Date: 6 April 1998
Solar physicists from the UK and other European countries are urging the European Space Agency (ESA) to build a new spacecraft to orbit the Sun. The Solar Orbiter would circle much closer to the Sun than any previous spacecraft, approaching the within 20 million km of the Sun's surface - 3 times closer than the innermost planet, Mercury. Following a special workshop sponsored by ESA and organised by Professor Eric Priest of the University of St Andrews, the scientists are suggesting that the Solar Orbiter project should be part of a coordinated international approach to learning more about the Sun. It would follow up the success of Ulysses and SOHO, two spacecraft currently being used to study the Sun.
The Sun is an important subject for research by ESA. As our nearest star, the Sun is of great significance for astronomy as a whole and provides the best opportunity for astronomers to study basic cosmical processes in great detail. In addition, it affects Earth's environment and can cause damage to satellites and disrupt electricity supplies.
A hundred of the world's leading solar physicists met at the ESA workshop, held at the Instituto de Astrofisica de Canarias on the Spanish island of Tenerife at the end of March. They considered various solar space missions being planned by different groups of scientists around the world and concluded that the Solar Orbiter, provided by ESA in collaboration with other space agencies, would be a key part of a coherent international programme.
"Space missions take many years to plan" says Professor Priest. "From time to time we have to stand back and decide what key scientific questions we should try to answer in the future. In what direction should we head? What might be technologically feasible in 10 or 20 years from now? Such exciting ideas for studying the Sun are on offer, it is hard to choose between them."
In addition to the Solar Orbiter, workshop participants looked at three other proposed space missions:
STEREO is a proposal to send several spacecraft to different positions in the Solar System. By viewing the Sun from different directions they could build up 3-D pictures (stereoscopic or tomographic). These would reveal clearly, for the first time, the complex contortions of gas and magnetic fields involved in solar eruptions. The consequences include solar flares and the mass ejections of gas which, arriving at the Earth, can cause satellite failures and power black-outs.
SOLAR-NET is a rival scheme, aiming to inspect the stormy surface and atmosphere of the Sun in much sharper detail. A combination of three telescopes in an interferometer could achieve a clarity of vision 40 times better than that of the present instruments in space. The immense magnetic explosions responsible for heating the Sun's atmosphere and for sparking solar flares occur in regions too small to be clearly resolved by available telescopes.
PROBE is a scheme to send a spacecraft into the Sun's hot atmosphere, where it could sample its atoms directly and measure the magnetic fields. Probe would fly by the Sun at a distance of only 2 million kilometres from the surface, compared with 150 million kilometres for the Earth-Sun distance. Remarkable heat shields would have to maintain the spacecraft at an operable temperature in sunshine 2500 times stronger than at the Earth.
After a lively debate, the following consensus emerged. A coherent international programme of missions should be set up. Stereo and Probe are likely to be realized within NASA's programme, but it was agreed that they would benefit greatly by participation of European scientists with ESA endorsement. Solar-Net was recognized to be of high scientific interest and was considered suitable for a small demonstration mission. Solar Orbiter was recommended as the main ESA element in the programme.
In contrast to Probe, which would fly right past the Sun in less than a day, Solar Orbiter would remain above the same region of the Sun for several days and would continue to orbit the Sun many times. At a later stage the aim would be to observe in detail, for the first time, the mysterious polar regions of the Sun. And Solar Orbiter would be able to see the regions through which Probe was flying.
Commenting on the choice of Solar Orbiter, Professor Priest said, "What tipped the scales was the exciting new science, its key role in the overall international programme, and the broad backing of the world's solar scientists. By selecting Solar Orbiter as the project for which we will all now be pressing, we can be certain that Europe will create a fitting successor to SOHO, which is already revolutionizing solar science.
As with any proposal like this, Solar Orbiter will have to go through ESA's procedures for mission selection before it can be authorised. Assessments by expert committees and detailed studies of the science, engineering and costs are required. Any collaboration with other space agencies has to be negotiated. However, the support of the specialists in solar physics helps strengthen the case for Solar Orbiter as ESA's next solar project, to be considered for possible launch between 2005 and 2010.
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