Solar Heliospheric Observatory (SOHO) Mission Overview
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  [Unless otherwise noted, the text in this file was compiled by Matthew
   Knight, University of Maryland.]

The Solar and Heliospheric Observatory (SOHO) is an unmanned scientific
space mission developed by ESA and NASA.  The spacecraft was launched
on Dec 2, 1995 aboard an Atlas IIAS rocket from Cape Canaveral, Florida.
Eventually, the SOHO spacecraft was placed in an orbit about the first
Lagrangian, or L1, libration point, that point along the Earth-Sun line
at which the gravitational attraction of the spacecraft to the Earth is
just balanced by its attraction to the Sun.  Located approximately 1.5
million kilometers from Earth, SOHO required a 'cruise phase' of about
4 months to settle into its orbit about L1.  The orbit allows for
observations of very low frequency helioseismologic oscillations, and
for continuous observation of the Sun's outer atmosphere, but suffers
some disadvantage in that the distance takes a toll on allowable
telemetry data rates.  The orbit is only quasi-stable, and periodic
station keeping is required.  The nominal lifetime for the SOHO mission
was two years after arrival on station.  Expendables were carried,
however, sufficient for at least 6 more years of operation.  The
mission was extended in 1997 and again in 2002; the nominal mission end
is March 2007.

On June 24, 1998, SOHO had a series of events that caused it to lose
Sun pointing on June 25.  The spacecraft started spinning around the
solar vector sufficiently fast that it flipped to place the angular
momentum vector with the highest moment of inertia pointing towards the
Sun.  This meant that the solar arrays and instruments were pointed
almost 90 degrees away from the Sun so that no power was being
generated.  This lack of power combined with the pointing change meant
that no communication with the spacecraft was possible.

Due to its orbit around the Sun, it took several weeks for the
spacecraft's solar arrays to be pointed at the Sun.  Contact with the
spacecraft was made on July 23, 1998 by bouncing a signal off the
spacecraft by the Arecibo and NASA/Deep Space Network RADAR.  This
confirmed the orbital position and the spacecraft spin rate.  This
information confirmed the results of detailed analyses that the
spacecraft was spinning at about 1 rotation per minute and in such a
way that the solar arrays would be pointed at the Sun for a maximum of
30 seconds each spin period.

On August 3, a very short carrier signal from the spacecraft was
received by the DSN, and 5 days later a short burst of telemetry was
received.  LASCO temperatures recorded in the telemetry were indicating
that the instrument was colder than -50C which was the lowest limit
that could be read out by an on-board thermistor.  An analysis by the
thermal engineers showed that the LASCO was actually between -80 to
-120C.  The CCD cameras were at the colder end of this range. On
September 16, 1998, SOHO reacquired the Sun.

In mid-2003, a malfunction in the pointing mechanism of the satellite's
high-gain antenna (HGA) was detected.  The antenna, which is used to
transmit the large amounts of data from SOHO's scientific observations
to Earth must be able to move along two axes, vertical and horizontal.
The horizontal (East-West) movement was no longer taking place
properly.  As a result, the spacecraft must be rolled by 180 degrees
every 3 months to cover both halves of the orbit.  Between each half,
there is a gap of 9 to 16 days where the HGA cannot be used, however
medium-rate telemetry continues through its omnidirectional Low-Gain
Antenna (LGA).  Thus, minimal data loss occurs during these blackouts,
and the SOHO spacecraft continues to operate as safely as before the
antenna anomaly occurred.

Mission Objectives Overview
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The Solar and Heliospheric Observatory (SOHO) supports two classes of
scientific investigations, centered around (1) helioseismology, or the
study of the Sun's natural seismic vibrational modes, with the
objective of advancing knowledge of the properties and structure of the
solar interior, and (2) the processes that account for the heating and
acceleration of the solar wind; more broadly, the nature and modes of
evolutionary change in the Sun's outer atmosphere.