Tim Furniss/LONDON
On 26 September, 92 days after being lost in deep space, the European Space Agency (ESA)/NASA Solar and Heliospheric Observatory (SOHO) began sending back images of the sun again.
The spacecraft's remarkable rescue owes much of its success to the initial location work completed by the large radio telescope at Arecibo, Puerto Rico, operated jointly with a NASA radio dish at Goldstone, California, the two combining as a powerful radar system.
The SOHO spun out of control on 25 June, and contact was lost as a result of a "calamitous sequence" of operational errors and decisions on the ground, which culminated in both spacecraft gyroscopes being turned off .
For almost a month, the SOHO was lost in space. It failed to respond to signals sent daily, and the worries of the project team multiplied. Without power from correctly oriented solar panels, the SOHO eventually would die. Its systems and instruments would have suffered from high or low temperatures for which they were not designed. Worst of all, the SOHO might have drifted away from its expected orbit, and never been heard of again.
The observatory was located on 23 July, however, by the Arecibo and Goldstone telescopes, which obtained echoes from the spacecraft at its predicted position. The signals confirmed a prediction that the spacecraft was rotating quite slowly. It still showed no sign of life.
The SOHO was turning once every 53s, with the solar panels edge-on to the sun. These were the worst possible conditions for receiving power from sunlight. Instruments on the side of the spacecraft facing the sun were being baked, while others on the shadowed side were losing heat to deep space.
The observatory was orbiting the sun without changing its spin axis. With every day that passed, the direction to the sun changed by 1° of arc and sunlight fell on the solar panels at a slightly more favourable angle.
On 3 August, with sunlight slanting on to the solar panels at better than 30°, the spacecraft responded to repeated transmissions. The slow and careful task of recovery could then begin. The first requirement was that the SOHO's batteries be charged, using the limited solar power available, to improve communications with the observatory.
The patient strategy worked. On 8 August, the first telemetry from the spacecraft was received. Most important for regaining control of the SOHO was the condition of the hydrazine fuel used by its thrusters. This was found to be partially frozen.
The priority then was to use the limited, but increasing, solar power to thaw out the hydrazine. That took nearly three weeks. Then the pipes carrying the fuel to the thrusters had to be warmed, too.
By 3 September, the propulsion system had been fully thawed, and sunlight was hitting the solar panels at a slant angle of nearly 60í. The need to keep the propulsion system unfrozen made battery charging a slow process, but it was completed by 8 September.
ESA and NASA experts, and former project engineers from industrial contractors, reviewed possible strategies for attitude recovery, which would orient the spacecraft to point at the sun.
On 16 September, the SOHO obeyed commands that turned its face fully towards the sun. For the first time since 25 June, when the craft spun out of control and communications were lost, it pointed the right way.
"It's a big step forward in our recovery plan for the SOHO," said ESA's Francis Vandenbussche, head of the SOHO recovery team at NASA's Goddard Space Flight Center. "This is the best news we have had from the SOHO in a long time," said Dr George Withbroe, director of the Sun-Earth Connection science team at NASA.
"Despite the gloomy early days after the loss, we always stayed hopeful that the resourceful people on the team could save the day. We are not there yet - we still have to see if the scientific instruments survived. But this gives us reason to hope," Withbroe added.
ESA project scientist for the SOHO, Dr Bernhard Fleck, said: "Now we start a comprehensive check of all the spacecraft's systems and scientific instruments. We shall take our time and go step by step, in consultation with the 12 scientific teams in Europe and the USA who provided the instruments.
"In some cases, the instruments have been through an ordeal of heat or cold, with temperatures approaching plus or minus 100°C. But I'm cautiously optimistic that the SOHO can win back much of its scientific capacity for observing the sun."
The hopes and optimism of the scientists were to be rewarded. From 16-26 September, a series of complicated systems checks and operations were completed and, finally, the SOHO was back at work.
Source: Flight International
