Apache Point Observatory Lunar Laser-ranging Operation
Encyclopedia
Apache Point Observatory
The Apache Point Observatory is located in the Sacramento Mountains in Sunspot, New Mexico 18 miles south of Cloudcroft. The observatory consists of the Astrophysical Research Consortium's 3.5-meter telescope, the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope,...
in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
. It is an extension and advancement of previous Lunar Laser Ranging Experiment
Lunar laser ranging experiment
The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...
, which uses retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
s on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
to track changes in lunar orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
al distance and motion.
Using telescopes on Earth, the reflectors on the Moon, and accurate timing of laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulses, by the early 2000s scientists could measure and predict the orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of the Moon to an accuracy of a few centimeters. This already impressive accuracy (the Moon is typically about 385,000 km away) provides the best known test of many aspects of our theories of gravity. APOLLO improves this even further, measuring the distance between the Moon to an accuracy of a few millimeters. Using this information, scientists will be able to further test various aspects of gravity: do the Earth and the Moon react the same to gravity despite their different compositions? Does the energy content of the Earth and the Moon react to gravity in the same way as Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
predicts? In general, does Einstein's General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
correctly predict the motion of the Moon, or are new theories required?
The APOLLO collaboration built their apparatus on the 3.5 meter telescope at Apache Point in southern New Mexico. By using a large telescope at a site with good atmospheric "seeing"
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
, the APOLLO collaboration gets much stronger reflections than any existing facilities. (Strong is a relative term here—APOLLO records approximately one returned laser photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
per pulse, as opposed to the roughly 0.01 photon-per-pulse average experienced by previous LLR facilities.) The stronger return signal from APOLLO translates to much more accurate measurements.
History and motivation
High precision Lunar Laser Ranging (LLR) started soon after the Apollo 11Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
astronauts left the first retroreflector on the Moon. Additional reflectors were left by the Apollo 14
Apollo 14
Apollo 14 was the eighth manned mission in the American Apollo program, and the third to land on the Moon. It was the last of the "H missions", targeted landings with two-day stays on the Moon with two lunar EVAs, or moonwalks....
and Apollo 15
Apollo 15
Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...
astronauts, and two French-built reflector arrays were placed on the Moon by the Soviet Luna 17
Luna 17
-External links:*...
(Lunokhod 1
Lunokhod 1
Lunokhod 1 was the first of two unmanned lunar rovers landed on the Moon by the Soviet Union as part of its Lunokhod program. The spacecraft which carried Lunokhod 1 was named Luna 17...
) and Luna 21
Luna 21
-External links:*...
(Lunokhod 2
Lunokhod 2
Lunokhod 2 was the second of two unmanned lunar rovers landed on the Moon by the Soviet Union as part of the Lunokhod program....
) lunar rover missions. Over the years since, many groups and experiments have used this technique to study the behavior of the Earth-Moon system, investigating gravitational and other effects.
For the first few years, the distance between the observatory and the reflectors could be measured to about
25 cm accuracy. Improved techniques and equipment lead to accuracies of 12–16 cm until about 1984. Then McDonald Observatory
McDonald Observatory
The McDonald Observatory is an astronomical observatory located near the unincorporated community of Fort Davis in Jeff Davis County, Texas, United States. The facility is located on Mount Fowlkes and Mount Locke in the Davis Mountains of West Texas...
built a special purpose system (MLRS) just for ranging, and achieved roughly 3 cm accuracies mid-to-late
1980s. In the early 1990s a French LLR system at the Observatoire de la Côte d’Azur
Côte d'Azur Observatory
The Observatoire de la Côte d'Azur originated in 1988 with the merger of two observatories:# Observatoire de Nice# The CERGA - External links :*...
(OCA) started operation, with similar precision.
The McDonald and OCA stations are collecting data that is about as good as possible, given the number of photons they collect back from the reflectors. Although minor improvements are certainly possible, getting significantly better data requires a larger telescope and a better site. This is the basic goal of the APOLLO collaboration.
The APOLLO laser has been operational since October 2005, and routinely accomplishes sub-millimeter level range accuracy between the Earth and the Moon.
