Mission Overview
    ================
      The Viking mission to Mars consisted of four spacecraft: the
      two orbiters VO1 and VO2, and the landers VL1 and VL2
      [SOFFEN1977].  During cruise to Mars the landers were attached
      to the orbiters; the combined spacecraft were then known as
      Viking 1 and 2.  The role of the orbiters was to transport the
      landers to Mars, to carry reconnaissance instruments for
      certifying the landing sites, to act as relay stations for
      lander data, and to perform their own scientific
      investigations.  The initial orbit periapses were placed over
      the candidate landing sites to allow for maximum viewing
      resolution and relay of the lander data.  After the primary
      lander missions were completed, the orbiters' orbits were
      allowed to drift so that the entire planetary surface could be
      systematically mapped by the three remote sensing experiments.
 
      The Viking 1 spacecraft was launched August 20, 1975, and
      arrived at Mars on June 19, 1976.  Lander 1 was deployed to the
      Mars surface on July 20, 1976.  The VO1 orbital inclination of
      38-39 degrees was chosen to optimize communication with  VL1.
      Viking 2 was launched September 9, 1975 and arrived at Mars
      August 7, 1976.  VL2 landed on September 3, 1976, at a more
      northerly site than VL1.  The VO2 orbit was correspondingly
      more inclined than VO1; initially 55 degrees, it was later
      adjusted to 80 degrees, providing particularly good coverage
      of polar regions.  The areocentric locations of VL1 and VL2
      have since been determined to be (22.270N, 48.264W) and
      (47.669N, 226.032W), respectively [YODER&STANDISH1997].
 
 
    Mission Phases
    ==============
      The timeline for the Viking Mission is divided into a number of
      mission phases in terms of the types of observations and level
      of activity.  The references [SNYDER1977], [SNYDER1979],
      [MOOREETAL1987], and [SNYDER&MOROZ1992] provide detailed
      descriptions of these mission phases.  A summary of the mission
      phases and the relevant dates are described below.  Before Mars
      encounter and orbit insertion the orbiter and lander spacecraft
      are considered as one spacecraft with the same mission phases.
      The primary missions for all four spacecraft (VL1, VO1, VL2,
      and VO2) are listed separately because each has a different
      starting date.  All mission phases after the primary mission
      are listed only once because all four spacecraft operated
      together.
 
 
      VIKING 1 MARS LAUNCH
      --------------------
        The Viking 1 spacecraft was launched on August 20, 1975 on a
        Titan Centaur 3 booster from Cape Canaveral, Florida.
 
         Spacecraft Id:                 : VO1, VL1
         Target Name                    : MARS
         Mission Phase Start Time       : 1975-08-20
         Mission Phase Stop Time        : 1975-08-20
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING 1 MARS CRUISE
      --------------------
        The Viking 1 spacecraft, consisting of the VO1 orbiter and
        VL1 lander, cruised to Mars for about 10 months, during which
        time the spacecraft was checked periodically.
 
         Spacecraft Id:                 : VO1, VL1
         Target Name                    : MARS
         Mission Phase Start Time       : 1975-08-20
         Mission Phase Stop Time        : 1976-06-19
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING 2 MARS LAUNCH
      --------------------
        The Viking 2 spacecraft was launched on September 9, 1975 on
        a Titan Centaur 3 booster from Cape Canaveral, Florida.
 
         Spacecraft Id:                 : VO2, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1975-09-09
         Mission Phase Stop Time        : 1975-09-09
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING 2 MARS CRUISE
      --------------------
        The Viking 2 spacecraft, consisting of the VO2 orbiter and
        VL2 lander, cruised to Mars for about 11 months, during which
        time the spacecraft was checked periodically.
 
         Spacecraft Id:                 : VO2, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1975-09-09
         Mission Phase Stop Time        : 1976-08-07
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING ORBITER 1 PRIMARY MISSION
      --------------------------------
        The Viking Orbiter 1 spacecraft entered Mars orbit on June
        19, 1976.  Operations commenced by supporting the selection
        of a landing site for VL1.  Throughout the Primary Mission,
        the VO1 spacecraft supported communications with the landers
        and made observations of the Martian surface and atmosphere.
        The Primary Mission ended at the start of the solar
        conjunction in November, 1976.
 
         Spacecraft Id:                 : VO1
         Target Name                    : MARS
         Mission Phase Start Time       : 1976-06-19
         Mission Phase Stop Time        : 1976-11-15
         Spacecraft Operations Type     : ORBITER
 
 
      VIKING LANDER 1 PRIMARY MISSION
      -------------------------------
        The Viking Lander 1 spacecraft separated from the VO1 orbiter
        and descended to the Martian surface on July 20, 1976.  The
        Primary Mission focused on the collection and analysis of
        soil samples and the characterization of the landing site and
        atmosphere.  The Primary Mission ended at the start of the
        solar conjunction in November, 1976.
 
