(Above) This painting is an artist’s impression of the Ulysses spacecraft mated with its solid rocket booster drifting away from the Space Shuttle Discovery. The booster was used to push Ulysses out of Earth orbit towards Jupiter. Ulysses used Jupiter’s gravity to hurl it into an orbit that takes it over the Sun’s polar regions, an area not visible to Earth-based observers.

(Above) Ulysses team members bid the spacecraft a fond farewell.

Ulysses, a joint NASA and European Space Agency mission, officially ceased operations today, after receiving commands from ground controllers to do so. The spacecraft, which operated for more than 18 years, charted the unexplored regions of space above the poles of the sun.

As planned via commands beamed to the spacecraft earlier in the day, Ulysses switched to its low-gain antenna at 1:09 p.m. PDT (4:09 p.m. EDT, or 2009 UTC). As a result, ground controllers were no longer able to pick up a signal from Ulysses, which had also been commanded to switch off its transmitter completely at 1:15 p.m. PDT (4:15 p.m. EDT, or 2015 UTC).

When space shuttle Discovery launched Ulysses on Oct. 6, 1990, it had an expected lifetime of five years. The mission gathered unique information about the heliosphere, the bubble in space carved by the solar wind, for nearly four times longer than expected.

“This has been a remarkable scientific endeavor,” said Richard Marsden, Ulysses mission manager and project scientist at the European Space Agency. “The results Ulysses obtained have exceeded our wildest dreams many times over.”

Ulysses made nearly three complete orbits of the sun. The probe revealed for the first time the three-dimensional character of galactic cosmic radiation, energetic particles produced in solar storms and the solar wind. Not only has Ulysses allowed scientists to map constituents of the heliosphere in space, its longevity enabled them to observe the sun over a longer period of time than ever before.

“The sun’s activity varies with an 11-year cycle, and now we have measurements covering almost two complete cycles,” said Marsden. “This long observation has led to one of the mission’s key discoveries, namely that the solar wind has grown progressively weaker during the mission and is currently at its weakest since the start of the Space Age.”

In addition to measuring the solar wind and charged particles, Ulysses instruments measured small dust particles and neutral gases from local interstellar space that penetrate into the heliosphere. Ulysses had an unprecedented three chance encounters with comet tails, registered more than 1,800 cosmic gamma-ray bursts, and provided findings for more than 1,000 scientific articles and two books.

“The breadth of science addressed by Ulysses is truly astonishing,” said Ed Smith, Ulysses project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The data acquired during the long lifetime of this mission have provided an unprecedented view of the solar activity cycle and its consequences and will continue to keep scientists busy for many years to come.”

Ulysses’ successes have not been confined to scientific data. The extended mission presented significant challenges to the NASA-European operations team. In particular, critical parts of the spacecraft became progressively colder with time. In recent years, a major effort was needed to prevent the onboard hydrazine fuel from freezing. The operations team continually created methods to allow the aging space probe to continue its scientific mission.

Earlier this month, the Ulysses mission team received a NASA Group Achievement Award. Another milestone was reached on June 10 when Ulysses became the longest-running ESA-operated spacecraft, overtaking the International Ultraviolet Explorer which logged 18 years and 246 days of operations.

“The Ulysses team performed exceptionally by building and operating a research probe that would return scientific data for analysis no matter what challenges it encountered,” said Arik Posner, Ulysses program scientist at NASA Headquarters in Washington. “The knowledge gained from Ulysses proves what can be achieved through international cooperation in space research.”

The Ulysses orbital path is carrying the spacecraft away from Earth. The ever-widening gap has progressively limited the amount of data transmitted. Ulysses project managers, with the concurrence of ESA and NASA, decided it was an appropriate time to end this epic scientific adventure.

ESA Ulysses Mission Operations Manager Nigel Angold points out that more than a year ago, “We had estimated Ulysses would not survive further than July 2008. However, the spacecraft didn’t stop surprising us and kept working a full year, collecting invaluable science data. It’s nice to be going out in style.”

After the spacecraft was placed into low Earth orbit in 1990, a combination of solid fuel motors propelled Ulysses toward Jupiter. Ulysses swung by Jupiter on Feb. 8, 1992. The giant planet’s gravity bent the spacecraft’s flight path southward and away from the ecliptic plane, putting the probe into a final orbit that would take it over the sun’s south and north poles.

The European Space Agency’s European Space Research and Technology Centre and European Space Operations Centre managed the mission in coordination with the Jet Propulsion Laboratory. Ulysses is tracked by NASA’s Deep Space Network. A joint ESA/NASA team at JPL oversaw spacecraft operations and data management. Teams from universities and research institutes in Europe and the United States provided the 10 instruments on board.

(Above) Artist’s concept of Ulysses moving towards the Sun

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 Solar & Planetary Missions Division at ESA’s European Space Operations Centre, Darmstadt, Germany.

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.