November 23, 1995. Copyright, 1995, Graphic News. All rights reserved

ON THE TRAIL OF THE SUN
By Nicholas Booth, Science Editor

LONDON, November 23, Graphic News- A spacecraft due for launch this week will help unravel many remaining mysteries about our daytime star, the Sun. The Solar & Heliospheric Observatory, or SOHO, will get an uninterrupted look for at least two years at the star which is at the centre of our planetary system, and is a triumph of international collaboration. The satellite itself was built in Europe and will be launched from Cape Canaveral by an Atlas booster, after which it will be controlled by NASA, with European astronomers having access to its observations.

The start of the SOHO missions comes at a busy time for the European Space Agency, which recently launched its Infrared Space Observatory to detect the heat and dust of stars being born elsewhere in our galaxy. ESA scientists have also taken the first observations from above and below the SunÕs poles with their Ulysses spacecraft which was launched more than five years ago. From its unique perspective, Ulysses has found that the SunÕs magnetic poles are not as strong as originally thought. Until now, there has not been a dedicated mission to map the Sun stationed sufficiently far from the Earth to avoid light reflected by our planet, which often ÔswampsÕ sensitive detectors.

The stage is set for SOHO to unravel many questions about the Sun, which seems to be a fairly typical middle-aged star and stable enough to sustain life in close proximity. It is a giant ball of hydrogen and helium some 150 million kilometres distant and roughly 700,000 kilometres in diameter. Pressures and temperatures at its centre are so great that hydrogen is fused into helium, releasing vast amounts of energy which keep the temperature as high as 15 million degrees. Beyond this central core, energy radiates outwards but is continually absorbed and reabsorbed, so that it takes millennia for the energy to be released. Above this sits the outer convective zone wherein gases rise and fall, and the top part of which forms the surface, which has a temperature of around 6,000 degrees. 

To understand the innards of the star, we must understand how its surface actually ÔworksÕ Ð and two enduring mysteries remain. The first concerns the way in which sound waves reverberate through the Sun, causing the surface to swell up and down by 50 kilometres every five minutes or so. By monitoring the way these patterns change Ð famously described as the Sun singing to itself, alas at frequencies too high to be heard by the human ear Ð we will learn more about the interior. And the second is the fact that within 500 kilometres above the SunÕs surface, its temperature rises from 8,000 degrees to half a million. In other words, the Sun appears hotter on its outside than on its surface, which seems to defy the laws of science. Unknown or unexplained mechanisms are believed to cause this heating in the extended atmosphere of the Sun which stretches roughly seven times its diameter into space.

The Solar & Heliospheric Observatory is scheduled for launch on November 23rd at 6:54AM GMT.

Source: The European Space Agency