SOHO is ready again to warn of solar storms
14 December 1999
With the Sun now entering its season of maximum sunspot counts, theworld's engineers have reason to be nervous. Blustery space weatherstirred up by the Sun can disrupt technological systems on the Earth andespecially in orbit, where 75 communications satellites worth about 15billion euros are at risk from solar storms. So the engineers will beglad to know that the world's chief watchdog for the Sun, the ESA-NASASOHO spacecraft, is now fully back on duty after a technicalinterruption from 28 November to 10 December that curtailed some of itsobservations.SOHO has already shown itself capable of giving early warnings of mass ejections heading from the Sun towards the Earth. It is even able to sense the presence of stormy regions on the Sun's far side, before the solar rotation brings them into view. SOHO's experts are also searching for signs of impending eruptions that will make them more predictable. At a meeting on space weather in London on 10 December, a clue was described by Richard Harrison, the scientist in charge of SOHO's Coronal Diagnostic Spectrometer.
"One sign of an impending mass ejection, recently discovered, is the observation of a dimming in the brightness of the region of the solar atmosphere which is about to erupt," Harrison said. "The dimming is detected in ultraviolet light emitted by gas at almost exactly a million degrees. Our CDS instrument tells us exactly how much gas has disappeared from the region that has gone dim, and how hot it is. Presumably the missing gas is heading for interplanetary space in the mass ejection."
When asked about the latest interruption in SOHO's observing programme, Richard Harrison was philosophical. "A spacecraft out there is bound to have troubles from time to time," he said. "Don't forget that unexplained glitches in SOHO may be due to solar particles. It's more exposed to them than the communications satellites we worry about."
SOHO is designed to take decisive action to look after itself if anything goes wrong with its control in orbit. On 28 November a still- unexplained computing error troubled SOHO and prompted it to go into a self-protective mode called Emergency Sun Reacquisition. Controllers on the ground had then to tiptoe around the safety systems and gently encourage SOHO to resume normal operations.
By 1 December the control team had reached near-normality and were trimming the orbit, using commands to the spacecraft's thrusters. SOHO did not like what was happening and automatically switched the thrusters off. Analysis revealed that the manoeuvre had caused SOHO to lose sight of a guide star. More serious was a failure of the onboard system to stop the spacecraft rolling excessively. In order to keep it in communication with the Earth, the controllers had to command SOHO to go back into the Emergency Sun Reacquisition mode.
Then they proceeded even more cautiously. Leaving the spacecraft in an intermediate condition called Coarse Roll Pointing, the team gave themselves a few days' thinking time. They wanted to make sure they fully understood SOHO's wayward behaviour, by reproducing it on a simulator at ESTEC in the Netherlands. The explanation was a slight mismatch of onboard computing systems that could occur in unusual circumstances. The controllers were then able to send additional software to SOHO to prevent it happening again, and by 10 December the spacecraft was back to its normal operating mode.
The emergency interfered with a celebration of SOHO's fourth birthday, remembering its launch on 2 December 1995. The champagne remained in the cooler while the spacecraft engineers from ESA, NASA and Europe's aerospace industry sorted out the problems. Before they finished they took the precaution of adding another manoeuvring option for SOHO to use if any Millennium Bug (Y2K) problem should affect communications from the ground.
SOHO brilliantly completed its nominal two-year mission early in 1998, but ESA and NASA decided to extend the mission to 2003, including the period of maximum solar activity around 2000. It remains the flagship of a multinational fleet of solar spacecraft, including the ESA/NASA Ulysses, and Cluster II due to be launched next year.
