Cluster II on the Homestretch — Launch in Mid-2000
Satellites Business Unit
DaimlerChrysler Aerospace AG
Munich, Germany
Press Release: November 24, 1999
System Testing of Cosmic Quartet:
Cluster II on the Homestretch — Launch in Mid-2000
Friedrichshafen — A few days ago the last of four Cluster II spacecraft left the integration center of Dornier
Satellitensysteme GmbH (DSS), a corporate unit of DaimlerChrysler Aerospace AG (Dasa, Munich). Like its three
identical “brothers” the satellite was integrated and tested for system capability in Friedrichshafen. Subsequently it was
transported to IABG in Ottobrunn near Munich, where it is currently being subjected to extensive tests under space
conditions. On the occasion of a press event DSS and the European Space Agency (ESA) presented the research project to
the public.
Cluster II consists of four satellites altogether, which are to investigate the effects of the solar wind on the Earth’s
magnetic field from the middle of next year onwards. DSS in Friedrichshafen is the industrial lead contractor for
development and manufacture. According to the current schedule the satellite quartet will be shipped to the Russian
cosmodrome Baikonur, Kazakhstan in April 2000 on board a cargo airplane. There, launch will be prepared by
approximately 30 engineers and technicians of Dornier Satellitensysteme GmbH. In June and July 2000 the four Cluster
satellites will finally be orbited in pairs by two Soyuz rockets and are to be in service for at least two years. Command
and control will be carried out by the European Space Operation Center (ESOC) in Darmstadt.
Phoenix from the Ashes
Originally the four Cluster satellites were to be orbited during the maiden flight of the European Ariane 5 booster in
June 1996. Launch failed however. As the Cluster project was unique, the scientists involved insisted on a replica. One
month after the failed launch, ESA initially decided to build only one further Cluster spacecraft. It was to be built mainly
from spare components of the Cluster program and from available parts of the structural model. By analogy with the
ancient fabulous creature which burns itself and always rises from the ashes, this Cluster satellite was called Phoenix.
In April 1997 ESA decided to build the other three spacecraft as well. In the technical jargon the four Cluster II
satellites are designated FM5 (for Phoenix), FM6, FM7 and FM8. FM stands for Flight Model.
New Start under Financial Constraints
It was clear from the beginning that Cluster II had to be financed from the current ESA budget. This was achieved
stretching the schedule of other projects. Approximately 125 million EURO were allocated to the industry consortium.
Additionally and also for cost reasons it was decided to refrain from a joint launch of the four spacecraft with an Ariane 5
booster. Instead, the spacecraft will be deployed into orbit in pairs, by two Soyuz rockets.
According to current plans, FM6 and FM7 will be deployed in June 2000; FM5 and FM8 will follow one month later. The
Soyuz boosters alone, however are not powerful enough to transport the spacecraft into the right altitude. They will
therefore be equipped with an additional upper stage, called Fregat, which is an evolution of the propulsion system for
the two Russian Phobos spacecraft which flew to Mars in 1988. Fregat is equipped with a digital onboard electronics
system allowing complex maneuvers in space. The Franco-Russian joint venture Starsem in Suresnes, France is
responsible for the delivery and launch of the boosters. The operating company of the Ariane launch vehicles,
Arianespace, also holds a stake in this joint venture.
Before the two Soyuz boosters with the Cluster satellites on board can be launched, Fregat has to prove its reliability and
successfully complete two qualification flights. Should one or both flights be aborted or fail, ESA would switch to an
Ariane 4 booster which could deploy all four Cluster spacecraft in a single launch. Another first: ESA has taken out an
insurance against the resulting financial loss in this case.
The primary prerequisite for Cluster II was that the basic design and all interfaces to the launch vehicle remained
unchanged. This led to various problems which however could be fully solved.
