Laser Ranging Retro-Reflector
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The Laser Ranging Retroreflector (LRRR) is the first ever deployable
lunar laser ranging experiment Lunar Laser Ranging (LLR) is the practice of measuring Lunar distance (astronomy), the distance between the surfaces of the Earth and the Moon using Lidar, laser ranging. The distance can be calculated from the Round-trip delay, round-trip time ...
. It was carried on
Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...
as part of the Early Apollo Scientific Experiments Package, and on
Apollo 14 Apollo 14 (January 31February 9, 1971) was the eighth crewed mission in the United States Apollo program, the third to Moon landing, land on the Moon, and the first to land in the Geology of the Moon#Highlands, lunar highlands. It was the las ...
and
Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the Apollo program and the fourth Moon landing. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a longer stay on the Moon and a greate ...
as part of the
Apollo Lunar Surface Experiments Package The Apollo Lunar Surface Experiments Package (ALSEP) comprised a set of scientific instruments placed by the astronauts at the landing site of each of the five Apollo program, Apollo missions to land on the Moon following Apollo 11 (Apollos Apol ...
(ALSEP). The LRRR consists of a series of corner reflectors set within a panel. Laser beams sent from Earth are bounced off the
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
and the timing of the return signal can be used to measure the distance from the signal source to the reflector. The reflector was conceived by James E. Faller in 1961. The experiment's principal investigator was initially Carroll Alley of the University of Maryland who was eventually succeeded by Faller.


Background

The motivation for a retroreflector came from a desire for a greater experimental basis for
general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
and specifically problems that arose with the
Brans–Dicke theory In physics, the Brans–Dicke theory of gravitation (sometimes called the Jordan–Brans–Dicke theory) is a competitor to Einstein's general theory of relativity. It is an example of a scalar–tensor theory, a gravitational theory in which the ...
of gravitation. A research group at Princeton University had been exploring the possibility of testing the
gravitational constant The gravitational constant is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...
by using corner reflectors carried on artificial satellites. At this time lasers had not been developed, and this approach would have required the use of
flashtube A flashtube (flashlamp) produces an electrostatic discharge with an extremely intense, incoherent, full-spectrum white light for a very short time. A flashtube is a glass tube with an electrode at each end and is filled with a gas that, when tr ...
s. With the development of the first functioning laser in 1960 at
Bell Labs Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
, this experiment was no longer restricted to being carried on
artificial satellites A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation (GPS), broadcasting, scientifi ...
, but the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, Earth's natural satellite, could also be used. The concept of using a corner reflector on the Moon came in 1961 from James E. Faller, who was a post-graduate doctoral candidate at the time. Conceived with NASA's
Surveyor Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. These points are usually on the ...
landing program in mind, his idea consisted of a corner reflector mounted within a rubber ball that could be dropped from a robotic lander; upon landing, the ball assembly would self-right and point the reflector upwards. Faller documented these ideas in a note titled "A Proposed Lunar Package (A Corner Reflector on the Moon)", but because he needed to complete his thesis, further development of the concept did not occur immediately. There were attempts in 1962 at precision ranging of the Moon using lasers without retroreflectors, most notably an attempt in 1962 by
Louis Smullin Louis Dijour Smullin (February 5, 1916 – June 4, 2009) was an American electrical engineer who spend most of his career at the Massachusetts Institute of Technology (MIT). He is best known for his work with Giorgio Fiocco to measure the dis ...
and
Giorgio Fiocco Giorgio Fiocco (13 June 1931 – 31 July 2012) was an Italian physicist, known for the development of the Lidar for the remote sensing of the atmosphere. In 1962 at MIT, together with Louis Smullin, Fiocco developed the first Lidar system, ...
from the
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...
. The Moon's surface can scatter a laser beam and produce a sufficiently strong enough signal to be detected on Earth, resulting in ranging measurements that were accurate to within . Beyond this though the effects of terrain became problematic, and when combined with a returned signal strength that is both weak and temporally dispersed, surface scattering was not sufficient for the purpose of precision ranging. After an assembly of Princeton staff and alumni at a Physical Society meeting in 1964, it was decided that an experiment based on this concept should be proposed to NASA. Plans for the experiment were laid out in a paper that was published in 1965 and the proposal to NASA was submitted later that year. This was led by Carroll Alley, a professor at the University of Maryland whose proximity to NASA's headquarters made him suited to taking on the role of the experiment's principal investigator. At the same time the Lunar Ranging Experiment (LURE) advisory committee was formed whose notable members included Robert H. Dicke, James E. Faller, David Todd Wilkinson, William M. Kaula, and Gordon J. F. MacDonald.


