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Ulysses
Mission Coming To A Natural End
Friday, February 22, 2008
Credits:
ESA
Ulysses, the mission to
study the Sun’s poles and the influence of our star on
surrounding space is coming to an end. After more than 17 years
in space – almost four times its expected lifetime –
the mission is finally succumbing to its harsh environment and is
likely to finish sometime in the next month or two.
Ulysses
is a joint mission between ESA and NASA. It was launched in 1990
from a space shuttle and was the first mission to study the
environment of space above and below the poles of the Sun. The
reams of data Ulysses has returned have forever changed the way
scientists view the Sun and its effect on the space surrounding
it.
Ulysses is in a six-year orbit around the Sun. Its
long path through space carries it out to Jupiter’s orbit
and back again. The further it ventures from the Sun, the colder
the spacecraft becomes. If it drops to 2ºC, the spacecraft’s
hydrazine fuel will freeze.
This has not been a problem
in the past because Ulysses carries heaters to maintain a
workable on-board temperature. The spacecraft is powered by the
decay of a radioactive isotope and over the 17-plus years, the
power it has been supplying has been steadily dropping. Now, the
spacecraft no longer has enough power to run all of its
communications, heating and scientific equipment simultaneously.
‘We expect certain parts of the spacecraft to reach
2ºC pretty soon,” says Richard Marsden, ESA’s
Ulysses Project Scientist and Mission Manager. This will block
the fuel pipes, making the spacecraft impossible to maneuver.
In an attempt to solve this
problem, the ESA-NASA project team approved a plan to temporarily
shut off the main spacecraft transmitter. This would release 60
watts of power that could be channeled to the science instruments
and the heater. When data was to be transmitted back to Earth,
the team planned to turn the transmitter back on. Unfortunately,
during the first test of this method in January, the power supply
to the radio transmitter failed to turn back on.
“The
decision to switch the transmitter off was not taken lightly. It
was the only way to continue the science mission,” says
Marsden, who is a 30-year veteran of the project, having worked
on it for 12 years before the spacecraft was launched.
After
many attempts, the Ulysses project team now consider it highly
unlikely that the X-band transmitter will be recovered. They
believe the fault can be traced to the power supply, meaning that
the extra energy they hoped to gain cannot be routed to the
heater and science instruments after all.
So, the
spacecraft has lost its ability to send large quantities of
scientific data back to Earth and is facing the gradual freezing
of its fuel lines. This spells the end of this highly successful
mission. “Ulysses is a terrific old workhorse. It has
produced great science and lasted much longer than we ever
thought it would,” says Marsden. “This was going to
happen in the next year or two, it has just taken place a little
sooner than we hoped.”
The team plan to continue
operating the spacecraft in its reduced capacity for as long as
they can over the next few weeks. “We will squeeze the very
last drops of science out of it,” says Marsden.
Source:
ESA
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