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Why does the Apollo Reflector experiment still work 50 years later?

An epic moon laser experiment is still in progress, five decades after the Apollo astronauts built it on the surface.

The moon-changing crew of Apollo 11, who landed on the moon 50 years ago this month, used special retroreflectors on the lunar surface, as did the later crews of Apollo 14 and 15 in 1971. (Another of the French built retro-reflector sits on the Soviet Lunokhod 2-Rover, which landed in 1973 without crew.)

  Apollo 11-Astronauts This retroreflector was left in 1969 on the lunar surface.

Apollo 11 astronauts left this retro reflector in 1969 on the lunar surface.

The NASA experiment, called the Laser Ranging Retroreflector, is "a special kind of mirror with the property of always reflecting an incoming light beam back in the direction it came from," the Lunar and explained Planetary Institute (LPI) in a statement. And the reflector is the key to measuring the distance between the earth and the moon, added the institute.

See also: Apollo 11 at 50: A Complete Guide to Historic Moon Landing

The work is an observatory – often the McDonald Observatory in Texas, although many others have participated – firing a laser beam at the moon. "They stay tight for long distances," the institute explained, though there are some differences. Until the beam bounces off the mirror and returns to Earth, it has a diameter of about 20 kilometers. This jet dispersion makes it difficult to observe the reflection.

The reflector itself needs no energy, which is why it is still in operation decades after the silence of other instruments on the moon. This instrument contains 100 "corner cubes" (or quartz glass cubes) arranged in an 18-inch (46 cm²) square aluminum plate, according to NASA's Jet Propulsion Laboratory (JPL). Due to the shape of these corner cubes, each laser shot reflects the light back directly. Over the years, lunar removal measurements have improved thanks to better laser and computer equipment.

"As soon as the laser beam strikes a reflector, scientists at the observatories use sensitive filtering and amplifying equipment to detect each return signal," said JPL. "The reflected light is too weak to be seen by the human eye, but under good conditions a photon, the base light particle, will be received every few seconds."

Observations must be made over several hours to account for the weak signal. However, averaging the signal allows researchers to accurately calculate the distance of the moon to less than 2 cm (1 inch), JPL said. This is not a bad error rate, as the moon is an average of 384,400 km from our planet or about 30 Earth's diameter away from us. The distance is so great that Apollo astronauts took three days to get there.

While the experiment is most famous for its lunar distance measurements, researchers have also used the data to show that the moon has a liquid core and the moon. The natural satellite is slowly moving away from the earth, James Williams said , a scientist at NASA's Jet Propulsion Laboratory who participated in the experiment, in a 2009 Space.com interview.

The timing of the laser tour provided evidence of Albert Einstein's theory of general relativity. This theory in part states that the speed of light in a vacuum is independent of the motion of an observer. This experiment also showed that Isaac Newton's gravitational constant is extremely constant, changing between 1969 and 2004 by less than 100 billion, NASA said.

Original article on Space.com.

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