As the fairing of the Soyuz booster is slightly smaller than that of Ariane, the articulated booms which extend beyond the
spacecraft body, had to be shortened by ten centimeters. This has no serious effect on the sensitivity of the experiments
accommodated on these booms which should be as far away as possible from the satellites’ disturbing environment.
During launch, the booms are folded. They unfold in space under the influence of spring and centrifugal force. The latter
is produced by the satellites’ rotation around their axis of symmetry.
The data system technology also required improvement. The data from the measuring instruments is buffered first and
relayed to the ground when the corresponding satellite comes within the receiving range of a ground station For cost
reasons ESA has planned fewer stations for Cluster II than for the original mission. DSS therefore had to use new
memories with a higher capacity. It was possible to replace the previous 2.25-Gbit memories by new 7.5-Gbit
memories. “This required a new development which however can be used for future missions such as Envisat”, explains
Günther Lehn, Project Manager of Cluster II at DSS.
The engineers were not surprised that some of the electronic parts and components were out of stock. In the space
segment for instance this concerned the transponders which ensure communication between the satellite and the ground
station (and vice versa). They were no longer manufactured in the original version and had to be replaced by new parts.
It was a fortunate coincidence that the same transponder was needed for the X-ray telescope XMM which was being built
at the same time under the leadership of DSS. Thus, the transponder was qualified under this project without additional
costs to the Cluster project.
At first sight, the fact that the Ariane 5 launcher is in a vertical position during spacecraft integration whereas the
Soyuz booster is prepared in a horizontal position and shipped to the launch pad where it is erected, seemed to be a
problem. The key issue was whether the spacecraft could be fueled in both the horizontal and vertical position. This is by
no means natural since the tank filling requires reliable venting. “A modified fueling procedure now makes it possible to
replenish the spacecraft without the need for a design change”, states Günther Lehn.
The “Weather in Space” — Forecasts are Welcome
Scientists draw a different picture of the Sun as we know it. To the naked eye, the Sun appears to be calm and unchanging.
In reality, violent bursts take place which propagate over the Sun’s surface like a surface fire and cover an area the size
of Europe, in a matter of hours. The energy released in this process within a few minutes or hours is equivalent to
current energy consumption on the Earth over a period of several thousand years. The US-European solar and
heliospheric observatory, SOHO, launched in 1995 has made major contributions to our current knowledge in this field.
In general, solar activity varies with an average duration of eleven years. An increase in solar spots and eruptions can be
observed when the activity reaches its maximum. During particularly powerful bursts the Sun ejects clouds of
electrically charged particles racing through the planetary system at speeds of over three million kilometers per hour.
When the particles hit the Earth’s magnetic field they are first trapped in It and then travel at high speed along the
magnetic field lines from pole to pole. However, when this magnetic cage overflows, the particles shoot down into the
atmosphere where they collide with atoms and molecules causing the latter to illuminate. This is the fascinating
phenomenon of the polar lights.
Particular strong particle storms have such an impact on the Earth’s magnetic field at an altitude of several thousand
kilometers that the effects are still felt on the Earth surface. In March 1989, for example, our planet was hit by an
extraordinary violent particle storm which caused the Earth’s magnetic field to swing, releasing intensive electric
currents. They, in turn induced high electric voltages in power lines so that a major part of Canada’s power supply broke
down. At outside temperatures of minus 15 degrees, six million Canadians were left in the dark and, in part, in the cold
for several hours.
According to new studies, minor gusts in the solar wind can also affect technical systems. Experts, for instance, attribute
the unexpected failure of the TV satellite Telestar in 1997 to a solar eruption. It is also assumed that electric currents
are induced in oil pipelines causing faster corrosion of the steel pipes. According to a study, the reject rate in the
production of semi-conductors rises with increasing solar wind.
For several years these “space weather” phenomena have increasingly come to the fore of solar research. A controversial
discussion is currently underway on perhaps the most pressing issue at present: Is there a long-term influence of the
Sun on the climate? Some researchers establish a causal relation between the exceptionally cold climatic phase in
Europe, the so-called “Small Ice Age” in the 17th century, and the virtual lack of solar spots between 1645 and 1715.
