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Apollo XI Instruments Still Providing Data

Jet Propulsion Lab Press Release, July 19, 1999

PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011

Contact: John G. Watson

FOR IMMEDIATE RELEASE                              July 19, 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 

     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 

   - 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.

JGW 7/19/99

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