After over 17 years of operation, the joint ESA/NASA mission Ulysses will officially conclude on 1 July this year. The spacecraft, which studied the Sun and its effect on the surrounding space for almost four times its expected lifespan, will cease to function because of the decline in power produced by its on-board generators.
Ulysses has forever changed the way scientists view the Sun and its effect on the surrounding space. The mission's major results and the legacy it leaves behind have been presented today at ESA Headquarters in Paris.
"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. "There will never be another mission like Ulysses," he continued.
"Many solar missions have appeared on the space scene in recent years, but Ulysses is still unique today. Its special point of view over the Sun's poles has never been covered by any later mission, making Ulysses's pioneering character still valid. This legendary spacecraft has served us extraordinarily well and it has certainly lived up to its mythical namesake's reputation."
"Ulysses has been a challenging mission since launch," said Ed Massey, Ulysses Project Manager at NASA's Jet Propulsion Laboratory, California, USA. "Its success required the cooperation and the intellect of engineers and scientists from around the world. The diversity of our team was one of its greatest strengths."
That strength and diversity spilled over into the Ulysses spacecraft itself. 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 two polar passes of the giant planet Jupiter.
"The main objective of Ulysses was to study, from every angle, the heliosphere, the vast bubble in space carved out by the solar wind," said Ed Smith, NASA's Ulysses Project Scientist. "The heliosphere separates the solar neighbourhood from the interstellar medium. Over its long life, Ulysses redefined our knowledge of the heliosphere and went on to answer questions about our solar neighbourhood we did not know to ask."
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 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.
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 detected and studied dust flowing into our Solar System from deep space and showed that it was 30 times more abundant than astronomers suspected. Perhaps most remarkably, the spacecraft detected helium atoms from deep space and confirmed that the Universe does not contain enough matter to eventually halt its expansion.
Hurtling through space at an average speed of 56 000 km/h, Ulysses has logged over 8.6 thousand million kilometres. The longevity of the mission is testament to a creative team of engineers who have risen to every challenge. As the power supply has weakened over the years, so they have come up with ingenious ways of conserving energy. Now, however, the power has dwindled to the point where fuel will soon freeze in the spacecraft's pipelines.
"When the last bits of data finally arrive, it will surely be tough to say goodbye to Ulysses," 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."
"It is with enormous affection that we bid farewell to Ulysses. It has been a story of remarkable success and collaboration," added David Southwood, ESA's Director of Science and Robotic Exploration.
Notes for editors: The Ulysses spacecraft was carried into low-Earth orbit in the cargo bay of Space Shuttle Discovery on 6 October 1990. It was then propelled towards Jupiter by a combination of solid fuel motors.
Ulysses flew by Jupiter on 8 February 1992; the giant planet's gravity bent the spacecraft's flight path downward and away from the ecliptic plane. This put it into a final orbit around the Sun that would take it past the Sun's north and south poles.
Ulysses is a joint mission between ESA and NASA. ESA provided the spacecraft, built by Astrium GmbH, Friedrichshafen, Germany (formerly Dornier Systems). NASA provided the Space Shuttle launch, the inertial upper stage and the payload assist module to put Ulysses into its correct orbit. NASA also provided the Radioisotope Thermoelectric Generator (RTG) to power the craft and its payload.
ESA's European Space Research and Technology Centre (ESTEC) and European Space Operations Centre (ESOC) have been managing the mission in coordination with NASA's Jet Propulsion Laboratory (JPL). Ulysses is tracked by NASA's Deep Space Network. A joint ESA/NASA team at JPL has overseen spacecraft operations and data management. Teams from universities and research institutes in Europe and the United States provided the 10 instruments on board.
Ulysses: the science legacy
During its 17.5 years in space, Ulysses has rewarded scientists with the unprecedented depth and breadth of its results. These have not just been about the Sun and its influence on nearby space. The mission has also provided surprising insights into the nature of our galaxy and even the fate of the Universe.
The joint ESA/NASA mission was designed to study the solar wind - a constant stream of particles emitted by the Sun, and the magnetic field that this carries through space. Because the Sun rotates, scientists believed that the magnetic field would wind up into a spiral.
Ulysses showed that the actual magnetic field is much more complex in shape and extent. This allows particles emitted by solar storms at low latitudes to climb up to higher latitudes, and vice-versa. 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.
Scientists originally thought that speed of the solar wind was typically 400km/s, with higher speed gusts. Ulysses soon showed that for much of the sunspot cycle, it is a fast wind emitted from the solar magnetic poles that dominates. The slow wind, whose origin remains somewhat mysterious, is a minor player.
When the mission was extended beyond the original goal of one orbit of the Sun, scientists were able to watch how the solar wind changed with time. "We designed the mission to give us a 3-D view of the solar wind, but we got the fourth dimension of time, as well," says Richard Marsden, ESA Ulysses Project Scientist.
The Sun does not emit solar wind steadily, but the emission varies through a cycle of magnetic activity lasting approximately 11 years. The cycle culminates in the reversal of the direction of the Sun's magnetic field.