         Spacecraft Id:                 : VL1
         Target Name                    : MARS
         Mission Phase Start Time       : 1976-07-20
         Mission Phase Stop Time        : 1976-11-15
         Spacecraft Operations Type     : LANDER
 
 
      VIKING ORBITER 2 PRIMARY MISSION
      --------------------------------
        The Viking Orbiter 2 spacecraft entered Mars orbit on August
        7, 1976.  Operations commenced by supporting the selection of
        a landing site for VL2.  Throughout the Primary Mission, the
        VO2 spacecraft supported communications with the landers and
        made observations of the Martian surface and atmosphere.  The
        Primary Mission ended at the start of the solar conjunction
        in November, 1976.
 
         Spacecraft Id:                 : VO2
         Target Name                    : MARS
         Mission Phase Start Time       : 1976-08-07
         Mission Phase Stop Time        : 1976-11-15
         Spacecraft Operations Type     : ORBITER
 
 
      VIKING LANDER 2 PRIMARY MISSION
      -------------------------------
        The Viking Lander 2 spacecraft separated from the VO2 orbiter
        and descended to the Martian surface on September 3, 1976.
        The Primary Mission focused on the collection and analysis of
        soil samples and the characterization of the landing site and
        atmosphere.  The Primary Mission ended at the start of the
        solar conjunction in November, 1976.
 
         Spacecraft Id:                 : VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1976-09-03
         Mission Phase Stop Time        : 1976-11-15
         Spacecraft Operations Type     : LANDER
 
 
      VIKING EXTENDED MISSION
      -----------------------
        The Viking Extended Mission began after solar conjunction.
        The two orbiters continued to observe the surface and
        atmosphere of Mars.  The two lander spacecraft analyzed
        additional soil samples and dug three deep holes in the
        surface.  All four spacecraft monitored the planet through
        the cycle of seasons.  During the winter season, the landers
        operated in an automatic manner designed to allow the
        spacecraft to survive the cold temperatures and still return
        some data.
 
         Spacecraft Id:                 : VO1, VL1, VO2, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1976-11-15
         Mission Phase Stop Time        : 1978-05-31
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING CONTINUATION MISSION
      ---------------------------
        Primary objectives of the Continuation Mission were to make
        orbital observations at times of the Mars year that were
        missed due to landing site selection and solar conjunction
        and to collect high resolution surface images when the
        atmosphere was clear.  A radio science solar conjunction
        relativity experiment was also done during the Continuation
        Mission.  Lander activities consisted of measurements by the
        imaging, meteorology, and XRFS instruments operating in a
        fully automated manner.  Viking Orbiter 2 developed a leak in
        its propulsion system and lost its attitude control gas.  VO2
        was turned off on July 25, 1978 after 706 orbits around Mars.
 
         Spacecraft Id:                 : VO1, VL1, VO2, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1978-05-25
         Mission Phase Stop Time        : 1979-02-26
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING INTERIM PERIOD
      ---------------------
        The Interim Period mission phase occurred during the time of
        the Voyager 2 encounter with Jupiter.  Thus, communications
        to and from the Viking spacecraft were limited.  The landers
        continued to operate in an automated manner making imaging
        and meteorology observations.  A final VL2 surface sampler
        sequence was conducted during this mission phase as an
        engineering test in the cold temperatures of mid winter.
        Orbital data stored on spacecraft tape recorders and not
        returned during the Continuation Mission were downlinked
        during the Interim Period.
 
         Spacecraft Id:                 : VO1, VL1, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1979-02-26
         Mission Phase Stop Time        : 1979-07-19
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING SURVEY MISSION
      ---------------------
        The prime scientific objective for VO1 during the Survey
        Mission was to obtain high resolution images of possible
        future landing sites.  The plan for the landers was to
        collect image and meteorology data for as long as possible.
        Because VL2 no longer had a direct downlink capability, it
        meant that VL2 could return data only as long as VO1 provided
        a relay link, once every seven weeks.  Communications with
        VL2 ended on April 11, 1980 after its batteries could no
        longer hold a charge.  VL2 operated on the surface of Mars
        for 1281 sols.  VO1 consumed the last of its attitude control
        gas on August 7, 1980 and was turned off after 1485 orbits
        around Mars.
 
         Spacecraft Id:                 : VO1, VL1, VL2
         Target Name                    : MARS
         Mission Phase Start Time       : 1979-07-19
         Mission Phase Stop Time        : 1980-08-07
         Spacecraft Operations Type     : ORBITER, LANDER
 
 
      VIKING COMPLETION MISSION
      -------------------------
        Viking Lander 1 continued to operated in its automatic mode
        during the Completion Mission.  The observation sequences
        were cyclic.  VL1 returned via direct downlink image and
        meteorology data about once a week with image sequences
        repeating every 37 sols.  The VL1 high-gain antenna was
        programmed to track the Earth until December, 1994.  However,
        communications were lost in November 1982 after a command
        sequence uplink.
 
         Spacecraft Id:                 : VL1
         Target Name                    : MARS
         Mission Phase Start Time       : 1980-08-07
         Mission Phase Stop Time        : 1982-11-19
         Spacecraft Operations Type     : LANDER