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. 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.

    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. 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 36 degrees Fahrenheit, the spacecraft's hydrazine fuel will freeze.

    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. 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.

    "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," stated Ed Massey, Ulysses project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "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."   

    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.   

    The primary mission of the Ulysses spacecraft was to characterize the heliosphere as a function of solar latitude. The heliosphere is the vast region of interplanetary space occupied by the Sun's atmosphere and dominated by the outflow of the solar wind. The periods of primary scientific interest is when Ulysses was at or higher than 70 degrees latitude at both the Sun's south and north poles.