Effort To Recover SOHO Spacecraft Continue (fwd)

Willie Koorts (wpk@saao.ac.za)
Thu, 16 Jul 1998 21:39:19 +0200 (GMT+0200)

Hi SeeSatters

I found this on ASTRO.  Hope it is not too far off topic,  but I thought 
some of you might be interested.

Willie
                        Willie Koorts   wpk@saao.ac.za   

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---------- Forwarded message ----------
Date: Thu, 16 Jul 1998 16:18:06 GMT
From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
To: astro@lists.mindspring.com
Subject: [ASTRO] Effort To Recover SOHO Spacecraft Continue

European Space Agency
Press Release Nr 26-98
Paris, France					16 July 1998

Effort to recover SOHO spacecraft continue as investigation board focuses
on most likely causes

ESA (European Space Agency) and NASA engineers, reasoning that over the
next two-to-three months the spacecraft's solar panels will increasingly
face the Sun and generate power, are continuing their efforts to contact
the Solar and Heliospheric Observatory (SOHO) spacecraft.

Meanwhile, the ESA/NASA investigation board concentrates its inquiry on
three errors that appear to have led to the interruption of
communications with SOHO on June 25. Officials remain hopeful that,
based on ESA's successful recovery of the Olympus spacecraft after four
weeks under similar conditions in 1991, recovery of SOHO may be
possible.

The SOHO Mission Interruption Joint ESA/NASA Investigation Board has
determined that the first two errors were contained in preprogrammed
command sequences executed on ground system computers, while the last
error was a decision to send a command to the spacecraft in response to
unexpected telemetry readings.  The spacecraft is controlled by the
Flight Operations Team, based at NASA's Goddard Space Flight Center,
Greenbelt, MD.

The first error was in a preprogrammed command sequence that lacked a
command to enable an on-board software function designed to activate a
gyro needed for control in Emergency Sun Reacquisition (ESR) mode.  ESR
mode is entered by the spacecraft in the event of anomalies.  The second
error, which was in a different preprogrammed command sequence, resulted
in incorrect readings from one of the spacecraft's three gyroscopes,
which in turn triggered an ESR.

At the current stage of the investigation, the board believes that the
two anomalous command sequences, in combination with a decision to send
a command to SOHO to turn off a gyro in response to unexpected telemetry
values, caused the spacecraft to enter a series of ESRs, and ultimately
led to the loss of control.

The efforts of the investigation board are now directed at identifying
the circumstances that led to the errors, and at developing a recovery
plan should efforts to regain contact with the spacecraft succeed.

ESA and NASA engineers believe the spacecraft is currently spinning with
its solar panels nearly edge-on towards the Sun, and thus not generating
any power.  Since the spacecraft is spinning around a fixed axis, as the
spacecraft progresses in its orbit around the Sun, the orientation of
the panels with respect to the Sun should gradually change.  The orbit
of the spacecraft and the seasonal change in the spacecraft-Sun
alignment should result in the increased solar illumination of the
spacecraft solar arrays over the next few months.  The engineers predict
that in late September 1998, illumination of the solar arrays and,
consequently, power supplied to the spacecraft, should approach a
maximum.  The probability of successfully establishing contact reaches a
maximum at this point.  After this time, illumination of the solar
arrays gradually diminishes as the spacecraft-Sun alignment continues to
change.

In an attempt to recover SOHO as soon as possible, the Flight Operations
Team is uplinking commands to the spacecraft via NASA's Deep Space
Network, managed by NASA's Jet Propulsion Laboratory, Pasadena, CA,
approximately 12 hours per day with no success to date.  A recovery plan
is under development by ESA and NASA to provide for orderly restart of
the spacecraft and to mitigate risks involved.

The recovery of the Olympus spacecraft by ESA in 1991 under similar
conditions leads to optimism that the SOHO spacecraft may be recoverable
once contact is re-established.  In May 1991, ESA's Olympus
telecommunications satellite experienced a similar major anomaly which
resulted in the loss of attitude, leading to intermittent power
availability.  As a consequence, there was inadequate communication, and
the batteries and fuel froze.  From analysis of the data available prior
to the loss, there was confidence that the power situation would improve
over the coming months.

A recovery plan was prepared, supported by laboratory tests, to assess
the characteristics of thawing batteries and propellants.  Telecommand
access of Olympus was regained four weeks later, and batteries and
propellant tanks were thawed out progressively over the next four weeks.
The attitude was then fully recovered and the payload switched back on
three months after the incident.  Equipment damage was sustained as a
result of the low temperatures, but nothing significant enough to
prevent the successful resumption of the mission.  The experience of
Olympus is being applied, where possible, to SOHO and increases the hope
of also recovering this mission.

Estimating the probability of recovery is made difficult by a number of
unknown spacecraft conditions.  Like Olympus, the hydrazine fuel and
batteries may be frozen.  Thermal stress may have damaged some of the
scientific instruments as well.  If the rate of spin is excessive, there
may have been structural damage.  SOHO engineers can reliably predict the
spacecraft's orbit through November 1998.  After that time, the
long-term orbital behavior becomes dependent on the initial velocity
conditions of the spacecraft at the time of the telemetry loss.  These
are not known precisely, due to spacecraft thruster activity that
continued after loss of telemetry, so orbital prediction becomes very
difficult.

More information on SOHO, including status reports, is available on the
internet at: http://sohowww.estec.esa.nl or via the new ESA science
website at: http://sci.esa.int/