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N° 33–1998: SOHO is pointing at the Sun again

17 September 1998

The SOHO solar spacecraft yesterday, 16 September 1998, obeyed commands that turned its face fully towards the Sun. For the first time since 25 June, when SOHO spun out of control and communication was lost, it points the right way. The SOHO flight operations team reported success in the maneuver, which is called attitude recovery, at 18:30 GMT (19:30 Central European Summer Time) on 16 September 1998.

"It's a big step forward in our recovery plan for SOHO," says ESA's Francis Vandenbussche, head of the SOHO recovery team at GSFC. "We were never quite sure that we would manage to make the spacecraft point back towards the Sun, which is essential for its proper operation." John Credland, ESA's head of Scientific Projects says "we have really got to congratulate our joint ESA/NASA team, helped by our industrial contractors, who have accomplished this great job."

"This is the best news we have had from SOHO in a long time", said Dr. George Withbroe, Director of the Sun-Earth Connection science theme at NASA Headquarters, Washington, DC. "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."

ESA's project scientist for SOHO, Dr. Bernhard Fleck, explains what will happen next. "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 degrees Celsius. But I'm cautiously optimistic that SOHO can win back much of its scientific capacity for observing the Sun."

SOHO, the Solar and Heliospheric Observatory, operates at a special vantage point 1.5 million kilometers out in space, on the sunward side of the Earth. It is a joint project of ESA and NASA. The spacecraft was built in Europe but it carries some American as well as European instruments. NASA launched SOHO and has responsibility for operations.

After its launch on 2 December 1995, SOHO worked a revolution in solar science by its special ability observe simultaneously the interior and atmosphere of the Sun, and particles in the solar wind and the heliosphere. Apart from amazing discoveries about flows of gas inside the Sun, giant "tornadoes" of hot, electrically charged gas, and clashing magnetic field-lines, SOHO also proved its worth as the chief watchdog for the Sun, giving early warning of eruptions that could affect the Earth. In April 1998, SOHO's scientists celebrated two years of successful operations, and the decision of ESA and NASA to extend the mission to 2003. The extension would enable SOHO to observe intense solar activity, expected when the count of sunspots rises to a maximum around the year 2000. It would remain the flagship of a multinational fleet of solar spacecraft, including the ESA/NASA Ulysses and Cluster II missions.

More details about the operations, and about SOHO in general, can be found on the Web at: http://sohowww.estec.esa.nl and http://sohowww.nascom.nasa.gov

For further information, please contact : ESA Public Relations 8/10, rue Mario Nikis F-75015-PARIS Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690

John Credland Head of Scientific Projects/ESTEC Tel :
+31.71.565.3430 Fax: +31.71.565.6048

Francis Vandenbussche Goddard Spaceflight Center SOHO Recovery Team Leader, ESA Tel: +1.301.286.4203 or +1.301.286.4098 Fax: + 1.301.286.0218

Don Savage NASA Headquarters, Washington, D.C. Tel:+1.202.358.1727

Bill Steigerwald Goddard Space Flight Center, Greenbelt, MD Tel:+1.301.286.50.17

Diary of a drama

In June 1998, during what should have been a rather standard series of maintenance procedures, things went badly wrong for SOHO.

25 June 1998: SOHO spun out of control. All contact with it was lost. As a joint ESA NASA investigation board concluded later, a "calamitous sequence" of operational errors and decisions culminated in leaving off both gyroscopes vital for SOHO's safety. No fault on the spacecraft contributed to the mishap. For nearly a month, 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, SOHO could be dying. Its systems and instruments might suffer from high or low temperatures for which they were not designed. Worst of all, SOHO might be drifting away from its expected orbit, and never be heard of again.

23 July 1998: SOHO was located. The large radio telescope at Arecibo, Puerto Rico, was operated jointly with a NASA radio dish at Goldstone, California, as a powerful radar system. It obtained echoes from SOHO at its predicted position, and the signals confirmed a prediction that the spacecraft would be spinning quite slowly. It still showed no sign of life.

With hindsight, mission experts know that SOHO was turning once every 53 seconds, 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. But SOHO was also orbiting around the Sun, without changing its spin axis. With every day that passed, the direction to the Sun changed by one degree of arc and sunshine fell on the solar panels at a slightly more favorable angle.

3 August 1998: cheeps from SOHO. Six weeks after contact was lost, and with sunshine slanting on the solar panels at better than 30 degrees, the spacecraft at last responded to the often repeated calls from the ground. SOHO's signals, received in Australia, lasted only 2 to 10 seconds and carried no information -- except the most important news that SOHO was alive and receiving the ground signals.

The slow and careful task of recovery could then begin. The first requirement was that SOHO's batteries should be charged by the very limited solar power, to improve communications with the spacecraft.

8 August 1998: messages from SOHO. The patient strategy worked. The first telemetry from the spacecraft confirmed the expectation that its service module was very cold. Next day, SOHO signaled low temperatures from some of its scientific instruments, high temperatures from others, and more normal temperatures from a third group.

Most important for regaining control of the spacecraft was the condition of the hydrazine fuel used by its thrusters. This was partially frozen. The top priority was to use the limited but ever-improving 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.

3 September: recharging the batteries again. By this time, the propulsion system had been fully thawed, and sunlight was hitting the solar panels at a slant angle of nearly 60 degrees. The need to keep the propulsion system unfrozen made battery charging a slow process, but it was completed by 8 September.

There followed a period of cautious thought. 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. 16 September: attitude recovered.


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