The continued importance of lunar laser ranging

Again, as mentioned many times here, news of the laser range experiment at MacDonald Observatory at Fort Davis, Texas shutting its lunar laser ranging experiment down, after nearly forty years of operation, has led some to believe the longest running (and only operational) experiment left on the lunar surface during the Apollo Era has come to an end.

The University of Texas announced the end of the laser ranging program at Fort Davis more than a month ago after the National Science Foundation turned down further funding requests. Not far away, however, as the world most famous laser range experiment was coming to an end, it was just getting started, and with better equipment and a larger telescope at Apache Point, under the supervision of New Mexico State University.

As reported briefly among Physics Today's news picks, Monday afternoon, lunar laser ranging is "Still going after 40 years." The Apache Point "APOLLO" station is well worth a browse, to contemplate the photon by photon count of reflected laser light originating in New Mexico as a pencil-thin beam that spreads to a kilometer in width by the time it reaches the reflectors left by the crews of Apollos 11, 14 and 15, and the French-built reflector on-board the Soviet unmanned expedition Luna 21.

The National Lunar Science Institute has picked up on the importance of resolving the distance to the Moon to within 3 millimeters, which may ultimately be accomplished at Apache Point but was outside the limits of the instruments devoted to LLRR observations at Fort Davis.

It does not reflect badly, so to speak, of the history racked up in this area at MacDonald, however. The distance to the Moon was determined within 30 centimeters at Fort Davis almost immediately after Apollo 11 returned to Earth, in late 1969. Several proposals have surfaced in recent years, though, theorizing on the possible importance of pegging the lunar distance down to a half millimeter. Establishing with this accuracy the distance between the Moon and Earth and the shared barycenter of their mutual center of gravity would add to the context of observations of the Cosmos at greater distances, eliminating from contention or confirming various grand "theories of everything."

According to a preview of his discussion to be held at the NSLI's 2009 forum, led by Dr. Jack O. Burns of the University of Colorado at Boulder, "LUNAR is one of seven teams recently funded by the NLSI, dedicated to the study of Astrophysics from the Moon and headquartered at the University of Colorado."

"In this talk," Burns writes, "I will describe the research and E/PO efforts of our team. LUNAR has defined four key projects: Gravitational Physics and Lunar Structure using Lunar Laser Ranging, Low Frequency Radio Heliophysics, Low Frequency Cosmology, and the Assessment of Other Potential Astrophysics from the Moon."

"The Moon is a unique platform for fundamental astrophysical measurements of gravitation, the Sun, and the Universe. Lunar laser ranging of the Earth-Moon distance provides extremely high precision constraints on General Relativity (GR) and alternative models of gravity. Current alternate theories for gravity, including those that explain dark matter and dark energy, predict deviations from GR at a level that is potentially within the grasp of the next generation of lunar laser retroreflectors. Lacking a permanent ionosphere and, on the lunar farside, shielded from terrestrial radio emissions, a low frequency (<100>

I will describe both the science and the technology of these new astrophysical observatories for the lunar surface along with an ambitious program of Education and Public Outreach that involves new planetarium shows, teacher workshops, undergraduate classes and interdisciplinary graduate seminars.

MacDonald LLR defunded by NSF

"After 40 years' reflection," the 0.8 meter laser ranging telescope at MacDonald Observatory in west Texas has lost its National Science Foundation (NSF) funding.

The NSF has notified MacDonald $125,000 in annual funding had been cut after an annual review of the scientific merits of its lunar laser ranger and other projects.

The famous facility near Fort Davis is home to a variety of large instruments. The laser ranging telescope was surpassed recently in accuracy by "APOLLO," the Apache Point Observatory Lunar Laser-ranging Operation, a modern project using lunar laser ranging (LLR) with a 3.5 meter telescope at Apache Point, New Mexico.

The laser reflectors arrays, left on the Moon by Apollo 11, 14 and the largest set down by Apollo 15 were each first detected at MacDonald, measuring the distance between Earth and Moon with a thin laser about a kilometer wide when it reaches the lunar surface.

The three reflector arrays absorb a small sample of that light and reflect it back to earth in amounts gathered up and counted in photons per hour.

The Soviet Union also attached smaller reflectors on two rovers, one which has never been detected and another that is perched on Luna 21. Though small, it is regularly detected by laser ranging station on Earth. Together, the four sets of arrays on the Moon continue to provide enough science to inspire designs for future reflectors.

Using MacDonald LLR telescope, the distance to the Moon has been measured within three inches, enough to determine the Moon is presently pulling away from Earth at a rate of a few inches per century. At Apache Point, that horizon has been brought down to within one inch, "sort of," anyway, according to their website.

The three Apollo laser reflector arrays are the only remaining active experiments from the Apollo field expeditions to the lunar surface between 1969 and 1972. The same should also be credited to the Soviet unmanned sampling and survey program.

If the MacDonald 0.6 lunar laser ranging is soon ended, these experiments using the arrays left on the Moon will continue to be monitored.

Close is no longer good enough.

Finer monitoring of the distance between Earth and Moon took on greater significance after it was proposed the measurements, even over even such a relatively small distance, might constrain the range of possible answers to great cosmic questions.

Apache Point, upgraded by 2005, scientists have improved the accuracy (and photon count) of measures of the Earth-Moon distance with an ultimate goal being within centimenters, a threshold some believe will prove or disprove certain theories of the cosmos.

According to those at Apache Point, "Einstein's Equivalence Principle, upon which General Relativity rests, claims all forms of mass-energy experience the same acceleration in response to an external gravitational force." The inertial mass and gravitational mass are equal for all forms of mass and energy.

"This is very difficult to verify for gravitational energy itself," they write, "because laboratory masses have no appreciable gravitational binding energy." They need masses with gravity they can detect.

"One needs bodies as large as Earth to have any measurable self-energy content. Even then, the self-energy contribution to Earth's total mass-energy is less than one part-per-billion." The contribution of Earth to the inertia of the Sun plus Earth is hard to detect.

"If Earth's gravitational self-energy does not precisely obey this Equivalence Principle, the orbits of Earth and the Moon, around the sun, would be slightly displaced from one another, a modification of Kepler's Third Law) which would show up as a signal in our lunar range data."

"Various Cosmic String-theories, Quintessence, and other alternatives to General Relativity almost all predict a violation of the Equivalence Principle at some level. Recent hints there may be some new and mysterious modification to the laws of large-scale gravitational attraction, indicated by supernovae and the unequal distribution of the cosmic background microwave radiation, make it important to probe every available aspect of the basic nature of gravity."

"Lunar Laser Ranging also provides the best test of the stability of Issac Newton's gravitational constant, G, at this time limited to a variation of less than one part in 10¹² each year."

"Relativistic geodetic precession is also best probed, currently by the more sensitive LLR installed in 2005, and verified at Apache Point t0 a 0.35% level of precision."

"The list goes on. Lunar Laser Ranging also provides the best test of the motional influence on gravitational attraction (called gravitomagnetism) to a 0.1% level of precision, and also sets the most stringent limits on deviations from the expected 1/r² law of gravity."