Science goals
The goal of APOLLO is to push LLR into the millimeter range precision, which then translates directly into an order-of-magnitude improvement in the determination of fundamental physics parameters. Specifically, assuming improvements of a factor of ten over prior measurements, APOLLO will test:- the Weak Equivalence PrincipleEquivalence principleIn the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
(WEP) to a part in
, - the Strong Equivalence PrincipleEquivalence principleIn the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
(SEP) to a few parts in
, - de Sitter relativistic precession to a few parts in
, and - the time variation of the Gravitational constantGravitational constantThe gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
G to a part in
per year.
Tests of the Equivalence Principles
The Weak Equivalence Principle says that all objects fall the same way in a gravity field, no matter what they are made of. The Earth and the Moon have very different compositions—for example, the Earth has a large iron coreInner core
The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...
, but the Moon does not. Furthermore, both are in orbit around the Sun, meaning they are both falling towards the Sun at all times, even as they revolve around each other. If the Earth and the Moon were affected differently by the gravity of the Sun, this would directly affect the orbit of the Moon around the Earth. But as closely as scientists can measure, the orbit of the Moon is just as predicted from assuming that gravity acts the same on each — to within 1 part in 1013, the Earth and the Moon fall towards the Sun in exactly the same way, despite their different compositions. APOLLO will lead to even tighter limits.
What about the Strong Equivalence Principle? According to Einstein's General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, the mass of any object consists of two parts—the mass of the atoms themselves, plus the mass of the energy that holds the object together
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
. The question is whether the energy portion of mass behaves like the traditional part—does it contribute to measured gravity of the object? To the inertia? In General Relativity, the self energy affects both the gravity field and inertia, and does so equally. This is the Strong Equivalence Principle (SEP).
Other modern theories, such as string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...
, quintessence
Quintessence (physics)
In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force...
, and various forms of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
, almost all predict a violation of the Strong Equivalence Principle at some level. Additionally, many puzzling experimental results, such as Galaxy rotation curve
Galaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....
s that imply dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
or supernova observations
Supernova Cosmology Project
The Supernova Cosmology Project is one of two research teams that determined the likelihood of an accelerating universe and therefore a positive Cosmological constant, using data from the redshift of Type Ia supernovae...
that imply dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
, could also potentially be explained by alternative theories of gravity (see, for example, MOND
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
). Therefore experimentalists believe it is important to make the most precise measurements of gravity possible, looking for any possible anomalies or confirming Einstein's predictions.
Precise ranging to the Moon can test the SEP since the Earth and the Moon have a different fraction of their mass in the energy component. Precision measurements are needed since this component is very small—if
is the self energy of the Earth — the energy needed to spread the atoms of the Earth out to infinity against the attraction of gravity — then the mass of the Earth is decreased by about 
×
of the Earth’s total mass. The self energy of the Moon is smaller yet, about 2×
of its mass. (The contribution for any object of laboratory size is negligible, about 
, so only measurements of planet-sized or bigger objects have any hope of seeing this effect.)If the Moon just revolved around the Earth, there would be no way to tell what fraction of the Moon's or the Earth's gravity was caused by each form of mass, since only the total can be measured. However, the orbit of the Moon is also strongly affected by the gravity of the sun—in essence, Earth and Moon are in freefall around the sun. If the energy portion of mass behaves differently than the conventional portion, then the Earth and the Moon will fall differently toward the Sun, and the orbit of the Moon around the Earth will be affected. For example, suppose the energy part of the mass does affect gravity, but does not affect inertia. Then:
From our perspective on Earth, this would appear as a displacement, or polarization, of the lunar
orbit away from the sun with an amplitude of 13 meters. If the violation went the other way, with the
self energy possessing inertial mass but not gravitational mass, the lunar orbit would appear to be
polarized toward the sun by the same amplitude. The calculation of the amplitude is complicated, but a crude estimate may be derived by multiplying the Earth’s orbital radius of
1.5×m by the
4.6×contribution to the Earth’s mass from the self-energy to yield 75 meters.
The signature of an EP violation is very simple, depending only on the distance of the Moon from the Sun. This repeats about every 29.5 days, somewhat longer than the time the Moon takes to go around the Earth once, which is 27.3 days. (This difference arises since the Earth moves along its orbit as the Moon goes around, so the Moon has to make a little more than one orbit to get back to the same position relative to the Sun.) This makes EP particularly easy to measure, since many confounding effects such as tides or weather will not repeat at 29.5 day intervals. Unfortunately, there is one effect—radiation pressure acting on the orbit of the Moon—that does repeat each 29.5 days. Fortunately, it is small, less than 4 mm, and fairly easy to model so it can be subtracted out.