Instrument

The experiment needed to be built to survive the challenging environmental conditions found on the surface of the Moon. This includes large temperature variations, cosmic and
solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...
, and
lunar dust Lunar regolith is the unconsolidated material found on the surface of the Moon and in the Moon's tenuous atmosphere. Sometimes referred to as Lunar soil, Lunar soil specifically refers to the component of regolith smaller than 1 cm. It differ ...
kicked up by both the arrival and departure of the
Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed sp ...
. Faller identified that an array of small-diameter retroreflector cubes would perform better thermally than one or more larger cubes of the same mass. This thermal performance was important because fused silica, the likely material for the reflectors, optically distorts with inputs of heat from solar energy. There was a strong desire for the experiment to be able to operate during the lunar day to avoid the loss of data collection opportunities during the daylight half of each
lunar month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...
. The Apollo 11 and 14 instruments consisted of 100 solid fused-silica corner reflectors set within a square panel. The corner reflectors were produced by
PerkinElmer PerkinElmer, Inc., previously styled Perkin-Elmer, is an American global corporation that was founded in 1937 and originally focused on precision optics. Over the years it went into and out of several different businesses via acquisitions and di ...
and Boxton-Beel Inc. Design and fabrication of the array package was completed by Arthur D. Little Inc. Each reflector is in diameter sitting below the panel's top surface and mounted between Teflon rings for greater thermal protection. The panel could be set at a slight incline so that the panel could present a more optimized cross-sectional area. The reflector enables the return of 10 to 100 times more powerful signal when compared with reflecting off the lunar surface. The expected life of the experiment was in excess of 10 years.


Missions


Apollo 11

The Apollo 11 LRRR was deployed on the lunar surface by the Apollo 11 astronaut,
Buzz Aldrin Buzz Aldrin ( ; born Edwin Eugene Aldrin Jr.; January 20, 1930) is an American former astronaut, engineer and fighter pilot. He made three extravehicular activity, spacewalks as pilot of the 1966 Gemini 12 mission, and was the Lunar Module Eag ...
, on July 21, 1969. The package was deployed approximately from
Lunar Module Eagle Lunar Module ''Eagle'' (LM-5) is the spacecraft that served as the crewed lunar lander of Apollo 11, which was the first mission to land humans on the Moon. It was named after the bald eagle, which was featured prominently on the mission ins ...
. Aldrin initially aligned the face of the array in an approximate fashion so that it faced the Earth, with more precise alignment provided by a sundial. Deployment of the instrument took around 5 minutes to complete. Detection of return signals from the lunar surface was done by teams from the University of California,
Wesleyan University Wesleyan University ( ) is a Private university, private liberal arts college, liberal arts university in Middletown, Connecticut, United States. It was founded in 1831 as a Men's colleges in the United States, men's college under the Methodi ...
and
Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C., in Greenbelt, Maryland, United States. Established on May 1, 1959, as NASA's first space flight center, GSFC ...
at the
Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...
; and from the University of Texas,
University of Mary The University of Mary (UMary or simply Mary) is a private, Benedictine university near Bismarck, North Dakota. It was established in 1959 as Mary College. The university is the largest degree-granting institution in western North Dakota. It h ...
and Goddard at the
McDonald Observatory McDonald Observatory is an astronomical observatory located near unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Locke in the Davis Mountains of West Texas, with additional faci ...
. The Lick Observatory's telescope was used for the initial signal detection period immediately after the landing of Apollo 11. The McDonald Observatory's telescope was used as part of a long-term observation program over a number of years. Each site used its own Q-switched
ruby laser A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960. Ruby lasers produce pulses of ...
to provide laser pulses, tens of nanoseconds in length and with energies of approximately 7 joules per pulse. Each telescope was used to reduce the divergence of the laser beam to only 2 arc seconds. This resulted in the laser pulses reaching the Moon with a diameter of rather than a diameter of . The returned signal diameter after a complete trip from the Earth to the Moon was wide resulting in only 1.6 detectable photoelectrons per laser shot by the telescope. To avoid risks to the operators of the telescope, an electronic viewing system was used to mitigate the risks posed by backscattered laser light to the human eye. This brought additional benefits such as to increase the perceived contrast which enable the identification of features during local daytime on Earth. Concerns about possible disruption to aircraft arriving at nearby
San Francisco Bay Area The San Francisco Bay Area, commonly known as the Bay Area, is a List of regions of California, region of California surrounding and including San Francisco Bay, and anchored by the cities of Oakland, San Francisco, and San Jose, California, S ...
airports resulted in the
Federal Aviation Administration The Federal Aviation Administration (FAA) is a Federal government of the United States, U.S. federal government agency within the United States Department of Transportation, U.S. Department of Transportation that regulates civil aviation in t ...
diverting aircraft. As soon as the instrument was deployed, attempts were made to detect returned signals with no success due to limited observation time and uncertainty in deriving the experiment package's precise location. Strong return signals would not be detected until August 1 by the team at the
Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...
achieving a measurement with an accuracy of approximately during the first observation. The detection aided other observatories by reducing the uncertainty of the instrument's location. The panel eventually enabled a measurement accuracy of approximately . The scientists operating the Lick telescope were puzzled by a persistent drift in the measured signal times versus those that were predicted. The explanation was that the Lick telescope's recorded location on Earth was incorrect by approximately . Additional detections were made by the McDonald Observatory, Pic du Midi Observatory, the Air Force Cambridge Research Laboratories Lunar Ranging Observatory and the Tokyo Astronomical Observatory.


References

{{reflist Apollo 11 Apollo 14 Apollo 15 Apollo program hardware