posted 02-25-2008 10:15 AM               

International Solar Mission to End Following Stellar Performance

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, 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."

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 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 36 degrees 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 U.S. 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.

posted 06-12-2008 11:28 PM               

International Mission Studying Sun to Conclude

After more than 17 years of pioneering solar science, a joint NASA and European Space Agency mission to study the sun will end on or about July 1.

The Ulysses spacecraft has endured for almost four times its expected lifespan. However, the spacecraft will cease operations because of a decline in power produced by its onboard generators. Ulysses forever has changed the way scientists view the sun and its effect on the surrounding space. Mission results and the science legacy it leaves behind were reviewed today at ESA Headquarters in Paris.

"The main objective of Ulysses was to study, from every angle, the heliosphere, which is the vast bubble in space carved out by the solar wind," said Ed Smith, Ulysses project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Over its long life, Ulysses redefined our knowledge of the heliosphere and went on to answer questions about our solar neighborhood we did not know to ask."

Ulysses ends its career after revealing that the magnetic field emanating from the sun's poles is much weaker than previously observed. This could mean the upcoming solar maximum period will be less intense than in recent history.

"Over almost two decades of science observations by Ulysses, we have learned a lot more than we expected about our star and the way it interacts with the space surrounding it," said Richard Marsden, ESA's Ulysses project scientist and mission manager. "Solar missions have appeared in recent years, but Ulysses is still unique today. Its special point of view over the sun's poles never has been covered by any other mission."

The spacecraft and its suite of 10 instruments had to be highly sensitive, yet robust enough to withstand some of the most extreme conditions in the solar system, including intense radiation while passing by the giant planet Jupiter's north pole. The encounter occurred while injecting the mission into its orbit over the sun's poles.

"Ulysses has been a challenging mission since launch," said Ed Massey, Ulysses project manager at JPL. "Its success required the cooperation and intellect of engineers and scientists from around the world."

Ulysses was the first mission to survey the environment in space above and below the poles of the sun in the four dimensions of space and time. It showed 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 to high latitudes and vice versa, sometimes unexpectedly finding their way out to the planets. Ulysses also studied dust flowing into our solar system from deep space, and showed it was 30 times more abundant than astronomers suspected. In addition, the spacecraft detected helium atoms from deep space and confirmed the universe does not contain enough matter to eventually halt its expansion.

Ulysses collected and transmitted science data to Earth during a 5.4-billion mile journey. As the power supply weakened during the years, engineers devised methods to conserve energy. The power has dwindled to the point where thruster fuel soon will freeze in the spacecraft's pipelines.

"When the last bits of data finally arrive, it surely will be tough to say goodbye," said Nigel Angold, ESA's Ulysses mission operations manager. "But any sadness I might feel will pale in comparison to the pride of working on such a magnificent mission. Although operations will be ending, scientific discoveries from Ulysses data will continue for years to come."

Ulysses was launched aboard space shuttle Discovery, Oct. 6, 1990. From Earth orbit, it was propelled toward Jupiter by solid-fuel rocket motors. Ulysses passed Jupiter on Feb. 8, 1992. The giant planet's gravity then bent the spacecraft's flight path downward and away from the ecliptic plane to place the spacecraft in a final orbit around the sun that would take it past our star's north and south poles.

The spacecraft was provided by ESA. NASA provided the launch vehicle and upper stage boosters. The U.S. Department of Energy supplied a radioisotope thermoelectric generator to provide power to 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.

posted 06-29-2009 06:25 PM               

Joint ESA/NASA Ulysses mission to end

Upon receipt of the last command from Earth, the transmitter on Ulysses will switch off on 30 June, bringing one of the most successful and longest missions in spaceflight history to an end.

After 18.6 years in space and defying several earlier expectations of its demise, the joint ESA/NASA solar orbiter Ulysses will achieve 'end of mission' on 30 June 2009. The final communication pass with a ground station will start at 17:35 CEST and run until 22:20 CEST (15:35-20:20 UTC) or until the final command is issued to switch the satellite's radio communications into 'monitor only' mode. No further contact with Ulysses is planned.

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 many other ground-breaking results, the hugely successful 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.

Expanding our understanding of the Sun

This is very important as 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.

Weakened power supply

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 realised that they could keep the fuel warm and circulating by performing a short thruster burn every two hours, an ingenious solution that has enabled Ulysses' science mission to continue. It was decided to maintain the spacecraft in operation using NASA's 70 m-diameter ground station network allocated on a 'spare-capacity' basis.

But as Ulysses has moved further from Earth, the communications bit-rate has gone down while other demands for the 70 m-diameter Deep Space Network stations have gone up. Most importantly, the overall return of scientific data has decreased to a level where it is hard to justify the cost of keeping Ulysses in operation.

Longevity a tribute to builders and operators

"We expected the spacecraft to cease functioning much earlier. Its longevity is a tribute to Ulysses's builders and the people involved in operations over the years," says Paolo Ferri, Head of the Spacecraft Operations Solar and Planetary Missions Division.

He added that, "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."

Final mission operations will be conducted from the Ulysses Mission Support Area (MSA) located at NASA's Jet Propulsion Laboratory in California, USA. The joint team will include Nigel Angold, ESA Mission Operations Manager, and Ed Massey, NASA Project Manager, as well as a number of engineers and analysts from both Agencies.

"Tuesday will be a very sad day when we send the last commands to Ulysses," said Angold. "But I'm very proud that we have overcome the many challenges that have come our way over the course of nearly two decades. Its longevity is an indication of the desire for international collaboration on space missions. ESA and NASA are to be commended for supporting this unique mission until the very end," he added.

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.

posted 06-29-2009 06:30 PM               

Ulysses - last communication - live from NASA/JPL
30 June 17:00 CEST (10:00 a.m. CDT)

Watch live web streaming of the last ground station communication with the joint ESA/NASA Ulysses spacecraft, Tuesday, 30 June. Final commands to end the mission will be issued from the Ulysses Mission Support Area at NASA/JPL, California.