Most recently Danish meteorologists claimed to have found a relation between the length of the activity cycle and the
mean annual temperature in the northern hemisphere. It is assumed that the fast particles of the Sun generate
electrically charged particles when penetrating the atmosphere, which promote the formation of clouds.
Systematic investigations of the complex relations between the solar wind and the Earth’s magnetosphere are only at the
beginning. For a more detailed and directed investigation, scientists from the major space agencies of the USA, Europe,
Russia and Japan, in the eighties, launched the most comprehensive international program on extraterrestrial research
so far: The International Solar-Terrestrial Program, ISTP. This research project is aimed at tracing the complete chain
of events from the processes inside the Sun via the emission and flow of the solar wind up to its effects on our planet.
Sometime in the future it will be possible to forecast the weather in space in order to warn against violent storms.
Under the ISTP program approximately ten spacecraft have been launched since 1992 which orbit the Earth at different
distances and inclinations to the Equator. They measure the flow of the solar wind, the strength of the Earth’s magnetic
field and a variety of other physical data.
The core component in this armada is SOHO and Cluster II. Together they are a cornerstone in ESA’s science program
Horizon 2000.
Cluster II — a Quartet Tacking in the Solar Wind
The four Cluster II spacecraft are to investigate the effects of this particle flow on the Earth’s magnetic field. The Fregat
upper stage will deploy them in an elliptical orbit at a distance to the Earth between 200 and 18,000 kilometers. The
satellites are equipped with a propulsion system which will transfer them to their final orbits over the two poles. Here
their distance to the Earth will vary between 25,500 and 125,000 kilometers and their orbit period will last 57 hours.
The orbits are selected such that the satellites are positioned at the corners of an imagined tetrahedron. That way, they
will always fly through a specific volume in which they measure changes in the magnetosphere. Cluster II will thus make
it possible for the first time to perform spatial and temporal explorations of the dynamic events taking place in the
Earth’s magnetosphere.
The measuring volume established by the four satellites has the form of a tetrahedron whose size is determined by the
satellites’ distance to each other. The Cluster spacecraft pass various areas of the Earth’s magnetosphere where
interesting events take place on completely different scales. To account for this problem, the distance between the
satellites will be varied between 1000 and 18,000 kilometers every six months.
The four Cluster satellites are equipped with identical instruments. German institutes were responsible for two out of
the eleven instruments on board of each spacecraft and were involved in another two instruments.
The optimum outcome of the Cluster II mission will be decided by the interaction with the other spacecraft currently in
Earth orbits, especially with SOHO. This solar observatory is therefore expected to remain in service at least until the
end of the Cluster mission. Counted from the launch date of the Cluster satellites that is another two years and three
months — a quarter for commissioning and two years of scientific measuring time. The other spacecraft like Geotail and
Polar launched in 1992 and 1996, respectively, are currently also still in service.
Solar Activity at its Maximum
From the engineering point of view all facts currently indicate that the scientists’ expectations which three years ago had
virtually vanished into thin air, can finally be met. The scientists made more than 100 improvements on their devices;
only the interfaces to the spacecraft had to remain unchanged.
Yet, one thing has changed since the failed launch of Cluster I: the Sun itself. Cluster II will be launched almost exactly
four years after the first mission. At that time the Sun had been in a medium activity stage; in the fall 2000 it will have
reached its peak. This means that there will be an increase in gusts and storms in the solar wind and the magnetosphere
will often alter its size and shape. “We will increasingly concentrate our measurements on the interaction with SOHO and
the phenomenon of space weather”, explains Patrick Daly of the Max Planck Institute for Aeronomy.
For further information:
Dornier Satellitensysteme GmbH
Mathias Pikelj
Tel.: +7545-8-9123
Fax: +7545-8-5589
e-mail: presse@dss.dornier.dasa.de
