Twenty years ago, SOHO, the solar and heliospheric observatory, set out to study the Sun – all the way from its deep core to the outer corona – and the solar wind. Launched on 2 December 1995 from Cape Canaveral in Florida, the joint ESA/NASA mission was originally intended for a mere two-year stint in space, charged with answering questions about the structure and dynamics of the solar interior, discovering why the solar corona exists and how it is heated, and finding out where the solar wind is produced.
Since then, its mission has been extended many times over, and today, SOHO celebrates 20 years of delivering significant insights into the nature of our Sun. Thanks to SOHO, we have the first images of a star’s convection zone and precise measurements of the Sun’s temperature structure, interior rotation, and gas flows. It has identified the source of the solar wind at the Sun’s poles and has revolutionised our understanding of the impact of space weather on the Earth’s climate.
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But it might all have ended much sooner. On 24 June 1998, after two and a half years of normal operation, from 1.5 million kilometres away from the Earth, the ground control team received a series of emergency messages, followed by silence. SOHO had disappeared. No one knew why.
A search effort was mounted by an international team, including engineers from Airbus Defence and Space (then Matra Marconi Space), who had been the lead contractor in building the spacecraft. The experts feared that SOHO was spinning uncontrollably and losing power fast. Commands were transmitted 12 hours a day in the hope of contacting it.
"There were very few people who thought we would get SOHO back,” says Philippe Temporelli, satellite system design manager.
The team’s single-minded dedication paid off. After four weeks of searching, a radio telescope in Puerto Rico locked onto SOHO. Finally, on 9 September, thanks to a strategy designed by the whole recovery team, SOHO stopped spinning and was pointed back at the Sun. By the end of October, most of its scientific instruments were up and running again.
“It was an incredible feeling. There were cheers and shouts in the control room.” Robert Harris, responsible for the Attitude and Orbit Control Subsystem
But SOHO’s difficulties were not over. Two of the three on-board gyroscopes responsible for attitude control had failed during its first disappearance. The remaining one failed a few weeks later, on 21 December. SOHO went into a slow roll.
The ground control team took over, using thrusters to keep it pointing at the Sun. This steady drain on the fuel reserves would have shortened SOHO’s lifespan, had it gone on for much longer. But, again, our engineers came up with innovative solutions, inventing strategies that turned the entire spacecraft into a giant gyro. In February 1999, new software that allowed for gyroless operation was installed.
“The second recovery involved a smaller team, but was just as exciting,” Michel Janvier, who instigated this innovative solution together with Harris.
In 2003 the whole mission team was honoured with the third ever International Academy of Aeronautics team award for the dedication and skill they showed in making possible a “world class mission”, and Harris himself was awarded an MBE (Member of the Order of the British Empire) by Queen Elizabeth II for his role in the two rescues.
SOHO has operated flawlessly ever since, delivering images and data that have substantially advanced our understanding of the Sun and the solar-terrestrial relationship. Its mission has been extended once again – to December 2016. When its work is done, it will be followed by Solar Orbiter, a new spacecraft which is also being built by Airbus Defence and Space. Scheduled for launch in 2018, it will take mankind closer to the Sun than any space probe to date.