Instrument Host Overview
    ========================
      The two Viking Lander spacecraft were the first spacecraft to
      operate successfully for an extended period of time on the
      surface of Mars.  Both spacecraft operated from 1976 through
      April 1980 and Viking Lander 1 (VL1) continued to operate until
      November 1982.  In 1981, Viking Lander 1 was renamed the Thomas
      A.  Mutch Memorial Station in honor of Tim Mutch, Lander
      Imaging Team Leader and later NASA Associate Administrator for
      Space Sciences.  Tim Mutch was killed in a climbing accident in
      1980.
 
      The Viking Landers were identical to each other and had the
      same instrument packages.  The main lander structure was a
      hexagonal prism body that housed the spacecraft computers, tape
      recorder, batteries, several science instruments, and controls
      for the surface sampler, thermal system, and data handling
      system.  The spacecraft body was supported above the surface by
      three legs, each with a curved footpad.  The legs were arranged
      in a triangle pattern with two at the front of the lander and
      one at the rear.  Mounted to the sides of the spacecraft were
      three terminal descent engines, two propellant tanks, and the
      extendible surface sampler arm and collector head.  Also
      mounted on the spacecraft body were two cameras, two
      radioisotope thermoelectric generators (RTG) with covers,
      sample entry ports for the biology, organic chemistry and
      inorganic chemistry instruments, the seismometer, the
      meteorology boom, a magnifying mirror, and three imaging
      reference test charts.  The spacecraft had three antennas for
      communications; a high-gain S-band antenna (the large disk
      antenna), a low-gain S-band antenna, and a UHF antenna.  The
      two S-band antennas were used to communicate with Earth,
      whereas the UHF antenna communicated with the orbiters.  The
      spacecraft had several redundant systems for computers,
      batteries, and communication systems [Moore et al., 1987].
 
      The lander was about 1.5 m across.  Clearance between the
      spacecraft body and the surface was about 22 cm.  After landing
      the spacecraft weighted about 610 kg.  Power for the spacecraft
      was generated by the two RTGs in which plutonium-238 produced
      about 70 watts of electric power and also charged a series of
      batteries.  Excess heat from the RTGs was used to heat the
      instruments and control systems in the spacecraft body [Soffen,
      1977].
 
 
    Science Packages
    ================
      Lander science packages included eight investigations.  The
      imaging system included two identical facsimile cameras.  The
      meteorology package was mounted on a mast and had instruments
      to measure atmospheric temperature, pressure, and wind speed
      and direction.  Each lander had a three-axis, short-period
      seismometer to measure Mars seismic activity.  The seismometer
      on VL1 failed to deploy and no data was returned from it, while
      the one on VL2 operated as planned.  A X-ray fluorescence
      spectrometer (XRFS) measured the inorganic elemental
      composition of soils at the landing sites.  A gas chromatograph
      mass spectrometer (GCMS) measured the composition of the
      atmosphere and searched for organic compounds in the soils.
      The biology investigation consisted of three experiments to
      search for biological metabolism, growth, or photosynthesis:
      carbon assimilation, labeled release of carbon-14, and gas
      exchange.  The physical properties investigation used
      information from many lander operations, such as sampling
      activities, digging trenches, pushing rocks, forming soil
      piles, and footpad penetration during landing to characterize
      the properties of rocks and soils at the landing sites.  The
      magnetic properties experiment was the only Viking experiment
      to directly study the mineralogy of soils.  This experiment
      consisted of magnets mounted on the surface sampler collector
      head and on the lander deck to collect magnetic minerals.
      Finally, lander communication systems were used for radio
      science experiments [Snyder and Moroz, 1992].