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Lunar Traverse Gravimeter

NSSDC ID: 1972-096C-03
Mission Name: Apollo 17 Lunar Module /ALSEP
Principal Investigator: Dr. Manik Talwani

Description

The primary objective of the traverse gravimeter experiment (S-199) was to make a high-accuracy relative survey of the lunar gravitational field in the Apollo 17 landing area and to use these measurements to obtain information about the geological substructure. A secondary goal was to make an Earth-Moon gravity tie. Specific objectives were to measure the value of gravity, relative to the value at a lunar base station, at selected known locations along the lunar traverse and to measure the value of gravity at a known point on the lunar surface (base station), relative to the gravity at a known point on earth.

The Traverse Gravimeter instrument package was rectangular in shape with a cylindrical surface at the front. It was approximately 51 cm high, 28 cm wide, and 25 cm deep, with a mass of 12.7 kg. The package was insulated by a multilayer blanket for thermal protection. A folding handle at the top of the instrument was used for hand carrying and for securing the instrument to the Lunar Roving Vehicle. It had three footpads at the base for surface operations. On top of the case was a nine digit display. A radiator for thermal control was also located at the top of the case. Inside the case was a 7.5 volt, 375 watt-hour battery pack and a two-axis gimbal system which contained the gravity sensor housed in a thermally protected and evacuated two-stage oven assembly, comprising an intermediate and a precision oven. The oven assembly is enclosed in an electronic frame and ceramic insulating ball supported by the gimbal assembly, which is attached through bearings to the housing. Pendulums act as level sensors, supplying signals to stepper motors and a gear train which position the gimbal assembly so that the gravity sensor is within 3 minutes of arc of vertical. The ovens maintain a constant temperature (within 0.01 K) near 322 K using electric heaters and a resistance thermometer.

The gravity sensor was a Bosch Arma D4E vibrating string accelerometer. Each of the two strings is energized and generates continuous vibrations with a frequency dependant on the local gravity. The difference between the frequencies is a simple function of gravity. The apparatus is a cylinder with ring magnets at either end. At the center of the cylinder is a soft spring attached to cyindrical masses above and below, the masses are attached to the insides of the cylinder by cross supports. At the center of each mass the electrically conducting vibrating strings are attached, passing through the ring magnets before extending out each end of the cylinder. When a voltage is applied across the string, the resulting current, running through the ring magnetic field, causes motion in the string and induces a voltage. The voltage is regenerated through a stable high-gain amplifier and fed back to the string. The signal is fed to a phase-lock loop module to determine the difference frequency. The difference frequency is measured using a precision 125-kHz clock.

The Traverse Gravimeter could be operated mounted to the back of the Lunar Roving Vehicle or placed directly on the surface. The vertical axis of the package had to be within 15 degrees of vertical to make a measurement, and required about 3 minutes of undisturbed operation. For a normal measurement, the instrument would level itself and then measure the difference frequency of the strings. The number of counts of the precision clock, from which the frequency can be determined, would be given as the first seven digits on the display. The eighth and ninth digit were for thermal monitoring. For calibration, the gravimeter could be inverted and a "bias" measurement would be made.

Facts in Brief

Mass: 12.7 kg

Funding Agency

  • NASA-Office of Applications (United States)

Discipline

  • Planetary Science: Geology and Geophysics

Additional Information

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

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Manik TalwaniPrincipal InvestigatorLamont-Doherty Geological Observatorymanik@rice.edu

Selected References

Talwani, M., et al., Traverse gravimeter experiment, In -- Apollo 17 Prelim. Sci. Rept., NASA SP-330, Wash., DC, 1973.

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