Finally, even if the experiments show no effect, there is a tiny theoretical loophole. The measurements show the sum of the WEP and SEP violations. If the experiments show no effect, the most natural explanation is that neither WEP or SEP are violated. But it is conceptually possible that both are violated, and by equal and opposite amounts. This would be an incredible coincidence since WEP and SEP depend on very different and arbitrary properties—the exact composition of the Earth and the Moon, and their self-energies. But this unlikely case cannot be completely ruled out until either other solar system bodies are measured to similar precision, or laboratory experiments reduce the bounds on WEP violations alone.
Variations in the gravitational constant
Existing ranging experiments can measure the constancy of the Gravitational constantGravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
, G, to about one part in
per year. The expansion rate of the universeMetric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
is approximately one part in
per year. So if G scaled with the size or expansion of the universe, existing experiments would already have seen this variation. This result can also be viewed as experimental verification of the theoretical result that gravitationally bound system do not partake in the general expansion of the universe. APOLLO will place much tighter bounds on any such variations.Other tests
At this level of accuracy, General Relativity is needed to predict the orbitOrbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of the Moon. Current tests measure geodetic precession to a 0.35% level of precision, gravitomagnetism
Gravitomagnetism
Gravitomagnetism , refers to a set of formal analogies between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein field equations for general relativity. The most common version of GEM is valid only far from isolated sources, and for slowly moving test...
at the 0.1% level, and checks whether gravity behaves as
as expected. APOLLO will improve on all these measurements.Principles of operation
a short-pulse laser reflected from a distant target—in this case the retroreflector arrays on the Moon. Each burst of light lasts 100 picoseconds (ps). One millimeter in range corresponds to only 6.7 ps of round-trip travel time.
However, the retroreflectors on the Moon introduce more than one mm of error themselves. They are not usually at an exact right angle to the incoming beam, so the different corner cubes of the retroreflectors are at different distances from the transmitter. This is because the Moon, although it keeps one face to the Earth, does not do so exactly—it wobbles from side to side and up and down, by as much as 10° in magnitude. (There is a nice animated GIF of this on the libration
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
page.) These librations occur since the Moon rotates at constant speed, but has an elliptical and inclined orbit. This effect may seem small, but it is not only measurable, it forms the largest unknown in finding the range, since there is no way to tell which corner cube reflected each photon.
The biggest array, the 0.6 m
Apollo 15 reflector, can have a corner-to-corner rangespread of ≈ 1.2 tan(10°) m, or 210 mm, or about 1.4 ns of round-trip time. The root-mean-square
(RMS) range spread is then about 400 ps. To determine the distance to the reflector to 1 mm precision, or 7 ps, by averaging, the measurement needs at least (400/7)2 ≈ 3000 photons. This explains why a much larger system is needed to improve the existing measurements—the current 2 cm RMS range precision requires only about 10 photons, even at the worst-case orientation of the retroreflector array.
APOLLO attacks this problem by using both a bigger telescope and better astronomical seeing. Both are considerably improved over existing systems. Compared to McDonald Observatory ranging station, the
Apache Point telescope has a factor of 20 greater light-collecting area. There is also a big gain from better seeing—the APO site and telescope combined can often achieve one arcsecond seeing, compared to the ∼ 5 arcseconds
typical for MLRS. The better seeing helps two ways—it both increases the laser beam intensity on the Moon and reduces the lunar background, since a smaller receiver field-of-view may be used, gathering light from a smaller spot on the Moon.
Both effects scale as the inverse square of the seeing, so that the signal-to-noise ratio of the lunar return is inversely
proportional to the fourth power of the seeing. APOLLO should therefore gain
about 20 (from the bigger telescope) × 25 (for better seeing) = 500 × in return signal strength over MLRS, and additional factor of 25 in
signal-to-noise (from fewer stray photons interfering with the desired ones). Likewise APOLLO should get a signal about
50 times stronger than the OCA LLR facility, which has a 1.5 m telescope and seeing of about 3 arcsec.
