Mission Overview
  ================
 
  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, FL. 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, and is ongoing as of 2010.
 
  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 RPM 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
  onboard 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.
 
  Author
  ------
 
  Unless otherwise noted, the text in this file was compiled by Matthew
  Knight, University of Maryland.

Mission Objectives Overview
  ===========================
    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 objectiv
    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.