Ulysses saw that on a large scale, the complexity of the magnetic field near the solar surface simplifies into a field created by a bar magnet inside the Sun. When solar activity is at a minimum, this bar magnet is aligned with the rotation poles. Six years later, at maximum, the bar magnet has moved to lie at 90o to the rotation poles. It then continues moving so that by the time of the next minimum, it is aligned with the rotational pole again, but in the opposite orientation.
The Sun's magnetic field creates a sphere of influence known as the heliosphere. "Prior to Ulysses, this was thought to be impenetrable to dust from deep space," says Ed Smith, NASA Ulysses Project Scientist; Ulysses showed that this was not true.
The spacecraft carried a superb instrument for diagnosing these invading particles. It found 30 times more dust from deep space in the vicinity of the Solar System than astronomers had previously expected.
Ulysses also detected heavy atomic nuclei racing into the Solar System. Known as cosmic rays, these are thought to have been accelerated by the explosion of high-mass stars. Ulysses estimated that the average age of a cosmic ray entering the Solar System is 10-20 million years and they have spent their lives streaming through the galaxy's outer regions before finding their way into the Solar System.
Ulysses collected rare samples of interstellar helium isotopes. These are especially interesting to cosmologists because theory predicts that their abundance was more or less fixed within a few minutes of the Big Bang. Ulysses measured these isotopes, supplying evidence to support the idea that the Universe will expand forever because insufficient matter was created in the Big Bang to halt its outward march.
Ulysses has been a big scientific effort, with many pay-offs, spread across Europe and America. "There must have been 150 to 200 scientists working on the instrument teams," estimates Smith. In addition, large numbers of scientists have accessed the mission's archived data, which is made available via the Internet. Around 1500 papers have been published so far using Ulysses data.
This rich treasure of unprecedented observations will keep the mission alive long after the actual spacecraft has died.
Ulysses: the engineering challenge
If Ulysses had been human, with a life expectancy of 70 years, it would have just celebrated its 245th birthday, thanks to a dedicated ESA/NASA engineering team. In spacecraft years, the mission's life expectancy was five years. This turned into 17.5 years, 3.5 times what was envisaged.
"This mission would not have been possible without excellent cooperation between ESA and NASA," says Ed Massey, NASA Ulysses Project Manager. The mission officially began in 1977 and was launched from the Space Shuttle's payload bay on 6 October 1990.
The first challenge for the team came soon after launch. When a 7.5-metre boom was deployed, the spacecraft started wobbling. This was potentially disastrous. The spacecraft was designed to point toward Earth and its scientific targets with an accuracy of 0.5o, but was wobbling by several degrees, threatening to severely degrade all observations.
"For a while it was touch and go as to whether we were going to have a mission," says Nigel Angold, ESA Ulysses Mission Operations Manager. The team analysed the problem, concentrating on the boom arm, and discovered that the Sun's heat was causing the boom to flex. This in turn induced the wobble.
Once they had diagnosed the problem, they devised a way to use a spacecraft thruster to correct for this anomalous motion, enabling accurate observations. The team got to know the wobble so well that they predicted its reappearance in February 2007 almost exactly.
After the first solar pass in 1994-95, the mission was extended and the team faced the challenge of keeping Ulysses alive. It was challenging because the power available to the spacecraft was constantly ebbing away. Ulysses uses a small Radioisotope Thermoelectric Generator (RTG). Due to the half-life of the radioactive heat source, the amount of power available gradually decreases with time. Over its lifetime, Ulysses has lost almost a third of its available power.
This meant that not all of the instruments and systems could remain switched on. Since 2002, the team has been running the spacecraft with one or more of the instruments turned off at any one time. Other systems have been turned off intermittently, too.
The lack of power has another effect, one that is potentially disastrous. The spacecraft must keep warm - otherwise its thruster fuel will freeze. Once this happens, as it inevitably will, there is no way to control the spacecraft.
But switching instruments off robs the spacecraft of heat. So the team had to be extremely careful to not switch anything off for too long and avoid creating cold spots within the spacecraft. Nursing Ulysses in this manner, they managed to coax it through a third solar orbit. "We've continued to wring out every drop of data that we can from this mission," says Angold.
But, as the spacecraft began its fourth journey into deep space last year, the power drop became too serious and the team tried switching off the main transmitter. Unfortunately, the power supply failed and it could not be turned back on again. This has left a cold spot critically near a fuel line. "We're very worried about that spot. Once the temperature falls below 2 deg C, the fuel will freeze," says Angold.
With the imminent loss of control caused by the freezing fuel, ESA/NASA officials have decided that it is time to end the mission and celebrate its successes - a good time to say goodbye to an old friend.
"From an engineering point of view, you want a routine mission but this one has been anything but routine. Ulysses has been a challenge and all the more interesting because of it," says Massey. "It has been like flying a different spacecraft each time the on-board conditions have changed throughout the mission," says Angold.
In recognition of the team's superb work, the Ulysses Mission Operations Team received the 2008 International SpaceOps Award for Outstanding Achievement.
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