The increased optical gain brings some problems due to the possibility of getting more than one returned photon per pulse. The most novel component of the APOLLO system is the integrated
array of Single-Photon Avalanche Diode
Single-Photon Avalanche Diode
In optoelectronics the term Single-Photon Avalanche Diode identifies a class of solid-state photodetectors based on a reverse biased p-n junction in which a photo-generated carrier can trigger an avalanche current due to the impact ionization mechanism...
s (SPADs) used the detector. This technology is needed to
deal with multiple photon returns within each pulse. Most single photon detectors can only record the time of the first photon if another arrives very soon thereafter (This effect is called dead time
Dead time
For detection systems that record discrete events, such as particle and nuclear detectors, the dead time is the time after each event during which the system is not able to record another event....
.). This means that if more than one photon comes back in a single pulse, a conventional single-photon detector would record the arrival time of only the first photon. However, the important quantity is the centroid of the time of all returned photons (assuming the pulse and reflectors are symmetrical), so any system that can return multiple photons per pulse must record the arrival times of each photon. In APOLLO, the incoming photons are spread over an array of independent detectors, which reduces the chance that two or more photons hit any one of the detectors.
Modeling station locations
Any laser ranging station, APOLLO included, measures the transit time, and hence the distance, from the telescope to the reflector(s). But for lunar ranging science, what is really wanted is the distance between the center of massCenter of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
of the Earth and the center of mass of the Moon. To do this, the positions of the telescope, and the reflectors, must be known to comparable precision (a few mm). Since both the telescope and the reflectors are stationary structures, it might seem they could be precisely measured, and then their position would be known thereafter. This assumption is not too bad for the Moon, which is a quiet environment. But for the Earth, the stations move quite a bit on this scale:
- The Earth's polar axis movesPolar motionPolar motion of the earth is the movement of Earth's rotational axis across its surface. This is measured with respect to a reference frame in which the solid Earth is fixed...
and the Earth's rotation is irregular. The polar axis moves due to various causes, some predictable (the Moon exerts a torque on Earth's tidal bulge) and some variable (rocks are rebounding from the last ice age, weather). Weather also affects the Earth's rotation, by moving large masses of water around. These effects, important to many other science projects as well, even have their own agency to keep track of them—the International Earth Rotation and Reference Systems ServiceInternational Earth Rotation and Reference Systems ServiceThe International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...
. - The stations move due to tideTideTides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s. The Moon, since it is tidally lockedTidal lockingTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
to the Earth, has relatively small and repeatable tides of about 10 cm. The solid Earth has larger tides, oscillating about 35 cm peak-to-peak, every 12 hours. - The Earth's crust changes in response to long term fluctuations such as post-glacial reboundPost-glacial reboundPost-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
and loading caused by sediment transport. - The Earth's short-term weather can also affect the location of the telescope, primarily vertically. Various weather effects can load local regions of the Earth's crust, depressing the crust by a few millimeters. These effects come from the atmosphere (high pressure systems press on the Earth's surface), and the ocean (water piles up on the coast depressing the crust). Ground water fluctuations, caused by rain, can also affect the telescope location.
- The pressure of sunlight pushes the Moon's orbit slightly off center. This is a small effect, about 3.65 mm, but it is particularly important since it mimics the effect of a EP violation.
- Even continental driftContinental driftContinental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
must be compensated for.
In addition, the Earth's atmosphere causes an additional delay, since the speed of light is slightly slower through the atmosphere. This amounts to about 1.6 meters when looking straight up at Apache Point. This delay is also affected by weather, primarily atmospheric pressure, which determines just how much air there is above the site.
Since many of these effects are weather-related, and also affect the more common satellite laser ranging
Satellite laser ranging
In satellite laser ranging a global network of observation stations measure the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors...
, ranging stations traditionally include weather stations, measuring local temperature, pressure, and relative humidity. APOLLO will measure all these, plus measure local gravity very precisely, using a precision
gravimeter
Gravimeter
A gravimeter or gravitometer is an instrument used in gravimetry for measuring the local gravitational field of the Earth. A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth...
. This instrument is capable of sensing vertical displacements as small as 0.1 mm, by measuring the change in gravity as the observatory moves closer to or further from the Earth's center.
