Lunar Surface Gravimeter

Lunar science experiment
Lunar Surface Gravimeter
The Apollo 17 Lunar Surface Gravimeter on the Moon with ALSEP central station in background.
AcronymLSG
UsesGravitational wave detection, Seismology
Notable experimentsApollo 17
InventorLucien LaCoste and Arnold Romberg
ManufacturerBendix Corporation, LaCoste Romberg, Arthur D. Little

The Lunar Surface Gravimeter (LSG) was a lunar science experiment that was deployed on the surface of the Moon by the astronauts of Apollo 17 on December 12, 1972. Conceived and led by Joseph Weber as principal investigator, the experiment aimed to measure changes in the local gravitational strength on the Moon's surface. These measurements were intended to provide insight into the internal structures of the Moon as it tidally deformed due interaction with the gravitational fields of the Earth and Sun. In addition the experiment hoped to contribute experimental evidence of the existence of gravitational waves.

The gravimeter unit that was deployed on Apollo 17 was not properly calibrated and could not be properly zeroed for use in lunar gravity. Whilst the experiment continued to be used as a one-axis seismometer, the data received back was noisy and required more modern analysis techniques before the experiments data was proven valuable. Later understanding of gravitational waves showed that even if experiment worked as intended, it would not have been sensitive enough to detect them. A conceptually similar experiment has been proposed, known as the Lunar Gravitational-wave Antenna.

Background

Gravitational waves are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves.[1] In 1916[2][3] Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.[4][5] By the time of the development 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.[6]: 1 

Following his attendance a general relativity conference at Chapel Hill in 1957, Joseph Weber started work on designing and building detectors that could experimentally prove the existence of gravitational waves. In 1960, he published his proposal for a mechanical detector that would consist of a suspended metal cylinder that would interact with gravitational waves and produce induced vibrations that could be detected.[7] Weber and other members of the scientific community were also exploring other experimental mechanisms to prove the existence of gravitational waves. In 1961, Weber theorised that gravitational waves could induce a resonant dilation in planetary bodies.[8] He suggested that Weber proposed both using that the Earth and Moon could both be leveraged to examine their interactions with gravitational waves. The work of Robert H. Dicke and Carl H. Brans supported this suggesting that changes in space-time caused by gravitational waves would result in a disturbance of the isostatic equilibrium of a planetary body as the waves propagated through it.[9] Weber presented at a National Aeronautics and Space Administration (NASA) organised conference on relativity in 1961 highlighting that using the Moon to detect gravitational ways was an attractive option due to being seismically quieter.[10] These events occurred in parallel to the early feasibility studies of the Apollo program and NASA had already identified a desire to send a gravimeter to the Moon as early as February 1960.[11]

Teams from University of California, Los Angeles, Princeton University and University of Maryland (led by Weber) would for several years attempt to measure the Earth's 0S0 mode with a gravimeter, effectively turning the entirety of the earth into a gravitational wave antenna.[9] The devices used for these explorations included various prototype La Coste-Romberg type gravimeters, that allowed extremely precise measurements.[9] These sensors proved difficult to use for detecting gravity waves due to thermal noise.[9] Despite teams trying to eliminate sources of noise in their devices[12] it was eventually found that due to the Earth being a seismically and geomorphically active body, the Earth's background noise for these type of devices could not be overcome at the time.[6]: 3  [13] Despite the setback, by 1964 plans for the development of an Apollo surface gravimetry experiment were underway leveraging the same technologies.[14] In addition to seeking evidence for gravitational waves, the gravimeter would aim to provide precise information on the tidal deformation of the Moon due to its interaction with the gravitational fields of the Earth and Sun.[14][15] Whilst La Coste gravimeters were not designed for operation in the harsher lunar environment, there was confidence in being able to adapt the device with the biggest expected challenge to overcome being the thermal sensitivity of the device.[14]

Weber had continued developing his bar detectors through the early 1960s iterating on their design and implementation to also reduce the amount of seismic and thermal noise the detector was exposed to, and by 1967 he believed his detectors were picking up signals indicative of gravitational waves.[16] In 1969 Weber published a paper formally declaring his detection of gravitational waves,[17] followed in 1970 with the claim that regular gravitational waves were being detected from the Galactic Center.[18] Whilst there was almost immediate doubt about the scientific validity of these observations,[16] it was intended that the Lunar Surface Gravimeter instrument would work in conjunction with the Weber bar instruments on Earth to better categorise the alleged observations seen by them.[6]: 3 

