Lunar Laser Ranging: Technological

Advances and New Applications

By: Rubi Almanza

Lunar Laser Ranging (LLR) started in 1969, when the crew of the Apollo 11 mission placed an initial array consisting of one-hundred corner cube retroreflectors (CCR).

Starting from this initial array, more additions have been emplaced during several space missions such as the Apollo 14, the Apollo 15, Lunokhod 1, and Lunokhod 2.

Precision represents a fundamental factor for

these measurements, so far they have attained a very high precision which has allowed for their applications in diverse fields such as geophysics, gravitational physics, and lunar planetology to mention a few.

This high precision was made possible thanks to the several improvements that have been introduced to LLR stations and the advances in solar system ephemeris modeling applied to the design for lunar equipment [1].

To be more specific, the precision of the observed range measurements has increased by more than an order of magnitude.

Historical accuracy of LLR Data [5].

In an attempt to further improve this precision figure several upgrades have been proposed. Some of them have been put to the test and are producing satisfactory results.

These upgrades fall into the following categories: LLR facilities on Earth LLR equipment to place on the Moon

In the past decade LLR facilities on Earth have

experienced substantial upgrades.

In a recent study its creators, T.W. Murphy, E.G. Adelberger and J.D. Strasburg, suggested that by incorporating the latest technology in lasers and detectors to the existing design of LLR facilities, precision could be improved by more than an order of magnitude over current LLR measurements [5].

LLR facilities on Earth

The APOLLO project started operating in 2006,

and at present it reports record-breaking photon return rates and the ability to successfully perform range measurements during full moon, none of which is attainable by any other LLR station [1].

This project has also initiated a campaign to improve ranging performance [1,5].

LLR facilities on Earth

The CCR arrays placed on the Moon’s surface

have not experienced any substantial upgrade. However, their functionality has been affected because of unpredictable factors such as lunar librations and the exposure to solar rays.

This has incited the development of modern retroreflector arrays featuring superior and more advanced measuring capacities.

LLR equipment to place on the Moon

The most promising upgrades for LLR lunar

equipment proposed so far fall into the following categories:

Targeting the thermal effects of solar rays exposure

Targeting optimal optical response for CCR

LLR equipment to place on the Moon

The exposure to thermal changes caused by solar rays

is a major challenge for CCR arrays at present since they absorb the solar energy which creates a flow of heat within them.

In a recent study, Douglas Currie, Simone Dell’Agnello and Giovanni Delle Monache suggested that this problem is solved with the inclusion of specific modifications in the housing design of the current LLR array systems [3].They proposed the LLRRA-21 design.

Targeting the thermal effects of solar rays

exposure

The LLRRA-21 design incorporates prominent

modifications such as a solar blanket and a sun shade [3].

The LLRRA-21 endured satisfactorily the effects of the adverse phenomena in series of thermal simulations [3].

Unfortunately, this project has not been put to the test yet.

The LLRRA-21

The size and type of the CCRs are the primary focus

points when testing and searching for optimal optical response [6].

In a recent study Toshimichi Otsubo et al. suggested that an excellent optical performance is achieved with large CCRs regardless of their type [6]. By a series of optical simulations they also suggest that one large CCR produces a return signal as strong as the one produced by the one-hundred CCRs currently in place [6].

Targeting optimal optical response for CCR

In all the years of LLR operation, the data collected

has been utilized in applications in diverse fields of study.

One of the most prominent of these applications is the supply of various tests of General Relativity [2-5]. Another promising application of LLR data in this field has arisen with the advances in the dynamics of cosmological models caused by the unfamiliar phenomena known as dark matter [4].

Innovative Applications for LLR

In a recent study, Yu V Dumin suggests that the local

Hubble expansion constant may be measured by interpreting the discrepancy ((dR/dt)) between the predicted rate of increase in the lunar orbit ((dR/dt)(tel)) and the LLR measurements of the Earth-Moon distance ((dR/dt)(LLR)) as a phenomenon attributed to the presence of the Hubble expansion [4].

He has tested his hypothesis only a few times because the test material is very inaccessible and rare.

Innovative Applications for LLR

As adverse phenomenon such as solar rays and lunar librations

threaten to impair the performance of the LLR arrays the design of more advanced equipment that resist the effects of such phenomenon is necessary. Current research on the field has targeted the effects of solar rays mainly with very satisfactory results. However, little to no significant work has been done to target the effects of lunar librations. The design of emplacement methods to take this equipment to the Moon’s surface is also needed.

A new application advocating the measurement of the local Hubble expansion has been proposed [4]. However, further research is needed to validate this theory because of the inaccessibility of study material. To do this additional samples of the FRW metric producing matter need to be examined.

Conclusions

[1] Battat, J. B. R., Murphy T. W., Adelberger, E. G., Gillespie B., et al. The

Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range. Publications of the Astronomical Society of the Pacific. 2009. 121/875: 29-40.

[2] Chapront, Jean, and François Mignard. The lunar laser ranging: operation and science. C.R. Acad. Sci. Paris. 2000. Volume IV:1233-1243.

[3] Currie, Douglas, Dell’Agnello, S., Delle Monache, G., et al. A Lunar Laser Ranging Retroreflector Array for the 21st Century. Acta Astronautica. 2011. 68:667-680.

[4] Dumin, Yu V. A new application of the lunar laser retroreflectors: searching for the “local” Hubble expansion. Advanced Space Research. 2003. 31/11: 2461-2466.

[5] Murphy, T.W., Adelberger, E. G., Strasburg, J. D., Stubbs, C. W., Nordtvedt, K. et al. Testing Gravity via Next-Generation Lunar Laser-Ranging. Nuclear Physics B (Proceeding Supplements). 2004. 134: 155-162.

[6] Otsubo, Toshimichi, Kunimori, H., Noda, H., Hanada H. et al. Simulation of optical response of retroreflectors for future lunar laser ranging. Advances in Space Research. 2010. 45:733-740.

References

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