Apollo 17 Lunar Surface Experiments Package (ALSEP)


Instrument Host Overview
========================
The Apollo Lunar Surface Experiments Package (ALSEP) was carried
on the Apollo 17 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 20.1921 North latitude, 30.7649 East longitude
(Davies and Colvin, 2000), was turned on at 02:53 UT on 12 December 1972
and shut down along with the other ALSEP stations on 30 September
1977.  For more information, see the ALSEP termination report (Bates 
et al., 1979).

ALSEP consisted of a set of scientific instruments emplaced near the
Apollo 17 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 17 ALSEP
was 2275.5 MHz.

A radioisotope thermoelectric generator (RTG, SNAP-27 model) produced
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 17 ALSEP
  instruments consisted of: (1) a heat flow experiment, designed to
  measure the rate of heat loss from the lunar interior and the thermal
  properties of lunar material; (2) a lunar surface gravimeter,
  designed to measure the lunar surface gravity and its temporal
  variations at a selected point on the surface; (3) a lunar mass
  spectrometer, designed to measure the composition of the tenuous
  lunar atmosphere; (4) a lunar seismic profiling experiment, to study
  the physical properties of lunar surface and subsurface materials and
  the structure of the local near-surface layers; and (5) a lunar
  ejecta and meteorites experiment, designed to measure the speed,
  direction, energy, and momentum of cosmic dust particles and lunar
  ejecta. The central station, located at 20.1921 North latitude,
  30.7649 East longitude, was turned on at 02:53 UT on 12 December 1972
  and shut down along with the other ALSEP stations on 30 September
  1977. See the Apollo 17 Preliminary Science Report (1973)
  for more information about the ALSEP experiments.


References
==========
Apollo 17 Preliminary Science Report, NASA SP-330, published by NASA,
Washington, D.C., 1973.

Bates, J.R., W.W. Lauderdale, and H. Kernaghan, ALSEP Termination Report,
NASA Reference Publication Series, NASA RF-1036, published by NASA Johnson
Space Center, Houston, Texas, Apr. 1979.

Davies, M.E., and T.R. Colvin, Lunar coordinates in the regions of the
Apollo landers, Journal of Geophysical Research, Volume 105, Issue E8,
pages 20,227-20,280, 2000.