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After more than 18 years studying the Sun, the plug is finally being pulled on the ailing spacecraft Ulysses.

Final communication with the joint European-US satellite will take place on 30 June.

The long-serving craft, launched in October 1990, has already served four times its expected design life.

The Esa-Nasa mission was the first to survey the environment in space above and below the poles of the Sun.

Data from the craft, published last year, also suggested that the solar wind - the stream of charged particles billowing away from the Sun - is at its weakest for 50 years.

"We expected the spacecraft to cease functioning much earlier," said Paolo Ferri of the European Space Agency (Esa).

"Although it is always hard to take the decision to terminate a mission, we have to accept that the satellite is running out of resources and a controlled switch-off is the best ending."

Long life

Ulysses has already defied the odds several times. In its 18-year life, the mission has been extended four times.

But its protracted mission has taken its toll. Ulysses’ main transmitter no longer works and its back-up systems are also beginning to fail.

Last year, the space agencies finally announced that they were finally ready to pull the plug after the satellite’s power supply had weakened to the point where the craft could no longer prevent its hydrazine fuel from freezing.

Engineers believed the craft would become uncontrollable and its end of life was scheduled for 1 July 2008.

However, mission scientists came up with a short-term fix whereby the fuel could be kept circulating by performing a short thruster burn every two hours.

The ingenious fix gave the craft another year of life. But, now, scientists believe it is time to switch off the mission.

In particular, they feel the scientific return has reached a level where it is hard to justify the operational costs.

Final communication with the craft will begin at 1635 GMT and run until 2120 GMT on 30 June, after which no further contact is planned. The craft will in effect become a man-made comet.

"[It] will be a very sad day when we send the last commands to Ulysses," said Nigel Angold, Esa Mission Operations Manager.

Source: BBC News