Exploring Earth's Moon
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The Moon is in an elliptical 28-day orbit ranging from 220,000 to about 252,000 miles from Earth. On average, the center of the Moon is about 238,700 miles from the center of Earth. Laser ranging techniques have allowed astronomers to narrow the exact measurement to within less than an inch.
There are retroreflectors on the Moon that are small mirrors called "corner cube retroreflector arrays." Each array is about the size of a suitcase. Because they can be seen from Earth, and used by scientists on Earth, the human-constructed retroreflectors often are cited as proof that Apollo astronauts actually visited the Moon.
Retroreflectors were conceived in the 1960s by Montana State University physicist Kenneth Nordtvedt who suggested to NASA that a series of reflectors on the surface of the Moon could be helpful in determining the exact distance between Earth and the Moon.
Laser ranging. Nordtvedt saw that light from a laser fired from Earth could hit a reflector on the Moon and bounce back to Earth. He understood that the time light took to travel to the reflector and back would reveal the distance. That information, in turn, would reveal a great deal of information about the lunar orbit, which could be used to test some of Albert Einstein's theories.
NASA sent three arrays of 100 to 300 prisms to the Moon during three flights in the Apollo Moon-landing program. The first retroreflector was positioned there in 1969 by the Apollo 11 astronauts. Two other arrays from the Soviet Union and France were delivered to the Moon aboard unmanned Lunakhod missions launched from the Soviet Union.
Batteries not included. Since retroreflectors require no electrical power, they continue to operate decades after Neil Armstrong set foot on the Moon. Scientists around our planet regularly fire off laser pulses to bounce off of the distant reflectors — to better understand the Moon's rotation, Earth's tides, and Einstein's theory of relativity.
The prisms reflect light back to its point of origin, allowing scientists to pin down the distance to the Moon to within about 10 inches by the early 1970s, and now to less than an inch.
Because the Soviet Moon probe Lunakhod 2 was not manned, its retroreflector was not placed as carefully on the lunar surface as when the Apollo astronauts were able to aim their retroreflectors toward Earth. As a result, its bigger mirror reflects a weaker laser echo than the smaller Apollo reflectors.
The laser beam. How powerful does the light beam have to be? One laser generator in use with a 3.5-meter telescope operated by the Astrophysical Research Consortium at Apache Point, near Sunspot, New Mexico, generates a peak power of a one billion watts (1 gigawatt) for a short time, but just long enough to fire off a one-inch bullet of light aimed through the telescope at the lunar surface.
The distance the light travels is calculated by measuring the light pulse's round-trip travel time and multiplying that figure by the speed of light.
Earth's atmosphere distorts the beam so that it is expanded out to 1.25 miles in diameter when it hits the Moon. Only one in 30 million of the original photons in the beam actually will hit the retroreflector. By the time the light makes it back to Earth, the beam will have expanded to 9.3 miles in diameter. Of the returning photons, only one in 30 million will hit the telescope on Earth.