Eruptions of gas from the sun, which disrupt power lines and navigation systems if they reach the earth, will be investigated by a solar observatory due for launch next Thursday.

The joint European Space Agency/NASA mission will float at the region in space where the sun's and the earth's gravitational fields balance each other. This region is 1.5 million kilometres from earth and will take three months to reach. From there it will watch the sun and find out about its internal structure and atmosphere, with its constant emissions of solar flares, gas eruptions and solar wind.

Soho - the Solar and Heliospheric Observatory - will carry 12 instruments. Richard Harrison, solar physicist at the Rutherford Appleton Laboratory and one of the 12 principal investigators on Soho, is in charge of the coronal diagnostic spectrometer, which will study the sun's atmosphere. He said: "The sun is just a big ball of gas in space." Its atmosphere is complicated and constantly changing under the influence of the sun's magnetic field, which is "a twisted-up thing, changing all the time".

One of the big mysteries of the sun's atmosphere is coronal mass ejection. "Enormous clouds just come crashing out of the sun at 2,000 km/second," said Dr Harrison. "They can engulf the earth."

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The most memorable recent ejection, in 1989, disrupted power distribution in Canada and changed compass readings in Scotland by seven degrees. Once the sun's atmosphere is better understood, he said, "we could then launch satellites that look for specific signs" and give early warnings.

Soho will also gather information on solar flares - streams of radioactive particles thrust into space. Solar flares have caused Concorde to be diverted and have "zapped spacecraft", says Dr Harrison. "You could lose a spacecraft as a result of them."

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And Soho will monitor the vibrations of the sun, which occur as its turbulent gases bump around inside. When the sun vibrates, its surface will vibrate towards and away from Soho. By measuring these shifts, scientists hope to build up a picture of what is happening in its interior.

The Particle Physics and Astronomy Research Council said Soho will cost the UK Pounds 50 million.