Airbus Defence and Space

SOHO: lost and found

Rescue mission at 1.5 million kilometres from Earth

In June 1998, after over two years of entirely satisfactory in-orbit performance, due to a series of unfortunate events in a sequence of ground operations which should have been an entirely routine manoeuvre, SOHO found itself 1.5 million kilometres from Earth, almost completely frozen, with no on-board power. All attempts to re-establish contact with the spacecraft failed and many believed the satellite to be lost for good.

But they had not counted on the tenacity of a team of engineers from the Airbus Defence and Space predecessor company Matra Marconi Space who, together with the European Space Research and Technology Centre (ESTEC), literally brought the spacecraft back to life and returned it to full operational service!

For the first four weeks, no-one knew where the satellite was. Extensive analysis of data from just prior to loss of the satellite showed that it was rapidly spinning on one of its own axes and would therefore have remained in the right place. In August 1998, a powerful radar signal from Earth produced a faint echo from the spacecraft. This confirmed that SOHO was indeed still in the right place and angled in such a way that sunlight would begin to fall on its solar cells again during the following months.


A race against the sun

Once this was established, and in a race against the sun before it moved away again, the batteries had to be sufficiently recharged and the propulsion system had to be thawed out, starting with the external pipes and then moving to the inner tanks, in order to prevent gas bubbles from bursting the pipes. This was the most delicate part of the operation.

The attitude control system was then reactivated and SOHO was once again ‘sun pointing’! A story of six-months of painstaking efforts which well and truly paid off …

As if this was not dramatic enough, soon after, all the gyroscopes used to control the spacecraft orientation were lost. A new gyro-less attitude control concept had to be developed urgently and SOHO became the first three-axis stabilised spacecraft to be operated without any gyroscopes.

Despite all these problems, engineers have kept SOHO functioning with all its instruments performing well. With several extensions to its original two-year lifetime, SOHO is now in its fifteenth year of operation, having delivered some extremely valuable scientific data, providing real insights into the inner workings of our sun.