Apollo 16 Lunar Surface Experiments Package (ALSEP) Instrument Host Overview ======================== The Apollo Lunar Surface Experiments Package (ALSEP) was carried on the Apollo 16 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 8.9754 S latitude and 15.4981 E longitude (Davies and Colvin, 2000), was turned on at 19:38 UT on 21 April 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 at the landing site by the astronauts. The instruments were arrayed around a central station and connected by cables 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 16 ALSEP was 2276.0 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 16 ALSEP instruments consisted of: (1) a passive seismometer, designed to measure seismic activity and physical properties of the lunar crust and interior; (2) an active seismometer to study the physical properties of lunar surface and subsurface materials and the structure of the local near-surface layers; (3) a lunar surface magnetometer (LSM), designed to measure the magnetic field at the lunar surface; and (4) a heat flow experiment, designed to measure the rate of heat loss from the lunar interior and the thermal properties of lunar material. However during ALSEP set up one of the astronauts tripped on the cable to the heat flow experiment and broke it, rendering the instrument inoperable. The central station, located at 8.9754 S latitude, 15.4981 E longitude, was turned on at 19:38 UT on 21 April 1972 and shut down along with the other ALSEP stations on 30 September 1977. References ========== Apollo 16 Preliminary Science Report, NASA SP-315, published by NASA, Washington, D.C., 1972. Apollo 16 - Expedition to Descartes (mission report), NASA MR-11, published by NASA, Washington, D.C., 1972. 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.