An experiment left on the lunar surface 20 years ago by the Apollo 11 astronauts continues to study the Earth-Moon system and return data to NASA's Jet Propulsion Laboratory and other scientific centers around the world.

Scientists who analyze the data from the Lunar Laser Ranging Experiment have observed, among other things, that the Moon is moving away from the Earth, plates of the Earth are slowly drifting and the length of day varies.

The Laser Ranging Retro-reflector was designed to reflect pulses of laser light fired from the Earth. The idea was to determine the round-trip travel time of laser pulse from the Earth to the Moon and back again, thereby calculating the distance between the two bodies to unprecedented accuracy. Unlike the other scientific experiments left on the Moon, this reflector requires no power and is still functioning perfectly after 20 years.

The laser reflector consists of 100 fused silica half cubes, called corner cubes, mounted in 46-centimeter (18-inch) square aluminum panel. Each corner cube is 3.8 centimeters (1.5 inches) in diameter. Corner cubes reflect beam of light directly back toward the point of origin; it is this fact that makes them so useful in Earth surveying.

The McDonald Observatory, Ft. Davis, Texas; the Lure Observatory atop the extinct Haleakala volcano on the island of Maui, Hawaii; and third observatory in southern France near Grasse, regularly send laser beam through an optical telescope and try to hit one of the reflectors.

The reflectors are too small to be seen from Earth, so even when the beam is correctly aligned in the telescope, actually hitting lunar reflector is quite challenging. At the Moon's surface the beam is roughly mile wide and scientists liken the task of properly aiming the beam to using rifle to hit moving dime two miles away.

Once the laser beam hits reflector, scientists at the observatories use sensitive filtering and amplification equipment to detect any kind of return signal. The reflected light is too weak to be seen with the human eye but under good conditions one photon -- the fundamental particle of light -- will be received every few seconds.

Three more reflectors have since been placed on the Moon, including two by later Apollo missions and one built by the French and left by the unmanned Soviet Lunakhod 2 lander. Each of the reflectors rests on the lunar surface in such way that its flat face points toward the Earth.

Continuing improvements in lasers and electronics over the years have led to recent measurements accurate to about three centimeters (approximately one inch). Scientists know the average distance between the centers of the Earth and the Moon is 385,000 kilometers (239,000 miles), implying that the modern lunar ranges have relative accuracies of better than one part in ten billion. This level of accuracy represents one of the most precise distance measurements ever made and is equivalent to determining the distance between Los Angeles and New York to one fiftieth of an inch.

During the last 20 years, scientists have been able to use the orbit of the Moon and the data they received through lunar ranging to study events happening on Earth.

There have been major scientific advances derived from lunar ranging:

* Lunar ranging has helped them determine the precise positions of the observatories that send the laser beams. Using these positions scientists can tell that the plates of the Earth are slowly drifting and the observatory on Maui is seen to be moving away from the one in Texas.

* The atmosphere, tides and the core of the Earth cause changes in the length of an Earth day -- the variations are about one thousandth of second over the course of year.

* The familiar ocean tides raised on the Earth by the Moon have direct influence on the Moon's orbit. Laser ranging has shown that the Moon is receding from the Earth at about 3.7 centimeters (1.5 inches) every year.

* Lunar ranging, together with laser ranging to artificial Earth satellites, has revealed small but constant change in the shape of the Earth. The land masses are gradually changing after being compressed by the great weight of the glaciers in the last Ice Age.

* Predictions of Einstein's theory of relativity have been confirmed using laser ranging.

* Small-scale variations in the Moon's rotation have been measured. They result from irregularities in the lunar gravity field, from changes in the Moon's shape due to tides raised in the Moon's solid body by the Earth and possibly from the effects of fluid lunar core.

* The combined mass of the Earth and Moon has been determined to one part in 100 million.

* Lunar ranging has yielded an enormous improvement in our knowledge of the Moon's orbit, enough to permit accurate analyses of solar eclipses as far back as 1400 BC.

The usefulness of continued improvements in range determinations for further advancing our understanding of the Earth-Moon system and the need for monitoring the details of the Earth's rotation will keep the lunar reflectors in service for years to come.

At JPL this lunar ranging analysis is done by Drs. Jean Dickey, James G. Williams and X Newhall and is sponsored by the Geodynamics Branch of NASA's Office of Space Science and Applications. Additional analysis is done at the Harvard/Smithsonian Center for Astrophysics and the Massachusetts Institute of Technology, both in Cambridge, Mass.; at the University of Texas in Austin, Tex.; and in France and China.

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