5-billion-km journey of the Astrium spacecraft on Oct. 6, 1990
Friedrichshafen — Exactly ten years ago, on October 6, 1990, the European spacecraft Ulysses embarked on its journey of by now almost five billion kilometers through our planetary system. This "milestone" of solar research was developed and built under the industrial leadership of Astrium in Friedrichshafen. Ulysses was the first craft ever to fly over the poles of the Sun and relayed valuable data on a hitherto unknown region of our planetary system to the research community on the ground. Ulysses is part of an international fleet of spacecraft designated to explore events inside and on the surface of the Sun. These new findings will help clarify the influence of the Sun on the Earth’s magnetic field and climate.
The magnetic fields on the Sun play a crucial role. On the one hand, they occur on a minor scale in the region of sunspots. Here magnetic field lines emerge at a spot onto the surface, form a type of bridge, and then vanish into the inside of the Sun at an adjacent spot. On the other hand, there is a large-scale field with two poles, similar to the one on our planet. This field is very difficult to observe from the Earth. It is far weaker than in the region of the sunspots, and, above all, the polar regions can hardly be seen.
Long journey with startup problems
For these reasons, solar investigators developed the idea of a spacecraft which should fly over the solar poles, already at the end of the fifties. The project, however, took a more concrete shape only in 1977 when the space agencies of the USA and Europe, NASA and ESA, decided to build two spacecraft which were to fly simultaneously over the south pole and the north pole, respectively. Launch of this International Solar Polar Mission was planned for 1983.
Budget cuts forced NASA however, to cancel the project, and ESA decided to go it alone. It renamed the mission Ulysses, after the legendary Greek hero who set out to explore the uninhabited world beyond the Sun. American institutes contributed various measuring instruments and NASA offered its space shuttle for launch of the spacecraft.
Today’s Astrium Group won the prime contract. Among other things, the company was responsible for the overall management and the integration tests. Additionally, it built the electric power distribution and the thrusters. Finally, Astrium engineers were also involved in the launch preparations in the USA.
Astrium delivered the spacecraft late in 1983, but launch was postponed to 1986 due to delays in shuttle development. In January 1986, a few months before the planned launch of Ulysses onboard a shuttle, the Challenger accident occurred. The American space program was grounded for some time and Ulysses had to wait another four years. On October 6, 1990 the moment had finally arrived.
Swing-by maneuver into the third dimension
Since even the most powerful launch vehicles would not have been able to hurtle Ulysses from the Earth’s ecliptic plane onto a trajectory over the solar poles, scientists had to revert to a trick. Initially the spacecraft flew towards Jupiter, crossed its north pole in 1992, and in the powerful Jovian gravity field, it was vertically hurtled from its previous trajectory. The craft then flew back and reached the Sun in 1994. At a distance of 300 million kilometers, it first flew by the south pole and then continued to the north pole which it passed in mid-1995.
According to the original plan, the mission would have ended then. Yet, all measuring instruments worked flawlessly, and hopes were justified that it would survive another pass of the Sun. ESA therefore decided to extend the Ulysses mission. On its further journey, the spacecraft departed from the Sun and returned to the Jovian trajectory. There it was tossed back again and this month it will reach the south pole of the Sun for the second time. In a wide circle, Ulysses will fly around the Sun and, next year, will pass over the north pole. The mission is funded until September 2004.
The European satellite control center, ESOC, in Darmstadt is responsible for this complicated "swing-by course".
The second pass around the Sun
For the solar research community, this second flyby is an invaluable scientific gift, because they can now observe the Sun in different phases of activity. Whereas its activity was at a minimum during the first pass in 1994/95, it is currently at its maximum.
Solar activity varies with a cycle of approximately eleven years. During minimum activity it is calm with only a few spots present. Furthermore, a particle wind flows off its surface which scientists call solar wind. During the phase of minimum activity, this wind blows rather uniformly into the planetary system. During maximum activity, however, numerous, large sunspots are formed and explosive eruptions occur quite frequently. Particles are then ejected at extremely high velocity and thus high energy. Within one or two days, this particle storm hits the Earth’s magnetic field with varying impact and consequences for our planet.
In the best of cases, colorful polar lights can be spotted. But a violent storm can also cause the entire magnetic field of the Earth to swing and release intensive electric currents. They, in turn, can induce high electric voltages in power lines and transformer substations. In an extreme case, high-voltage mains can break down. This occurred in Canada in March 1989. Worldwide radio communications are disturbed, voltage fluctuations occur in submarine cables, chip manufacturers report production problems, and a host of other trouble. The influence of the solar wind on our climate has also become an increasingly important subject of discussion.
All these phenomena are causally related to the magnetic field of the Sun. The primary objective of Ulysses is to study these phenomena in detail. The findings are of extraordinary importance to the scientific community since they provide information on how the Sun generates the magnetic field inside, and how the particles are then accelerated in the outer magnetic field.
