SOHO Reveals the Secrets of the Sun; Magnificent Cosmos; Scientific American Presents; by Lang; 6 Page(s)

From afar, the sun does not look very complex. To the casual observer, it is just a smooth, uniform ball of gas. Close inspection, however, shows that the star is in constant turmoil-a fact that fuels many fundamental mysteries. For instance, scientists do not understand how the sun generates its magnetic fields, which are responsible for most solar activity, including unpredictable explosions that cause magnetic storms and power blackouts here on Earth. Nor do they know why this magnetism is concentrated into so-called sunspots, dark islands on the sun's surface that are as large as Earth and thousands of times more magnetic. Furthermore, physicists cannot explain why the sun's magnetic activity varies dramatically, waning and intensifying again every 11 years or so.

To solve such puzzles-and better predict the sun's impact on our planet-the European Space Agency (ESA) and the National Aeronautics and Space Administration launched the two-ton Solar and Heliospheric Observatory (SOHO, for short) on December 2, 1995. The spacecraft reached its permanent strategic position-which is called the inner Lagrangian point and is about 1 percent of the way to the sun-on February 14, 1996. There SOHO is balanced between the pull of Earth's gravity and the sun's gravity and so orbits the sun together with Earth. Earlier spacecraft studying the sun orbited Earth, which would regularly obstruct their view. In contrast, SOHO monitors the sun continuously: 12 instruments examine the sun in unprecedented detail. They downlink several thousand images a day through NASA's Deep Space Network antennae to SOHO's Experimenters' Operations Facility at the NASA Goddard Space Flight Center located in Greenbelt, Md.