Instrument Host Overview
  ========================
    The Apollo 15 subsatellite (PFS-1) was a small spacecraft released into
    lunar orbit from the Apollo 15 Service Module.  Its main objectives
    were to study the plasma, particle, and magnetic field environment of
    the Moon and map the lunar gravity field.  Specifically it measured
    plasma and energetic particle intensities and vector magnetic fields,
    and facilitated tracking of the satellite velocity to high precision.
    A basic requirement was that the satellite acquire fields and particle
    data everywhere on the orbit around the Moon.  A virtually identical
    subsatellite was also deployed by Apollo 16.  The Moon's roughly
    circular orbit around the Earth at about 380000 km (60 Earth radii)
    carried the subsatellite into both interplanetary space and various
    regions of the Earth's magnetosphere.  The satellite orbited the Moon
    and returned data from 4 August 1971 until January 1973.
 
    The subsatellite spacecraft was deployed on 4 August 1971 at 21:00:31
    UT by launching it from the scientific instrument module of the Service
    Module at a relative velocity of approximately 1.2 m/s using a spring
    loaded device which also imparted a spin of 140 rpm to the satellite.
    After release, the booms were deployed, lowering the spin rate to 12
    rpm.  The spin axis was normal to the ecliptic plane within 1 degree.
    The orbital period was approximately 120 minutes, clockwise as viewed
    from the north.  The perilune of the first orbit was 102 km, the
    apolune 139 km, and the inclination 28.5 degrees with respect to the
    Moon's equator, but the orbit was rapidly altered by gravitational
    perturbations.  The geocentric ecliptic longitude of the Moon at the
    time of launch was 155 degrees.  The subsatellite provided about 6
    months of data coverage before two successive electronic failures in
    February 1972 caused the loss of most of the data channels.  The
    surviving data channels were monitored intermittently until June 1972
    and then more or less continuously until late January 1973, when ground
    support was terminated.  It is assumed the subsatellite orbit decayed
    and it impacted the Moon sometime after this, the impact site is
    unknown.
 
 
    Spacecraft and Subsystems
    -------------------------
      The Apollo 15 subsatellite spacecraft was a hexagonal cylinder 78 cm
      in length and approximately 36 cm across opposite corners of the
      hexagon with a mass of 36.3 kg.  Three equally-spaced 1.5-meter-long
      deployable booms were hinged to one of the end platforms.  A fluxgate
      magnetometer was on the end of one boom and the other two carried tip
      masses to provide balance.  A short cylinder was attached to the end
      platform opposite the booms and was used for the initial deployment
      and spin-up of the satellite, which was achieved using a spring
      loaded mechanism in the SM bay.  A wobble damper inside the satellite
      removed precessional and nutational motions.  An S-band antenna
      protruded from the panel opposite the booms.  Solar panels covering
      the six sides provided about 24 W of energy in sunlight and an
      average power of 14 W over one orbit of the Moon.  The power
      subsystem also included a battery pack of 11 silver cadmium cells. An
      S-band transmitter was capable of sending 128 bits/s to the Earth. A
      magnetic core memory unit provided a storage capacity of 49,152 bits
      when the spacecraft could not transmit directly.  Two solid state
      particle telescopes were mounted on an end panel of the spacecraft
      and four particle analyzer devices were attached to the sides.
 
 
    Scientific Experiments
    ----------------------
      The following scientific experiments were performed on board or at
      the Apollo 15 subsatellite spacecraft:
 
      - The Lunar Plasma Shadows and Boundary Layer Experiment measured the
        density and energy of electrons and protons near the Moon.
 
      - The Biaxial Fluxgate Magnetometer measured the strength and
        orientation of the magnetic field near the Moon.
 
      - The S-Band Transponder allowed accurate tracking of the subsatellite
        spacecraft from Earth for mapping the gravitational field of the
        Moon.
 
    For more information about the subsatellite spacecraft and its
    experiments, see the Apollo 15 preliminary science report (1972)
    [APOLLO15A1972].
 
 
  This instrument host description was provided by the NASA National Space
  Science Data Center (NSSDC).