Lunar Laser Ranging

When the Apollo 11 astronauts lifted off from the Moon on July 21, 1969, they left behind a small package of basic experiments comprising a device for the LLR experiment, which is basically an angled tray containing a hundred 1.5 in (3.8 cm) corner-cube quartz prisms. A corner-cube prism, also known as a retroreflector, is cut in such a way that a light beam shone into it will be reflected in a series of right angles and then back out straight along the path it came in on. In the LLR a powerful laser pulse is addressed to the moon: the laser pulse flies from the transmitter on the Earth to the Moon and back and the distance is measured simply by measuring time of flight of a laser pulse and multiplying it by the half of the light speed (the light makes the path back and forth).

A clever and successful method that became well known by NASA’s Apollo program: actually, there are 3 retroreflectors on the moon left behind by Apollo 11, 14 and 15 missions… and sometimes this direct measurement is cited as proof that the human being was on the moon.

By the way it should be remarked that the very first LLR experiment was performed in 1962, i.e. soon after the invention and the first operation of a laser in 1960 and several years before the first human step on the moon!

The first LLR measurement, named Lunasee, was made by Louis Dijour Smullin and Giorgio Fiocco at the Lincoln Laboratory’s telescope in Lexington in May 1962, firing with a 50-joule ruby laser (the most powerful ever built at that time) to the moon and receiving “echoes” with a 48-inch telescope. After the first night, the New York Times wrote on May 10, 1962: “Last night, for the first time, man illuminated another celestial body”.

The 3 LLR experiments with retroreflectors on the moon are still operative but deep analyses on the registered data and comparisons with theoretical simulations lead to very different opinions ranging from  “these retroreflector arrays … are still operating normally after 25 years” (Dickey, 1994) to “possible degradation of lunar reflectors” (Murphy, 2007). Someone, according to the number of return photons, goes even further concluding “that in all lunar laser ranging experiments the measurements were taken to the bare surface of the Moon” (M?rki, 2018).

By the way, the very first LLR station which measured directly onto the surface of the Moon in 1962 presented the best consistent data. Having had the honor and the great pleasure of having worked (learned) with Giorgio Fiocco I want to remember this great achievements.io