Experiment history

Schematic diagram of the Lunar Surface Gravimeter

Four geophysics teams for the Apollo lunar surface exploration program were proposed by Willis Foster, Director of the Office of Manned Space Science in 1964 which included one dedicated to gravimetry consisting of Joseph Weber and Gordon J. F. MacDonald who was at that time at University of California, Los Angeles.[19] Approval of the Lunar Surface Graviter came late in the planning process for Apollo 17.[19] Bendix Corporation was the prime contractor for the Apollo Lunar Surface Experiments Package and were responsible for managing the design, manufacture and testing of all experiments and their integration with the Apollo Lunar Surface Experiments Package (ALSEP) central station.[20] Each experiments instrument had separate contractors and the flight article of the LSG was manufactured by Bendix and was responsible for the flight hardware's electronics. [19][21]: 12-4  The LSG's sensor was developed and built by LaCoste Romberg, a notable producer of gravimeters; and the instruments thermal control was provided by Arthur D. Little.[21]: 12-4  Concerns were raised by Marshall Space Flight Center about whether the experiment could be delivered by July 1972. Rather than being integrated at Bendix with the rest of the ALSEP experiments, integration of the experiment was done at Kennedy Space Center.[19]

Instrument description

The instrument is a gravimeter, based on the LaCoste & Romberg D-meter,[22] that primarily consists of an adjustable mass on a sprung lever attached to the instrument's measurement electronics.[6]: 4 [22] The experiment had a total mass of 12.7 kg, a volume of 26,970 cm3, and utilised a maximum of 9.3 W of power. [23][24]: 2-9  It was capable of measuring gravity to 1 part in 105.[24]: 2-9  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.[6]: 5 [25] 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.[6]: 4  The gravimeter was designed to measure seismic responses in a range between 0 and 16 Hz.[6]: 4  The experiment required its operational temperature to be maintained at 323 K (50 °C; 122 °F) to within 1 millidegree.[23] To regulate the instrument temperature a static sunshield was attached to reduce excess heat buildup, a hole on the top of the instrument allowed the radiation of heat into space, insulation on the bottom of the instrument prevented the transfer of heat from the lunar surface and an internal electric heater prevented to device from cooling.[23][24] Power and communications with the experiment was provided via a ribbon cable connected to the ALSEP central station, through which all other active ALSEP experiments were provided with power.[23] Other experiments that flew as part of ALSEP included the Heat Flow Experiment, Lunar Ejecta and Meteorites Experiment, Lunar Atmospheric Composition Experiment, and Lunar Seismic Profiling Experiment.[21]: 2-1 

Surface operations

The experiment was deployed on the Moon's surface by Apollo 17 astronauts Gene Cernan and Harrison Schmitt on December 12, 1972.[23] Deployment consisted of levelling the instrument, deploying the sunshield and attaching the power and communications cable to the ALSEP central station.[23]

Following deployment of the LSG, it was discovered that the instrument's calibration weights were not sufficiently heavy enough to allow the sprung level to be properly balanced. Apollo 17 astronauts made several attempts to debug the issue on the Moon.[25] This included releveling the instrument, rock the instrument side to side, tapping "sharply" with increasing force and sending a variety of commands to the device.[6]: 7  Eventually it was accepted the LSG was no longer able to operate as a gravimeter. Adjustments were made to the sensor via remote commands by placing the weight mechanism in contact with the sprung level, applying a small amount of for.[25] The hope was that it could be used as a low fidelity seismograph which is how it functioned for the remainder of its operational life.[25]

The instrument experienced large deviations from its desired operational temperatures in both March 1974 and July 1975, when the instruments internal heater became stuck in a mode which kept the heater active.[24]: 2:9 – 2:10  In both instances, the heater was manually commanded to cycle the instrument returned to its operational temperature and continued to return data.[24]: 2:9 – 2:10 

Failure cause

The experiments researchers from University of Maryland explained that this was due to an arithmetic error, known by the manufacturers La Coste and Romberg, resulting in balance weights that were insufficient for use in the Moon's gravity and unable to provide the necessary adjustments to the instrument's sensor.[19][25] Apollo 17's Schmitt later claimed that Weber's team had refused to test the device on a slope in a way that would emulate lunar gravity, in order to protect the proprietary nature of the instrument.[26][27]

