ESA Science & Technology - INFO 08-1996: Cluster - Four identical satellites investigating the Earth's turbulent relationship with the Sun

Once in space, the four satellites will manoeuvre to an eccentric polar trajectory along which they will fly in tetrahedral formation for the next two years. They will take highly precise and, for the first time, three- dimensional measurements of the extraordinarily dynamic phenomena that occur where the solar wind meets the near- Earth environment.

They will gather an unprecedented volume of very high- quality information on the magnetic storms, electric currents and particle accelerations that take place in the space surrounding our planet, which give rise to all manner of events, such as the aurorae in the polar regions, power cuts, breakdowns in telecommunication systems, or satellite malfunctions, and perhaps even changes in climate.

The Cluster mission will also gather a host of fundamental information on the ionised gases whose behaviour physicists are trying to reproduce under laboratory conditions with the ultimate aim of generating thermonuclear energy. A cosmic battlefield The Sun's flames are lapping at the Earth's doorstep. In its constant state of effervescence/evaporation, it emits into space a wind charged with ions, electrons and protons which reach Earth at speeds of 1.5 to 3 million km per hour. Fortunately, our planet is armed with a natural shield against this onslaught: the magnetosphere, a distant magnetic, ionised extension of our atmosphere which slows and deflects the bulk of the stream of particles emitted by the Sun.

This shield does not provide complete protection, however. Under constant buffeting from the interplanetary wind, the "fluid" magnetic screen is buckled, distorted and occasionally torn, causing small holes. When this happens, intense electric currents, magnetic storms and particle accelerations immediately develop. The overall interaction between the solar wind and the magnetosphere is so violent that the energy transferred can be as much as 1013 watts - equivalent to worldwide power consumption - and the currents induced run to millions of amps. A full-scale electrodynamic battle, in other words.

The side-effects are well known. The aurorae, which occur within about 20 of the Earth's magnetic poles, are caused by the precipitation of charged particles through the atmosphere at very high altitude. Magnetospheric storms can also cause power surges in electricity transmission lines; in March 1989, for instance, there was a power failure throughout the province of Quebec.

In orbit, the presence of charged particles can affect the performance of satellite components and represents a threat to astronauts' health. This accounts for the loss of control of two communication satellites, Anik E1 and E2, in January 1994. It also explains why stringent precautions against harmful radiation have to be built into the plans for operation of the inhabited international space station Alpha.

Finally, these fast-moving flows of particles also penetrate the Earth's upper atmosphere, where they generate such thermal and dynamic effects that scientists wonder whether they are not having a long-term influence on the atmosphere as a whole and climate patterns.

As all these examples testify, the stormy relationship between the Sun and Earth across the interplanetary medium has significant repercussions on human activity. That is why detailed study of the phenomena involved has gradually come to be seen as a major objective of modern science. The satellites launched during the first forty years of the conquest of space have revealed that the magnetosphere is a structured medium, dominated by the Earth's magnetic field and stretching more than 60 000 kilometres towards the Sun, but our knowledge of the physics remains fairly sketchy.

"What we are now aiming for is a precise understanding of how the Earth's magnetic shield interacts with the solar wind flow. We are hoping to identify the mechanisms that move matter and propagate energy, and thus try to see whether the consequences of magnetic storms can be forecast," explains Rudolf Schmidt, Cluster Project Scientist at ESA.

With its four satellites and the ultrasensitive measuring instruments on board, Cluster offers unprecedented facilities for finding answers to these questions.

Unlike all the previous probes that have travelled through the magnetosphere individually or two at a time at most, Cluster will take simultaneous measurements from four points in space. It will therefore deliver information on the three-dimensional structures of the phenomena it records and separate data on changes in them over time. Scientists are therefore expecting to obtain an infinitely more detailed description of the system of gases, currents and fields in perpetual motion making up the Earth's electromagnetic environment.

The satellites will be on a 125 000 km x 25 000 km orbit, flying in tetrahedral formation, the distances between them varying from 200 to 20 000 kilometres.

Four Identical Scientific Jewels

Each of the spacecraft is cylindrical in shape, 2.9 metres in diameter and 1.3 metres in height, with a mass of 1.2 tonnes of which 54% is accounted for by propellant, most of which will be consumed in hoisting the craft up to its working orbit.

During the mission the satellites will be spin-stabilised, at 15 rpm. Under the effect of this movement, four 50 m wire booms carrying electrical field instruments will be deployed transversely, along with two 5 m booms carrying magnetic sensors.

Each of the craft will be a high-performance laboratory flying eleven instruments to measure electromagnetic fields, radio waves and noise, electrons and ionised atoms in the medium. Waves emitted by the magnetosphere at frequencies between 10 and 400 kilohertz will be recorded and analysed. Electric and magnetic fields will be measured to within a few microvolts per metre and 0.25 nanoteslas (one millionth of the magnetic field detected by a compass on the Earth's surface). Electrons with energies of up to 400 kiloelectronvolts and ions up to 1500 kiloelectronvolts per nucleon will be detected.

One of the instruments, it should be noted, will not itself be probing the surrounding medium but helping the others to do so more accurately by stabilising the satellite's electrostatic potential. It will actively eliminate parasite electrostatic charges by emitting a current of indium ions of variable intensity up to 20 microamps.

