July 20, 1999 An experiment left on the lunar surface 30 years ago by the Apollo 11 astronauts continues to return valuable data about the Earth-Moon system to scientific centers around the world, including NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Scientists who analyze the data from the Lunar Laser Ranging Experiment have measured, among other things, that the Moon is moving away from the Earth and that the shape of the Earth is changing at an unprecedented accuracy level. They have also used the experiment to test the validity of several predictions of Einstein's Theory of Relativity.
The lunar laser ranging reflector is designed to reflect pulses of laser light fired from the Earth. The idea was to determine the round-trip travel time of a laser pulse from the Earth to the Moon and back again, thereby calculating the distance between the two. Unlike the other scientific experiments left on the Moon, this reflector requires no power and is still functioning perfectly after 30 years.
The reflector consists of a checkerboard mosaic of 100 fused silica half cubes (roughly the size of the average computer monitor screen), called corner cubes, mounted in a 46-centimeter (18-inch) square aluminum panel. Each corner cube is 3.8 centimeters (1.5 inches) in diameter. Corner cubes reflect a beam of light directly back toward the point of origin; it is this fact that makes them so useful in Earth surveying.
"The Lunar Laser Ranging project cuts across disciplinary and international boundaries, measuring characteristics of the Earth, the Moon and gravitational physics," said Dr. James Williams, a research scientist at JPL. "Data analysis has been conducted around the world, including Germany, France and the U.S."
The McDonald Observatory Laser Ranging Station near Ft. Davis, Texas, and the Observatoire de la Cote d'Azur, operated by the Centre d'Etudes et de Recerche en Geodynamique et Astronomie near Grasse, France, regularly send a 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 a lunar reflector is quite challenging. At the Moon's surface the beam is roughly one mile wide; scientists liken the task of properly aiming the beam to using a rifle to hit a moving dime two miles away.
Once the laser beam hits a 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 left on the Moon, including two by later Apollo missions and one (built by the French) by the unmanned Soviet Lunakhod 2 lander. Each of the reflectors rests on the lunar surface in such a way that its flat face points toward the Earth.
Continuing improvements in lasers and electronics over the years have lead to recent measurements that are accurate to about two centimeters (less than 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 10 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- hundredth of an inch.
During the course of the last 30 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:
- The familiar ocean tides raised on the Earth by the Moon have a direct influence on the Moon's orbit. Laser ranging has shown that the Moon is receding from the Earth at about 3.8 centimeters (1.5 inches) every year. - Lunar ranging, together with laser ranging to artificial Earth satellites, has revealed a 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 from the effects of a fluid lunar core. - The combined mass of the Earth and Moon has been determined to one part in 200 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 atmosphere, tides and the core of the Earth cause changes in the length of an Earth day -- the variations are about one thousandth of a second over the course of a year.
Researchers say that lunar reflectors will remain in service for years to come, because of 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.
At JPL, this lunar ranging analysis, sponsored by NASA's Office of Space Science, is conducted by Drs. James G. Williams, Dale Boggs, J. Todd Ratcliff and Jean O. Dickey. JPL is a division of the California Institute of Technology, Pasadena, CA.
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