LaCoste & Romberg addressed the devices failure nearly 30 years later. The challenge the company described is that the weights of the masses for use on the Moon could not simply be assumed to be 1/6th the mass of those for testing on Earth. Due to the need to factor in the mechanical properties of the sprung lever and its center of gravity, no two devices were perfectly alike.[22] Each instrument would normally be assessed by comparing read outs against known gravity values at test locations, and the devices weights adjusted to account for the variations inherent to each instrument. This was done for the test article, but wasn't done for the flight article to speed up the manufacturing process to meet the tight deadlines.[22] Mass values that had been calculated for the flight-like test article were used for the weights on the actual flight hardware, and subsequently only found to be incorrect after deployment during the mission.[22]

This problem was compounded by that the fact that when modifying the D-meter design for use on the Moon, a decision was taken to reduce the extent with which fine adjustments could be made since the instrument was going to be static. The result was an instrument that had half of the fine-adjustment capability. If this change had not been made, then the wrong mass values could have been accounted for and the device operated normally.[22]

Science

Gravimetry

Due to the instrument's failure, the LSG did not manage to achieve two of its primary objectives, to identify any gravitational wave–induced oscillations in the Moon and to measure the isostatic response to tidal forces from the Sun and Earth.[24][28]: 4–5  Since the deployment of the instrument, it has become known that Weber bars were not capable of detecting gravitational waves nor was the LSG sensitive enough to detect gravitational waves.[29]

Seismography

To get the instrument to operate as a seismograph, there was a need to use the weight caging mechanism to apply a force to the sprung level in order to balance it. This had significant negative effects on the instrument. It set up substantial new harmonic resonances in the instrument, reduced its sensitivity to some frequencies and increased its sensitivity to frequencies now occupied by the devices own resonance.[6]: 7–8 

The data received from ALSEP experiments would typically be sent as tapes to the principal investigator of each experiment for analysis. It was then expected the principal investigator would forward on the tapes to be archived by the National Space Science Data Center.[25] Weber's team did not transfer any of the measurements taken between 1972 and 1976 to the National Space Science Data Center,[30] release any of the data themselves and no examination of the instruments operation as a seismometer occurred.[25] One PhD was completed by Russell Tobias based on this experiment.[28] In the instrument's final report, it was stated that the computers used for analysing the experiment's data were taken back by NASA,[6] although this applied to many lunar science experiments.[25]

Even though the data between 1972 and 1976 remains unavailable, from March 1976 there were operational changes in how data was received and processed from the ALSEP station, with all data being sent to the University of Texas. Data was stored in a raw form and required decoding for each instrument.[25] In 2015, an evaluation of this data found that the instrument detected deep and shallow moonquakes, and impact events.[25] The investigation suggested that combining the LSG and other Apollo seismograph data might increase the resolution and understanding of the Moon's internal structure.[25]

Future experiments

Whilst the LSG failed in its original objectives, the potential for the Moon as an environment conducive to the study of gravitational waves remains. The Moon in a general sense exhibits very little seismic activities and but the frequency bands that are suitable for the study of gravitational waves exhibit levels of noise orders of magnitudes lower than on Earth.[31][32][33] Areas in permanent shadow at the lunar poles are thermally stable which reduce any sensor noise cause by thermal variability.[34] As well as being thermally stable, they also happen to be some of the coldest locations in the solar system. Cool temperatures reduce sensor noise caused by the thermal motion of particles.[35] With NASA and the European Space Agency (ESA) working towards a new period of human exploration on the Moon, both agencies have sought input on science goals.[36][37] Two experiments that would study the theorised responses of a planetary body to gravitational waves were proposed in response to ESA, with the Italian Gran Sasso Science Institute proposing the Lunar Gravitational-Wave Antenna[38] and the french Astroparticle and Cosmology Laboratory proposing the Lunar Seismic and Gravitational Antenna.[39] The Lunar Gravitational-Wave Antenna would leverage an array of highly sensitive seismometers into to assess how gravitational waves would interact with the Moon.[38]

References

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External links

  • Media related to Lunar Surface Gravimeter at Wikimedia Commons


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