Instrument Host Overview
  ========================
    The Apollo 12 Lunar Surface Experiments Package (ALSEP) was carried on
    the Apollo 12 lunar module spacecraft.  It contained scientific
    experiments to be deployed and left on the lunar surface, and other
    scientific and sample collection apparatus.  The ALSEP central station
    located at 3.00942 degrees South latitude, 23.42458 degrees West
    longitude [DAVIES&COLVIN2000], was turned on at 14:21 UT on 19 November
    1969 and shut down along with the other ALSEP stations on 30 September
    1977.  For more information, see the ALSEP termination report by Bates,
    et al. (1979) [BATES1979].
 
    ALSEP consisted of a set of scientific instruments emplaced near the
    Apollo 12 landing site by the astronauts.  The instruments were arrayed
    around a central station which supplied power to run the instruments
    and communications so data collected by the experiments could be
    relayed to Earth.  The central station was a 25 kg box with a stowed
    volume of 34,800 cubic cm.  Thermal control was achieved by passive
    elements (insulation, reflectors, thermal coatings) as well as power
    dissipation resistors and heaters.  Communications with Earth were
    achieved through a 58 cm long, 3.8 cm diameter modified axial-helical
    antenna mounted on top of the central station and pointed towards Earth
    by the astronauts. Transmitters, receivers, data processors and
    multiplexers were housed within the central station.  Data collected
    from the instruments were converted into a telemetry format and
    transmitted to Earth.  The ALSEP system and instruments were controlled
    by commands from Earth.  The uplink frequency for all Apollo mission
    ALSEP's was 2119 MHz, the downlink frequency for the Apollo 12 ALSEP
    was 2278.5 MHz.
 
    A radioisotope thermoelectric generator (RTG, SNAP-27 model) the power
    to run the ALSEP operations. The generator consisted of a 46 cm high
    central cylinder and eight radiating rectangular fins with a total
    tip-to-tip diameter of 40 cm. The central cylinder had a thinner
    concentric inner cylinder inside, and the two cylinders were attached
    along their surfaces by 442 spring-loaded lead-telluride thermoelectric
    couples mounted radially along the length of the cylinders. The
    generator assembly had a total mass of 17 kg. The power source was an
    approximately 4 kg fuel capsule in the shape of a long rod which
    contained plutonium-238 and was placed in the inner cylinder of the RTG
    by the astronauts on deployment. Plutonium-238 decays with a half-life
    of 89.6 years and produces heat. This heat would conduct from the inner
    cylinder to the outer via the thermocouples which would convert the
    heat directly to electrical power. Excess heat on the outer cylinder
    would be radiated to space by the fins. The RTG produced approximately
    70 W DC at 16 V. (63.5 W after one year.) The electricity was routed
    through a cable to a power conditioning unit and a power distribution
    unit in the central station to supply the correct voltage and power to
    each instrument.
 
    ALSEP Scientific Instruments
    ----------------------------
      All ALSEP instruments were deployed on the surface by the astronauts
      and attached to the central station by cables.  The Apollo 12 ALSEP
      instruments consisted of: (1) a passive seismometer, designed to
      measure seismic activity and physical properties of the lunar crust
      and interior; (2) a suprathermal ion detector, designed to measure
      the flux, composition, energy, and velocity of low-energy positive
      ions; (3) a cold cathode ion gauge, designed to measure the
      atmosphere and any variations with time or solar activity such
      atmosphere may have; (4) a lunar dust detector, to measure dust
      accumulation, radiation damage to solar cells, and reflected infrared
      energy and temperatures; (5) a lunar surface magnetometer (LSM),
      designed to measure the magnetic field at the lunar surface; and (6)
      a solar wind spectrometer, which measured the fluxes and spectra of
      the electrons and protons that emanate from the Sun and reach the
      lunar surface.  See the Apollo 12 preliminary science report (1970)
      [APOLLO12A1970] for more information about the ALSEP experiments.
 
  This instrument host description was provided by the NSSDC.