After more than 18 years of dedicated service, the solar orbiter Ulysses is due to end its tenure June 30.
The joint ESA/NASA spacecraft will finally switch off its transmitter, after defying several earlier expectations of its demise.
Ulysses is the first spacecraft to survey the environment in space above and below the poles of the sun in the four dimensions of space and time.
Among a number of groundbreaking results, the mission showed that the sun's magnetic field is carried into the solar system in a more complicated manner than previously believed. Particles expelled by the sun from low latitudes can climb up to high latitudes and vice versa, even unexpectedly finding their way down to planets.
This means that regions of the sun not previously considered as possible sources of hazardous particles for astronauts and satellites must now be taken into account and carefully monitored.
"Ulysses has taught us far more than we ever expected about the sun and the way it interacts with the space surrounding it," said Richard Marsden, ESA's Ulysses project scientist and mission manager. The shutdown of the satellite is a joint decision of the two agencies and comes a year after the mission was expected to end.
A year ago, the satellite's power supply had weakened to the point that it was thought the low temperatures would cause the fuel lines to freeze up, rendering Ulysses uncontrollable. This didn't happen immediately and spacecraft controllers realized that they could keep the fuel warm and circulating by performing a short thruster burn every two hours.
But as Ulysses has moved further from Earth, the communications bit-rate has gone down, and the mission managers decided they could no longer justify the cost of keeping Ulysses in operation.
"We expected the spacecraft to cease functioning much earlier," said Paolo Ferri, head of the Spacecraft Operations Solar and Planetary Missions Division. "Its longevity is a tribute to Ulysses's builders and the people involved in operations over the years. 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."
After shutoff, Ulysses will continue to orbit the sun, becoming in effect a man-made 'comet'.
"Whenever any of us look up in the years to come, Ulysses will be there, silently orbiting our star, which it studied so successfully during its long and active life," said Marsden.