ESA Science & Technology - INFO 16-1999: Cluster II quartet take the stage together

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

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 kms^-1.

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: jellwoodestec.esa.nl

Dr. Philippe Escoubet
ESA - Cluster II Project Scientist
tel: +21 (0)71 565 3454
email: cpescoubestec.esa.nl

ESA Public Relations Division
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