After more than 18 years of dedicated service, the solar
orbiter Ulysses is due to end its tenure Jun 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 ground-breaking 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 shut-down 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 shut-off, 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.
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