Meanwhile, another instrument will emit two electron beams on a circular path 1 to 40 kilometres in diameter in the magnetosphere and then pick up and analyse the returning beams. This will provide precise measurements of normally inaccessible components of ambient electric and magnetic fields.

Each of the satellites and each instrument on board will have to meet unprecedented demands, and their development presented correspondingly unique technical and industrial challenges. As John Credland, ESA's Cluster Project Manager, explains: "This is the very first time that four identical scientific satellites will have been built, launched and simultaneously operated in space. More specifically, it is the first time that Europe has built spacecraft in small series production. It has also been a big challenge for the research institutes, which have had to develop and supply four models, all of them with the same sensitivity and precision, of 11 scientific instruments."

The satellites were built, under ESA's overall responsibility, by a consortium of 15 European companies led by Dornier-DASA of Friedrichshafen, Germany. The instruments were developed by European and American laboratories, over 200 scientists having played an active part.

4.8 Tonnes Stacked at the top of Ariane-5

Since last summer the four satellites have been at Europe's Spaceport at Kourou in French Guiana, where their powerful new launch vehicle is going through final preparations. Once the launch date was confirmed the filling of the satellites tanks with a total of 2.6 tonnes of propellant was started, a very delicate operation that took four weeks to complete. Then the four craft, stacked at the top of Ariane-5, will be shut away under the launcher's fairing, and that will be the last the scientists will see of their technological prodigies; afterwards, they will only be able to communicate with them by radio.

Half an hour after lift-off, a system of precompressed springs and separation bolts will eject the four passengers from the launcher one by one, delivering them into a geostationary transfer orbit.

Three weeks of complex manoeuvres will follow, putting the spacecraft through a sequence of four intermediate orbits to escape the Earth's equatorial plane and reach the very eccentric mission orbit, at an inclination of 90 to the equator. During these manoeuvres, each satellite will fire its onboard rocket system five times to achieve cumulative acceleration of 1.8 million kilometres per hour, burning up two-thirds of its fuel supply.

On completion of these orbital manoeuvres, testing of the 44 instruments flown will start. Three months after the launch, the mission ought to be able to enter its operations phase. ESA's Operations Centre (ESOC) in Darmstadt will then have the difficult task of simultaneously controlling the scientific activity and movements of the four satellites in flight. ESOC will plan all operations in advance. It will command and constantly verify the operations of each spacecraft and each instrument. It will control the manoeuvres for adjusting distances between satellites. The Centre will be equipped with all the heavy computing facilities required for such a quadruple mission.

Radio communications will be conducted by ESA's ground stations at Odenwald in Germany and Redu in Belgium, their two 15 m antennas being used in parallel, each dedicated to links with a given pair of satellites.

The mission has also presented major problems in the areas of batching commands to the 44 instruments, distribution of the harvest of data received back, coordinated analysis of the data, storage and final distribution of results. These have been dealt with by setting up a distributed management system that is unique in Europe. As John Credland explains, "With this system, which is based on the information superhighway infrastructure, each Principal Investigator will be able to receive his experiment data on his personal computer in his laboratory, which he can also use to control the instrument settings."

All the mission scientists will also receive a daily CD-ROM containing a copy of the raw data transmitted a week previously by all the instruments. The system will be complemented by "fixed" national data centres in Austria, Scandinavia, France, Britain, Germany, Hungary, the United States and China for long-term archiving and general distribution of results. The backbone of the system will be Esanet, within the worldwide Internet.

Extending the Boundaries of Meteorology to Solar-terrestrial Relations?

The arrangements made should therefore give scientists throughout the world ready access to Cluster's output of matchless data. This is going to be particularly useful over the years to the turn of the century, a period of unprecedented international activity devoted to investigation of solar- terrestrial physics**.

The aims of this activity? To make a general study of the physics, which cannot be reproduced on Earth, of the ionised gases that surround the planet and represent a sample of the plasmas making up 95% of the visible matter in the Universe. More specifically, to study the storms that regularly occur between the Sun and Earth in order to test the feasibility of developing, within 10 to 15 years, meteorological forecasting of these sometimes extremely disruptive phenomena.

Efforts in both these directions are going to benefit from the Cluster mission, whose unique contribution will eventually take the form of more than 1000 CD-ROMs carrying three thousand billion bits of original data on the three-dimensional structure and dynamics of the space surrounding our planet. A fundamental and irreversible addition to scientific knowledge, no less.

*Cluster and Soho together make up the Solar-Terrestrial Science Programme (STSP), the first cornerstone of ESA's Horizon 2000 science programme. The STSP is being conducted jointly with NASA, the European and American contributions being 70% and 30%. ESA's investment is estimated at EURO 845 million. For Cluster, it is supplying the four satellites, the Ariane-5 launch and mission operations control. The United States has contributed to development of the 4 x 11 onboard instruments, in cooperation with ESA Member States.

**As many as 400 scientists from about 20 countries are currently engaged in what amounts to a massive international campaign of research on solar-terrestrial relations, which is deploying radars on the ground, networks of magnetometers, optical telescopes, and sounding rockets and atmospheric balloons. As to space-borne research, there could be up to 25 missions between now and the end of the century. Of these, the STSP, comprising Soho and Cluster, is the most important contribution in terms of the number and specific characteristics of satellites, the other major missions being Interball (Russia), Wind and Polar (United States) and Geotail (Japan).