliver the 25 milliamps needed for its Geiger tube telescope, the least power-hungry of the 10 scientific instruments on board. 鈥淭he power problem has sealed Pioneer 11鈥檚 fate,鈥� says Wirth. 鈥淣ASA says, why commit resources to a probe that鈥檚 doing no science?鈥�
Wirth says the team will still observe the spacecraft from time to time, 鈥渂ut only out of engineering interest to watch it slowly die鈥�. The team will also continue to track Pioneer 10, which has not lost as much power as its twin. It had been hoped that the two Pioneers would pass through the 鈥渉eliopause鈥� 鈥� the 鈥渂ow shock鈥� where the solar wind slams into the interstellar medium 鈥� and then sample the gas and dust between the stars. But Pioneer 10, currently the most distant manmade object at 9.5 billion kilometres from the Earth 鈥� or 63 times the distance from the Earth to the Sun 鈥� has only three years to reach the heliopause before its power supply fades.
If, as some planetary scientists now believe, the edge of the Sun鈥檚 influence extends to at least 100 times the distance between the Sun and Earth, the prize will not go to Pioneer 10 but to one of the two Voyager space probes, which are expected to survive until 2015.