NASA plans to launch the High Energy Spectroscopic Imager - an $85 million mission to study solar flares - on 5 February, after more than 18 months of delays.

Solar flares are the most powerful explosions in the solar system. Although the flares occur 150 million kilometers away, their powerful emissions can disrupt the upper atmosphere, interfering with terrestrial communications. They can also damage satellites, and induce strong magnetic storms that can disrupt electric power grids as well as communications.

Over a period of seconds to minutes, one solar flare can release as much energy as a billion megatons of TNT. The energy twists magnetic fields and accelerates particles to high speeds. These particles generate X rays and gamma rays as they pass through the solar atmosphere.

HESSI will detect the X rays and gamma rays, recording images and spectra with better resolution than ever before possible.

Second peak

Originally, NASA planned to launch the craft in July 2000 - timed to match a peak in the 11-year cycle of solar activity. But in March 2000, faulty test equipment shook the spacecraft so hard that it suffered serious damage and required extensive repairs.

The launch was rescheduled for March 2001. But concerns about the reliability of the Pegasus booster rocket pushed the date back until June. Then one week before that the June launch date, a Pegasus launcher malfunctioned, destroying NASA's X-42 test vehicle.

But Robert Lin, principal HESSI investigator at the University of California at Berkeley, told New Scientist: "We've been fortunate". Although solar activity peaked as expected in mid-2000, it reached a second peak at the end of last year.

That second peak "means that things still look pretty active," he says. But HESSI will have fewer flares to study during its lifespan, which will be at least two years.

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