Using all these measurements, scientists try to model and predict the exact location of the telescope, and the delays through the atmosphere, so they can compensate for them. The tides are fairly predictable, and the Earth's rotation is measured by the IERS
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...
and can be accounted for. Atmospheric delay is fairly well understood, and is dominated by the pressure measurement alone.
Early models had uncertainties in the 5–10 mm range for reasonable elevation angles, though more recent efforts have produced a model claiming 3 mm accuracy down to 10 degrees above the horizon, and sub-millimeter performance above 20–30° elevation. The weather is perhaps the biggest error source. Atmospheric loading is estimated from the barometric pressure at the telescope and the average pressure within a 1000 km radius. Ocean loading has been handled strictly by empirical models, and ground water has been largely ignored. APOLLO will probably require improvements in all these models to reach the full accuracy of the measurements.
Status
APOLLO has been up and working to various degrees since October 2005, with science-quality data beginning April 2006. The current (as of mid 2011) status is:- All 5 reflectors (three Apollo and two Lunokhod) ranged routinely.
- As many as 12 photons in a single pulse (limited by detector - might have been more)
- Sustained rate of about 3 photons per pulse over several minutes. This about 65 times more photons detected than previous efforts.
- As many as 50,000 return photons detected in a single lunation (during 5 hours total operation)
As of mid 2011, the range precision (per session) appears to be about 1.8-3.3mm per reflector, while the orbit of the Moon is being determined to roughly the 15mm level. The gap between the measurements and the theory could be due to systematic errors in the ranging, insufficient modeling of various conventional effects that become important at this level, or limitations of our theory of gravity. More observations and better modeling will help decide between these alternatives, though insufficient modeling is the primary suspect, since this is known to be both complex and difficult.
The APOLLO collaboration has discovered that the optical efficiency of the lunar reflectors decreases at full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
. This effect was not present in measurements from the early 1970s, was visible but not strong in the 1980s, and is now quite significant (about 10x). The cause is unclear but one possibility is that dust on the arrays leads to temperature gradients, distorting the returned beam.
Measurements during the total lunar eclipse of December 2010 have confirmed thermal effects as the cause.
In April 2010, the APOLLO team announced that with the aid of photos from the Lunar Reconnaissance Orbiter
Lunar Reconnaissance Orbiter
The Lunar Precursor Robotic Program is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite , were launched in June 2009...
, they had found the long-lost Lunokhod 1
Lunokhod 1
Lunokhod 1 was the first of two unmanned lunar rovers landed on the Moon by the Soviet Union as part of its Lunokhod program. The spacecraft which carried Lunokhod 1 was named Luna 17...
rover and had received returns from the laser retroreflector. By the fall of 2010, the location of the rover had been determined to about a centimeter. The location near the limb of the moon, combined with the ability to range the rover even when it is in sunlight, promises to be particularly useful for determining aspects of the Earth-Moon system.
The collaboration
APOLLO is collaboration between:University of California, San Diego
University of California, San Diego
The University of California, San Diego, commonly known as UCSD or UC San Diego, is a public research university located in the La Jolla neighborhood of San Diego, California, United States...
(Tom Murphy
Tom Murphy (physicist)
Tom Murphy is a Professor of Physics at the University of California, San Diego. He is the project investigator for the Apache Point Observatory Lunar Laser-ranging Operation Project...
(PI), Eric Michelsen),
University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
(Eric Adelberger,
Erik Swanson),
Harvard
(Chris Stubbs,
James Battat),
Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
(Jim Williams,
Slava Turyshev,
Dale Boggs),
Lincoln Laboratory
Lincoln Laboratory
MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense research and development center chartered to apply advanced technology to problems of national security. Research and development activities focus on long-term technology development as well as...
,
(Brian Aull,
Bob Reich),
Northwest Analysis
(Ken Nordtvedt),
Apache Point Observatory
Apache Point Observatory
The Apache Point Observatory is located in the Sacramento Mountains in Sunspot, New Mexico 18 miles south of Cloudcroft. The observatory consists of the Astrophysical Research Consortium's 3.5-meter telescope, the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope,...
(Bruce Gillespie,
Russet McMillan),
and Humboldt State
(C. D. Hoyle).