Airbus Defence and Space

Ulysses – longest serving spacecraft goes into retirement

Friedrichshafen, June 11, 2008 – For over 17 years, the European spacecraft Ulysses explored the Sun and its influence on the Earth.


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It was the first spacecraft ever to fly across the two poles of our central star, and represents a milestone in the study of the Sun and its influence on the Earth. Ulysses far exceeded its nominal service life of five years. But now the onboard power supply is failing. The spacecraft can no longer be heated, and the hydrazine fuel that powers the attitude control system is freezing solid. This marks the end of an enormously successful mission. The engineers and technicians at Airbus Defence and Space, under whose leadership the spacecraft was built under contract to the European Space Agency ESA, are proud that Ulysses has proved so reliable.

The Sun is not such a peaceful celestial body as it appears to us from Earth. There are frequent eruptions of radiation and particles, and the dark sunspots are evidence of an eleven-year cycle of activity. A constant particle wind blows into space, also impacting the Earth. All these phenomena are due to the same cause: a magnetic field. This can meanwhile be observed very well from Earth, but the view of the poles is obscured from this position. Yet the polar regions are of particular interest because these are where the magnetic field lines of the dipole field enter and leave the body of the Sun.

Because of this fact, the desire to send a spacecraft across the Sun’s poles arose a very long time ago. The project started to materialise in 1977, when the space agencies in the USA and Europe, NASA and ESA, decided to build two spacecraft. These were to fly simultaneously across the south and north poles of the Sun. However, budget cutbacks forced NASA to pull out of the project. ESA thus decided to build its own spacecraft, which it named ‘Ulysses’ (the mythical seafarer Odysseus).

The main contract for the project was awarded to Airbus Defence and Space GmbH of Friedrichshafen. The company was responsible for the entire management, for the integration tests, and for supervising the launch and the mission itself. It also built various components including the propulsion module, the attitude control system and the nutation dampers, these latter being designed to prevent the spacecraft from going into a spin.

Airbus Defence and Space delivered the finished spaceship at the end of 1983. However, the launch was subsequently postponed until 1986 due to problems associated with the space shuttle development programme. Just a few months before the scheduled launch date, the space shuttle Challenger exploded just after lift-off. This disaster led to the suspension of the American space programme, and Ulysses had to wait a further four years for launch. Not until 6 October 1990, seven years after its completion, was the spacecraft finally borne into space on the space shuttle Discovery.

Although the technology on the spacecraft is now over 25 years old, it still works perfectly and would probably continue to do so for many years more. However, the end of the power supply by the Radioisotope Thermoelectric Generator (RTG) has brought about a ‘natural’ end to the mission.

Once Ulysses had left the space shuttle’s cargo bay, a rocket booster was ignited. This accelerated the spacecraft to the highest speed ever reached by an artificial space vehicle. Yet it was still not sufficient to catapult Ulysses out of the Earth’s orbital plane and into a trajectory across the sun’s poles. This called for some ingenious thinking. First of all, the spacecraft travelled away from the Sun to the outer planet Jupiter. In 1992 it crossed Jupiter’s north pole, where the strong gravitation field catapulted it vertically out of the planet’s orbital plane. After this swing-by manoeuvre, the spacecraft flew back to the Sun, which it reached in the summer of 1994. It began by passing the south pole at a distance of 300 million kilometres, then flew in a wide arc to the north pole, which it passed over in mid-1995.

Because the measuring instruments and the spacecraft were functioning perfectly, ESA extended the mission. On its further travels the spacecraft moved away from the Sun again, and flew back to its orbit around Jupiter. On arrival it turned back again and reached the Sun’s south pole for the second time in autumn 2000. It orbited the Sun, passing across the north pole in autumn 2001. It finally made a third flight over the south pole in winter 2006/2007, and over the north pole a year later.

Ulysses consists primarily of two large units: the instrument platform and the antenna. The platform is roughly square in shape, with a side length of 3.2 metres and a height of 2.1 metres. It carries nine scientific instruments that measure the particles of the solar wind and the electric and magnetic fields. Two of these devices were built under German direction at the Max Planck Institutes for Solar System Research in Katlenburg/Lindau and for Nuclear Physics in Heidelberg. The entire spacecraft weighs 367 kilograms, only 55 kilograms of which is accounted for by the instruments.

Airbus Defence and Space was responsible for the overall design, which included several unique features. The highly sensitive measuring instruments, for instance, had to be mounted in such a way that they would not interfere with one another. To make this possible, two magnetometers, plasma wave sensors and measuring instruments for x-ray and gamma ray eruptions had to be mounted on a 5.5-metre arm. In addition, a 72.5-metre dipole antenna and a 7.5-metre monopole antenna for measuring plasma waves were unfolded in space.

These antennas gave the spacecraft a slight imbalance which could have sent it into a spin during flight. This risk had to be prevented because the antenna used for data transmission had to point constantly towards Earth with an accuracy to within 0.2 degrees. Perfect orientation was ensured by a nutation damper designed by Airbus Defence and Space, which operated purely mechanically and did not require any fuel.

The unusual orbit also made special demands on the spacecraft’s technology. Ulysses travelled in an elongated ellipse, circled the Sun and then flew to an orbit around Jupiter. Its distance from the Earth varied between about 50 million and 900 million kilometres, or between 0.3 and 6 times the Earth’s distance from the Sun. The signals between the spacecraft and the Earth took up to 50 minutes to reach their destination. This meant that it was essential for Ulysses to be capable of operating largely autonomously. In other words, the onboard computer had to decide for itself when and how to actuate the nutation dampers, for example. During the swing-by manoeuvre, too, Ulysses was left to its own devices. It was also capable of seeking contact with Earth of its own accord if it was no longer receiving any signals from there.

