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.