The spacecraft
Ulysses essentially consists of two main elements: the instrument platform and the antenna. The platform is a box-like structure, each side measuring 3.2 meters, and 2.1 meters high. Nine scientific instruments are onboard to measure the particles of the solar wind, as well as electric and magnetic fields. Two instruments were built by the Max-Planck Institutes for Aeronomy in Katlenburg/Lindau and for Nuclear Research in Heidelberg, as the principal investigators. Two magnetometers were mounted on the ends of 5.6-meter-long booms to avoid interference from the spacecraft’s disturbing environment. The total spacecraft weighs 367 kilograms, with the instruments accounting for only 55 kilograms. The power supply is another point of interest. During the mission, Ulysses is up to 800 million kilometers away from the Sun. At that distance, solar cells cannot provide sufficient power. Therefore, spacecraft power is supplied by a radioisotope thermoelectric generator built in the USA. It carries material which disintegrates radioactively producing heat in the process. A special generator directly converts the heat into current, supplying the spacecraft with a maximum of 250 watts. This type of generator was already
successfully used in the US planetary spacecraft, Pioneer and Voyager, and recently also in the Saturn orbiter, Cassini.
Scientific highlights
For the first time, Ulysses explored the magnetic field and the particle wind of the Sun, "on site", from all directions The instruments thus delivered a three-dimensional image of the Sun’s environment which is impossible to capture with ground-based instruments. Additionally, the two solar passes, in 1994/1995 and 2000/2001 respectively, will give scientists
the opportunity to directly compare the phases of minimum and maximum solar activity, for the first time.
During the first approach to the Sun, scientists detected that the velocity of the solar wind increased the higher Ulysses rose over the ecliptic plane of the Earth. The instruments registered low particle velocity of 400 km/s in the region of the Sun’s equator which however, rose to 800 km/s near the poles. Apparently, this high-velocity wind escapes through "holes" in the solar atmosphere, the so-called corona which is several million degrees hot. These coronal holes are more common in the polar regions. It has not yet been fully clarified how the particles are accelerated in both the low-velocity wind and the high-velocity wind. But magnetic fields definitely play the decisive role.
Ulysses surprised the experts during its first two passes over the poles. They had expected the magnetic field to have the form of a dipole, similar to the Earth. In that case, the field lines at the poles, where they come to or go into the surface, would be bundled closer than in low latitudes. Ulysses actually detected a virtually uniform field as if there were no magnetic poles at all. Scientists think that this phenomenon is attributable to the close interaction between the magnetic field and the solar wind. Apparently, the particle wind from the polar regions flows towards the equatorial plane, bending the magnetic field in the process. The field lines coming from or entering the poles are thus drawn apart.
Another unexpected result of the Ulysses mission is that the Sun is highly effective in warding off fast, charged particles from space. Scientists had previously assumed that these particles from interstellar space could easily penetrate the planetary system over the poles of the Sun. Ulysses proved that this is not the case. The magnetic field constantly flutters in the solar wind, effectively warding off such charged particles. The situation is similar to that of a swimmer fighting against a strong surf.
Furthermore, the swing-by maneuver at Jupiter in February 1992 made it possible to also explore the magnetosphere of this gigantic planet which extends over millions of kilometers. Electrically charged particles are trapped in it and accelerated. Among other things, particles from the surface of the Jovian moon Io also get trapped in the magnetosphere, flowing through a type of tunnel towards the planet. Five million amperes with a power of 2,500 billion watts flow through this "umbilical". Ulysses found out that this flow is not uniform but shows areas of higher density. These areas are probably attributable to volcanic eruptions on the Jovian moon. Such an event releases large particle clouds which are channeled through this "tunnel".
Scientists from the Max-Planck-Institute for Nuclear Physics in Heidelberg made an unexpected discovery. A cosmic dust experiment on board
registered minuscule grains with a diameter of less than one ten thousandth of a millimeter. Particles of comparable size are found in cigarette smoke. The analysis showed that these particles were already present when the Sun and the planets first began to form. This discovery offers the opportunity to analyze real "interstellar dust".
Solar research with Astrium
From the outset, the Astrium Group was actively involved in Europe’s extraterrestrial solar research activities. As early as the sixties, the two Heos probes, the first spacecraft of the newly formed European Space Organization, ESRO, were built, followed by the German satellites, Azur and Aeros. The two German/American probes, Helios 1 and 2, orbited the Sun at approximately one third of the Earth’s distance and delivered valuable data between 1974 and 1986. ISEE was the next satellite to observe the solar wind far away from the Earth.
In the nineties, Ulysses, the large-scale projects SOHO (Solar and Heliospheric Observatory), and the satellite quartet, Cluster, provided enormous impetus for science. Together they are one of the four
"cornerstones" of the European Space Agency’s science program. SOHO was launched on December 2, 1995 and since the spring of 1996, has been supplying an incessant stream of images and data of the Sun. The first set of the four Cluster satellites was slated for launch by Europe’s new launcher vehicle, Ariane 5, on its maiden flight. But the booster exploded and the four satellites were lost. On account of the unique nature of the Cluster project, ESA gave green light for the construction of another set. In less than three years, a virtually identical satellite quartet was built under the system leadership of Astrium. In July and August 2000
respectively, the Cluster II satellites were launched in pairs into Earth orbit, by two Russian Soyuz boosters. For the first time, these satellites will explore in three dimensions the effect of the solar wind on the Earth’s magnetic field.
The Sun and its effects on our planet will continue to be one ESA’s science objectives. Currently, two projects are being discussed as candidates for Flexi-missions: on the one hand, the "Solar Orbiter", a space telescope designated for high-resolution observation of the Sun; on the other hand, three identical satellites specifically designed to explore the effects of magnetic storms on the Earth’s magnetic field.
For further information:
Earth Observation & Science
Mathias Pikelj
Phone: ++7545-8-9123
Fax: ++7545-8-5589