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Lunar Surface Magnetometer

NSSDCA ID: 1969-099C-04

Mission Name: Apollo 12 Lunar Module / ALSEP
Principal Investigator:Dr. Charles P. Sonett

Description

The Lunar Surface Magnetometer (LSM) consisted of a triaxial fluxgate magnetometer and was part of a three-station network (Apollo 12,15,16) designed to measure the magnitude and temporal variations of the lunar surface magnetic field to yield information on the internal electromagnetic characteristics of the Moon, including the lunar gross electrical diffusivity and the existence of a molten core. It was also intended to help elucidate the interaction between the solar plasma and the lunar surface, the behavior of the Earth's magnetic tail, and the nature of local magnetic anomalies.

Instrument Description

The instrument consisted of three fluxgate magnetometers (designated x-axis, y-axis, and z-axis sensors) mounted on orthogonal 100-cm booms, protruding from the top of a base box with support legs. The booms extend from the box at an angle of 35 degrees with the horizontal, with the sensors about 70-75 cm above the lunar surface and approximately 150 cm from each other. The sensors were mounted on gimbals allowing their measurement axes to be flipped for purposes of calibration and site survey. The magnetometers were capable of measuring magnetic fields in the three ranges of plus to minus 100, 200, or 400 gammas as selected by ground command, with a resolution of 0.2 gamma. Frequency response was from 0 (d.c.) to 3 Hz. Sensor orientation was initially determined by the astronauts' using a bubble level and a shadowgraph, and has been subsequently monitored (with an accuracy of 0.2 degrees) by gravity-level sensors. The package was set up with the z-axis sensor pointing east and the x-axis sensor pointing towards the northwest.

Each sensor consisted of a flattened toroidal magnetic permalloy core drive to saturation by a sinusoidal current of 6000 Hz passing through a drive winding wrapped around the core. Also wrapped around the core were sense windings and feedback windings. The sense windings measure the superposition of the drive winding magnetic field and the total local lunar surface field, a second harmonic of the driving frequency is generated in the sense winding with a magnitude proportional to the strength of the surface field. The phase of the second harmonic signal with respect to the drive signal indicates the direction of the surface field with respect to the sensor axis.

The output signal was amplified and synchronously demodulated to drive a voltage to an analog-to-digital converter and then transmitted to Earth. Magnetometer electronics were self-contained, in the base box below the booms. Thermal control was achieved by multi-layer insulation blankets, control surfaces, parabolic reflectors, radiators, and heaters. The package had a mass of 8.9 kg and used 3.5 W average power in the daytime and 7.5 W at night. The average data rate was 116 bits/second.

Instrument Operation

The LSM was deployed at 14:00 UT and turned on at 14:39 UT on 19 November 1969 at 3.01 S, 23.42 W. The first site-survey sequence was started at 22:50 on 22 November 1969. The instrument measured the magnetic field constantly for about three weeks after deployment, then on the dayside continuously for several months, then on the dayside very intermittently until data became static and invalid on 29 June 1970. The instrument was commanded off on 14 June 1974. The reason for the failure was thought to be due to open welds in the circuitry. Repairs and improvements in thermal control were made to the Apollo 15 and 16 LSMs to alleviate this problem.

Explorer 35

Concurrent with the LSM operation, the ambient steady-state and time-dependent magnetic fields in the lunar environment were monitored from lunar orbit by the Explorer 35 magnetometers. The spacecraft had an orbital period of 11.5 hrs., with an apolune of 9390 km and a perilune of 2570 km. More on these instruments can be found at:

https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1967-070A-03

https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1967-070A-04

Alternate Names

  • Apollo12ALSEP/LunarSurfaceMagnetometer
  • LSM
  • S034

Facts in Brief

Mass: 8.9 kg
Power (avg): 5.5 W
Bit rate (avg): 0.116 kbps

Disciplines

  • Space Physics: Magnetospheric Studies
  • Planetary Science: Fields and Particles
  • Space Physics: Heliospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Palmer DyalOther InvestigatorNASA Ames Research Center
Dr. Charles P. SonettPrincipal InvestigatorUniversity of Arizona

Selected References

  • Dyal, P., et al., Apollo 12 magnetometer. Measurement of a steady magnetic field on the surface of the moon, Science, 169, No. 3947, 762-764, Aug. 1970.
  • Dyal, P., et al., Lunar surface magnetometer, IEEE Trans. Geosci. Electron., GE-8, No. 4, 203-215, Oct. 1970.
  • Dyal, P., and C. W. Parkin, Electrical conductivity and temperature of the lunar interior from magnetic transient response measurements, J. Geophys. Res., 76, No. 25, 5947-5969, Sept. 1971.
  • Dyal, P., et al., Lunar surface magnetometer experiment (Apollo 12 prelim. sci. rept.), in Apollo 12 Prelim. Sci. Rept., NASA SP-235, Chapter 4, 55-73, Wash., D.C., 1970.
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