Organization:Lunar Surface Gravimeter
 The Apollo 17 Lunar Surface Gravimeter on the Moon | |
| Acronym | LSG |
|---|---|
| Notable experiments | Apollo 17 |
The Lunar Surface Gravimeter (LSG) was a lunar science experiment that was hoped to find evidence for gravitational waves and to measure the viscoelastic tidal deformation of the Moon by the Earth and Sun. The experiment, which did not work as planned, was deployed on the surface of the Moon by the astronauts of Apollo 17 on December 12, 1972.
Background
The LSG's primary goal was to facilitate the detection of gravitational waves and verify the results from an Earth-based instrument. At the time of the Apollo program, it was believed only the largest objects (in size rather than mass) in the universe such as stars and galaxies would generate gravitational waves of sufficient magnitude to be detectable.[1] The instrument was built based on a prediction that the deformation and elastic response of a planetary scale object to gravitational waves passing through it and any harmonic responses induced would be detectable by a gravimeter.[1] Whilst the Earth could operate as an "antenna", it is a seismically and geomorphically active body resulting in an extremely "noisy" environment.[1] The principal investigator of the LSG experiment invented the Weber bar type of gravity wave instrument, of which two were in operation during the time of deployment by Apollo 17. It was intended that the LSG instrument would work in conjunction with the Weber bar instruments on Earth to better categorise apparent observations seen by them.[1] The instrument would in effect turn the Moon into a gravitational wave antenna.[2]
Instrument
Description
The instrument is a gravimeter that primarily consists of an adjustable mass on a sprung lever attached to the instrument's measurement electronics.[1] The gravimeter was designed to measure seismic responses in a range between 0 and 16 Hz.[1] The mass was adjustable through the addition or removal of weights which would allow the experiment to both be tested in Earth gravity to prove out its functionality, and also be operated in lunar gravity without modification to the device.[1] The measurement electronics were primarily driven by a trio of capacitor plates. Two plates were affixed to the experiment's frame in parallel with a third plate between them attached to the sprung lever.[1] To regulate the instrument temperature a static sunshield was attached to reduce excess heat buildup, an internal heater prevented extreme cold, a hole on the top of the instrument allowed the radiation of excess heat into space and insulation on the bottom of the instrument prevented the transfer of heat from the lunar surface.[3] The experiment required its operational temperature to be maintained at 323 K (50 °C; 122 °F).[3]
Operations
The experiment was deployed on the Moon's surface by Apollo 17 astronauts Gene Cernan and Harrison Schmitt on December 12, 1972. Following deployment of the LSG, it was discovered that the instrument's calibration weights had been manufactured using an incorrect value for the Moon's gravity meaning they were not heavy enough to be able to provide the necessary adjustments to the instrument's sensor. Despite efforts by Apollo 17 astronauts, the LSG's capability to measure gravitational waves was rendered inoperable. Schmitt subsequently revealed that Joseph Weber's team had refused to test this device in a way that would emulate lunar gravity, in order to protect the proprietary nature of the instrument.[4]
Adjustments to the sensor meant that it could be used as a low fidelity seismograph which is how it operated for the remainder of its operational life.[1] The instrument experienced large deviations from its desired operational temperatures in both March 1974 and July 1975.[5] In both instances, the instrument returned to its operational temperature and continued to return data.[5]
Science
Due to the instrument's failure and because it was only able to operate as a low-fidelity seismometer, the instrument did not manage to identify any gravitational wave–induced oscillations in the Moon nor did it correlate any observations made by the detectors on Earth.[5][6] Weber's team did not transfer any of the measurements taken between 1972 and 1976, and no papers were released by his research group although one PhD was completed by Russell Tobias based on data from this experiment. In the instrument's final report, it was stated that the computers used for analysing the experiment's data were taken back by NASA,[1] although this applied to many lunar science experiments.
A 2015 investigation explored the LSG's value as a surface gravimeter. A reevaluation of the data found that the instrument detected deep and shallow moonquakes, and impact events. The investigation suggested that combining the LSG and Apollo seismograph data might increase the resolution and understanding of the Moon's internal structure.[7]
Future experiments
Whilst the experiment failed in its objectives, a conceptually similar experiment based on the theorised responses of a planetary body to gravitational waves has been proposed, known as the Lunar Gravitational-wave Antenna.[8]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Giganti, John J.; Larson, J.V.; Richard, J.V.; Tobias, R.L.; Weber, J (January 1977). "Lunar Surface Gravimeter Experiment - Final Report to the National Aeronautics and Space Administration". https://ntrs.nasa.gov/api/citations/19770012037/downloads/19770012037.pdf.
- ↑ "Apollo 17 Preliminary Science Report". https://history.nasa.gov/alsj/a17/a17psr.html.
- ↑ 3.0 3.1 "NASA - NSSDCA - Experiment - Details". https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1972-096C-09.
- ↑ "Chapter 4 – 30 Days and Counting" (in en-US). 2017-11-06. https://www.americasuncommonsense.com/1-apollo-17-diary-of-the-12th-man/b-chapters-1-9/5-chapter-4-30-days-and-counting/.
- ↑ 5.0 5.1 5.2 Bates, J.; Lauderdale, W. W.; Kernaghan, H. (1979-04-01). "ALSEP termination report.". https://nssdc.gsfc.nasa.gov/misc/documents/b32116.pdf.
- ↑ Tobias, Russell Lawrence (1978). The lunar surface gravimeter and the search for gravitational radiation (Thesis). OCLC 18232780. ProQuest 302901798.[page needed]
- ↑ Kawamura, Taichi; Kobayashi, Naoki; Tanaka, Satoshi; Lognonné, Philippe (2015). "Lunar Surface Gravimeter as a lunar seismometer: Investigation of a new source of seismic information on the Moon" (in en). Journal of Geophysical Research: Planets 120 (2): 343–358. doi:10.1002/2014JE004724. ISSN 2169-9097. Bibcode: 2015JGRE..120..343K. https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2014JE004724.
- ↑ Harms, Jan et al. (1 March 2021). "Lunar Gravitational-wave Antenna". The Astrophysical Journal 910 (1): 1. doi:10.3847/1538-4357/abe5a7. Bibcode: 2021ApJ...910....1H.
External links
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