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PD0RDD > NASA 21.09.98 18:41l 172 Lines 8701 Bytes #-10072 (0) @ WW
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To : NASA@WW
Onderwerp: ESA Press Release - "SOHO is pointing at the Sun again"
N? 33-98 - Paris 17 September 1998
---------------------------------
SOHO is pointing at the Sun again
---------------------------------
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.
end
.
Greetings from Hans at Maarssenbroek ³~ The Netherlands
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