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.

Friedrichshafen, November 24, 1999/99037

For further information:

Dornier Satellitensysteme GmbH
Mathias Pikelj,
Tel.: +7545-8-9123
Fax: +7545-8-5589
e-mail: presse@dss.dornier.dasa.de