Visitors to the test and integration facilities at IABG near Munich, Germany, on
24 November may be excused for thinking they are suffering from multiple
vision. On display there, in a giant clean room, will be not one but four
identical cylindrical spacecraft.

This is the only occasion on which all four of ESA’s Cluster II spacecraft will
be on display together in Europe.

Four Spacecraft, One Mission

The unique event takes place near the end of the lengthy assembly and test
programme, during which each individual spacecraft is being assembled in
sequence, one after the other.

Two have already completed their assembly and systems testing and are about
to be stored in special containers at IABG prior to shipment to the Baikonur
launch site in Kazakhstan next spring. In the case of the other two, flight
models 5 and 8, installation of the science payloads has finished, but their
exhaustive series of environmental tests at IABG have yet to begin.

Following delivery to the launch site next April, the satellites will be launched
in pairs in June and July 2000. Two Soyuz rockets, each with a newly designed
Fregat upper stage, are being provided by the Russian-French Starsem
company. This will be the first time ESA satellites have been launched from the
former Soviet Union.

Cluster II is a replacement for the original Cluster mission, which was lost
during the maiden launch of Ariane 5 in June 1996. ESA, given the mission’s
importance in its overall strategy in the area of the Sun-Earth connection,
decided to rebuild this unique project.

ESA member states supported that proposal. On 3 April 1997, the Agency’s
Science Programme Committee agreed. Cluster II was born.

European Teamwork

Scientific institutions and industrial enterprises in almost all the 14 ESA
member states and the United States are taking part in the Cluster II project.

Construction of the eight Cluster / Cluster II spacecraft has been a major
undertaking for European industry. Built into each 1200 kg satellite are six
propellant tanks, two pressure tanks, eight thrusters, 80 metres of pipework,
about 5 km of wiring, 380 connectors and more than 14 000 electrical
contacts.

All the spacecraft were assembled in the giant clean room at the
Friedrichshafen plant of prime contractor Dornier Satellitensysteme. On
completion, they were sent to IABG in Ottobrunn, near Munich, for intensive
vibration, thermal, vacuum and magnetic testing.

The European ground segment for the mission is just as important. A vast
amount of data — equivalent to 290 million printed pages — will be returned
to Earth over the mission’s two-year lifetime. Signals to and from the
spacecraft will be sent via a 15 metre antenna at Villafranca in Spain and
processed at the European Space Operations Centre (ESOC) at Darmstadt,
Germany.

The main control room at ESOC will be used during the launch and early phases
of the mission, with teams of operators working round the clock. About two
weeks after the second Cluster II pair are placed in their operational orbits,
mission operations will switch to a smaller, dedicated control room at ESOC.

The Joint Science Operations Centre at Rutherford Appleton Laboratory in the
UK will co-ordinate the scientific investigations. Its main task will be to
combine all requirements from the 11 science instrument teams into an
overall plan.

The flow of information returned by the 44 instruments will be distributed to
eight national data centres, six in Europe, one in the USA and the other in
China.

Solar Maximum

Cluster II is part of an international programme to find out more about how the
Sun influences the Earth. The four Cluster II satellites will join an armada of
spacecraft from many countries, which are already studying the Sun and high
speed wind of charged particles (mainly electrons and protons) which it
continually blasts into space. Ulysses and SOHO, both joint ESA-NASA
missions, and ESA’s Cluster II , when it will be there, are the flagships of this
armada.

The timing of the mission is ideal, since it will take place during a period of
peak activity in the Sun’s 11-year cycle, when sunspots and solar radiation
reach a maximum.

Cluster II will measure the effects of this activity on near-Earth space as
incoming energetic particles subject the magnetosphere — the region
dominated by the Earth’s magnetic field — to a buffeting.

Each spacecraft carries an identical set of 11 instruments provided by
scientific institutions in different countries.

Formation Flying

Cluster II will be the first space science mission ever to fly four identical
spacecraft simultaneously. Once the quartet have been inserted into highly
elliptical polar orbits, ranging from 19 000 to 119 000 km above the Earth,
they will spend the next two years travelling from the magnetosphere into
interplanetary space and back again.

Sometimes they will be within a few hundred kilometres of each other,
sometimes 20 000 kilometres apart, depending on the physical phenomena to
be studied. By orbiting in a tetrahedral (triangular pyramid) formation, they
will be able to make the first detailed three-dimensional study of the changes
and processes taking place in near-Earth space.

As the satellites orbit the Earth, they will investigate the rapid changes which
occur in the Earth’s magnetosphere when large numbers of
electrically charged particles (electrons and protons) in the solar wind reach
the Earth. Huge amounts of data will be returned which will help scientists
unravel the physical processes and small-scale variations taking place in the
near-Earth environment.

“Cluster II will give us the best information yet on how the Sun affects the
near-Earth environment,” said Cluster II project scientist, Philippe Escoubet.
“For the first time we will be able to study the Earth’s magnetic field from
four viewpoints with identical instruments.”

“It will be like having four cameras at a football match — one behind the goal
and three others at different angles,” he explained. “This is very exciting
because it will help us to understand the space environment which surrounds
our planet.”

How The Sun Affects Our Planet

Such studies are not just of academic interest. The Sun affects our world in
many ways. Apart from its familiar output of light, heat and ultraviolet
radiation, our nearest star also emits a continuous stream of atomic particles
— the solar wind — that sweeps out into space at speeds ranging from 280 to
1 000 km/s (1 800 times faster than Concorde).

Sometimes, explosions on the Sun send millions of tonnes of gas towards the
Earth. These clouds of high-energy particles can travel the 150 million km
between the Sun and Earth in a few days. The most energetic particles of all,
created by solar flares, can reach the Earth in just 30 minutes.

This activity is particularly noticeable at times of solar maximum. When
charged particles from the Sun enter the Earth’s upper atmosphere, they
create shimmering curtains of coloured light, known as auroras, in the polar
night sky.

Other effects can be much more serious:

* Solar storms affect the Earth’s ionosphere, causing disruption of
short-wave radio communications, navigation systems on ships and aircraft,
and military radar systems.

* Surges in electricity transmission lines can cause widespread power
blackouts, as happened in Quebec, Canada, in March 1989 when 6 million
people were left without electricity as a result of a huge solar-induced
magnetic storm.

* Damage to microchips and electrical discharges can cause satellites to stop
operating, disrupting telephone, TV and data communication services. (Aware
of the potential dangers, the designers of the Cluster II spacecraft have built
them to survive collisions with high-energy particles from the Earth’s
radiation belts and the solar wind.)

* Radiation levels can become hazardous to astronauts and occupants of
high-flying aircraft.

* Variations in solar energy output cause global climate changes which affect
plant growth, crop production and food supply.

* High-energy particles hitting the Earth’s upper atmosphere can damage the
ozone layer which protects us from harmful ultraviolet radiation.

For more information, please contact:

Mr. John Ellwood
ESA – Cluster II Project Manager
tel +31 (0)71 565 3507
email: jellwood@estec.esa.nl

Dr. Philippe Escoubet
ESA – Cluster II Project Scientist
tel: +21 (0)71 565 3454
email: cpescoub@estec.esa.nl

ESA Public Relations Division
Tel: +33 (0)1.53.69.71.55
Fax: +33 (0)1.53.69.76.90

Further information on Cluster II and the ESA science programme can be found
at: http://sci.esa.int