Due to the tremendous variations in the vehicle’s distance from the Sun, huge temperature fluctuations occurred on board. The spacecraft therefore had to be cooled when it was close to the Sun and heated when it was a long distance away. This was particularly important for the fuel lines, where the temperature could not be allowed to drop below two degrees Celsius because otherwise the hydrazine would freeze. This task was performed by the RTG, which had been supplied by the American project partners. The RTG contains radioactive material which generates heat as it decays. The heat thus released is directly converted into electricity for the instruments, and is also used for heating. Solar cells were not an option because they cannot deliver sufficient power at a distance of 900 million kilometres from the Sun. The initial power output of 285 watts delivered by the RTG has meanwhile dropped to less than 200 watts. This is no longer sufficient to heat the spacecraft, which has just passed over the Sun’s north pole and is on its way back to orbit around Jupiter.

Ulysses was the first-ever mission to investigate the Sun’s magnetic field and particle wind from all directions in space. This is not possible from Earth. The scientific readings obtained constitute the foundation of the first-ever three-dimensional image of the solar environment, which radically changed the scientists’ conception of the Sun and its surroundings.

Ulysses’ unexpectedly long service life also made it possible to observe the activity of the Sun throughout almost two cycles of solar activity. Solar activity fluctuates in cycles, each lasting about eleven years. Times of minimum activity are peaceful, and very few sunspots are formed. At times of maximum activity, however, numerous large sunspots are formed, and there are frequent explosive eruptions in which particles are emitted at extremely high speeds. Such particle storms can hit the Earth’s magnetic field within one or two days and, in extreme cases, cause nationwide power cuts on our planet and interfere with the radio contact between aircraft and ships.

Ulysses has allowed these phenomena to be observed from minimum to maximum activity and back to minimum. The scientists watched how the condition of the Sun changed and how the polarity of the magnetic field was reversed during the course of a cycle. This process took several months to complete, as Ulysses found out.

The collected treasury of data will give the scientists plenty to think about for many years to come. Here are just a few selected scientific highlights:

When Ulysses first approached the Sun, the researchers discovered that the velocity of the solar wind increased the higher Ulysses rose above the Earth’s orbital plane. While the instruments registered low particle speeds of about 400 km/s in the vicinity of the solar equator, the speeds rose to 800 km/s close to the poles.

On its first two polar crossings, Ulysses baffled the experts. They had expected the magnetic field to have a dipole shape like that on Earth. If this were true, the field lines would be closer together at the poles, where they enter and leave the surface, than at lower latitudes. But in actual fact Ulysses discovered an almost uniformly shaped field. The scientists assume that this is due to the effect of the particle wind, which distorts the magnetic field.

However, Ulysses not only took solar research a great leap forwards, but was also able to chalk up a number of other discoveries. It managed for the first time to prove the existence of dust particles reaching us from interstellar space – that is, from distant stars – and entering our solar system. Never before had it been possible to capture stardust.

Ulysses also illustrates the fact that a basic research experiment can reveal phenomena that nobody had ever even thought about. On 1 May 1996, an instrument on the spacecraft unexpectedly registered a stream of electrically charged particles for a period of several hours. Careful analysis revealed that this must have been caused by a comet. The source proved to be the comet Hyakutake, through whose tail Ulysses had flown. This must have been almost 500 million kilometres long, or about 3½ times the distance between the Earth and the Sun. It is thus the longest comet tail known to man.

Airbus Defence and Space (Germany) has been involved in European extraterrestrial solar research for several decades. Back in the 1960s, the company built the two Heos spacecraft for the European space agency of the time, ESRO. This was followed by the German satellites Azur and Aeros and the two German-American spacecraft Helios 1 and 2. They orbited the Sun at about one-third of its distance from Earth and delivered valuable readings from 1974 to 1986. Airbus Defence and Space (France) is the prime contzractor for the Solar Observatory SOHO, which was launched in December 1995).

A highlight in more recent times was Airbus Defence and Space GmbH’s lead management in the construction of ESA’s four Cluster satellites. This quartet is orbiting the Earth and investigating the effect of the solar wind on the Earth’s magnetic field. ESA’s next major solar mission, the Solar Orbiter, is scheduled for launch in 2015. Airbus Defence and Space’s wealth of experience makes it eminently suitable for the task of building this spacecraft.

Polar flyovers by Ulysses

South pole July 1994 – October 1994, September 2000 – January 2001, November 2006 – April 2007

North pole June 1995 – September 1995, September 2001 – December 2001, November 2007 – March 2008

About Airbus Defence and Space

Airbus Defence and Space, a wholly owned subsidiary of AIRBUS Group, is dedicated to providing civil and defence space systems and services. In 2007, Airbus Defence and Space had a turnover of €3.5 billion and 12,000 employees in France, Germany, the United Kingdom, Spain and the Netherlands. Its three main areas of activity are Airbus Defence and Space for launchers and orbital infrastructure, and Airbus Defence and Space for spacecraft and ground segment, and its wholly owned subsidiary Airbus Defence and Space for the development and delivery of satellite services. AIRBUS Group is a global leader in aerospace, defence and related services. In 2007, AIRBUS Group generated revenues of €39.1 billion and employed a workforce of more than 116, 000.


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