Computer Science – Sound
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996xmm..pres....8.&link_type=abstract
XMM Press Release INFO 8-1996
Computer Science
Sound
Scientific paper
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 kph. 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 fai
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