Ulysses Mission Nearing End Of 17-Year Run
The joint NASA and
European Space Agency Ulysses mission to study the sun and its
influence on surrounding space is likely to cease operations in the
next few months. The venerable spacecraft -- which has lasted more
than 17 years or almost four times its expected mission lifetime --
is succumbing to the harsh environment of space.
Ulysses was the first mission to survey the space environment
above and below the poles of the sun. The reams of data Ulysses
returned have forever changed the way scientists view our star and
its effects.
"I remember when we got those first pictures of Ulysses floating
out of the space shuttle Discovery's payload bay back in October of
1990 and thinking we had a great five years ahead of us," said Ed
Massey, Ulysses project manager at NASA's Jet Propulsion Laboratory
in Pasadena, CA. "I never dared think that we would be receiving
invaluable science data on a near continuous basis for more than 17
years. Ulysses has set the bar on solar science data collection
quite high."
Science findings and discoveries from the mission were numerous
and unprecedented. Examples include taking the first direct
measurements of interstellar dust particles and interstellar helium
atoms in the solar system and the discovery that the magnetic field
leaving the sun is balanced across latitudes.
"The data and science output of this mission truly deserves to
be named after the legendary explorer in Greek mythology," said
Arik Posner, Ulysses program scientist, NASA Headquarters,
Washington. "My compliments go out to the international team of
scientists and engineers who built a spaceship and instrument
payload that is highly sensitive, yet durable enough that it
withstood the most extreme conditions in the solar system,
including a polar passage of the giant planet Jupiter."
Since its Jupiter flyby
in 1992, Ulysses has been in a six-year orbit around the sun. Its
long path through space carries it out to Jupiter's orbit and back.
The farther it ventures from the sun, the colder the spacecraft
becomes. If it drops to 2 degrees Celsius (36 degrees Fahrenheit),
the spacecraft's hydrazine fuel will freeze. This has not been a
problem in the past because Ulysses carries heaters to maintain a
workable onboard temperature.
The spacecraft is powered by the decay of a radioactive isotope.
Over its 17-plus years, the power has been steadily dropping. The
spacecraft no longer can run all of its communications, heating and
scientific equipment simultaneously. "We expect certain parts of
the spacecraft to reach 2 degrees Celsius pretty soon," said
Richard Marsden, ESA project scientist and mission manager. This
temperature drop will block the fuel pipes, making the spacecraft
impossible to maneuver.
The NASA/ESA project team approved a plan to temporarily shut
off the main spacecraft's X- band transmitter. This would release
60 watts of power, which could be channeled to the science
instruments and the heater. The team planned to turn the
transmitter back on when data was to be transmitted back to Earth.
This would have made it possible to run Ulysses for up to another
two years.
Unfortunately, during the first test of this approach in
January, the power supply to the radio transmitter failed to turn
back on. Engineers believe the fault can be traced to the
transmitter's power supply, meaning that the extra energy they
hoped to gain cannot be routed to the heater and science
instruments. "The decision to switch the transmitter off was not
taken lightly. It was the only way to continue the science
mission," Marsden said.
After many attempts,
the Ulysses project team now considers it highly unlikely that the
X-band transmitter will be recovered. As a result, 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.
The team plans to continue operating the spacecraft in its reduced
capacity, using the alternate S-band transmitter, for as long as
they can over the next few weeks.
"We will squeeze the very last drops of science out of it that
we can," Marsden said. "Ulysses is a terrific old workhorse. It has
produced great science and lasted much longer than we ever thought
it would."
The Ulysses spacecraft was built by Dornier Systems of Germany
for ESA. NASA provided the launch and the upper stage boosters. The
US Department of Energy, Washington, supplied the generator that
powers the spacecraft; science instruments were provided by both
U.S. and European investigators. The spacecraft is operated from
JPL by a joint NASA/ESA team and has employed NASA's Deep Space
Network for communications.