PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
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Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE July 21, 1994
During their brief moon walk 25 years ago, the Apollo 11 astronauts deployed a variety of scientific experiments, including a reflector array left in the fine powder of the Sea of Tranquility that continues to measure the moon's orbit around Earth to unprecedented accuracy.
Scientists who analyze data from the Lunar Laser Ranging Experiment have reported some watershed results from these long-term experiments, said Jet Propulsion Laboratory team investigator Dr. Jean Dickey. The team's findings appear in this week's issue of Science magazine, which commemorates the silver anniversary of the Apollo 11 lunar landing.
"Using the Lunar Laser Ranging Experiment, we have been able to improve, by orders of magnitude, measurements of the moon's rotation," Dickey said. "We also have strong evidence that the moon has a liquid core, and laser ranging has allowed us to determine with great accuracy the rate at which the moon is gradually receding from the Earth."
The laser ranging retro reflector was positioned on the moon in 1969 by the Apollo 11 astronauts so that it would point toward Earth and be able to reflect pulses of laser light fired from the ground. By beaming laser pulses at the reflector, scientists have been able to determine the roundtrip travel time of a laser pulse and provide the distance between these two bodies at any given time down to an accuracy of about 3 centimeters (about 1 inch).
The laser reflector consists of 100 fused silica halfcubes, called corner cubes, mounted in a 46-centimeter (18inch) 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 and, thus, allow scientists to measure the Earth-moon separation and study the dynamics of the Earth, the moon and the Earth-moon system.
Once the laser ranging experiments began to yield valuable results, more reflectors were left on the moon. A reflector identical to the Apollo 11 mission reflector was left by the Apollo 14 crew, and a larger reflector using 300 corner cubes was placed on the moon by the Apollo 15 astronauts. French-built reflectors were also left on the moon by the unmanned Russian Lunakhod 2 mission.
Several observatories have regularly ranged the moon with these reflectors: one is located at McDonald Observatory near Fort Davis, Texas; another is located atop the extinct Haleakala volcano on the island of Maui in Hawaii; another is located in southern France near Grasse.
The Lick Observatory in northern California also has been used in the past for the lunar laser ranging experiments and ranging programs have been carried out in Australia, Russia and Germany. Despite the difficulty of detecting reflected laser light from the moon, Dickey said, more than 8,300 ranges have been measured over the last 25 years.
"Lunar ranging involves sending a laser beam through an optical telescope," Dickey said. "The beam enters the telescope where the eye piece would be, and the transmitted beam is expanded to become the diameter of the main mirror, then bounced off the surface toward the reflector on the moon."
The reflectors are too small to be seen from Earth, so even when the beam is precisely aligned in the telescope, actually hitting a lunar retro reflector array is technically challenging. At the moon's surface the beam is roughly four miles wide. Scientists liken the task of 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 ranging observatories use extremely sensitive filtering and amplification equipment to detect the return signal, which is far too weak to be seen with the human eye. Even under good atmospheric viewing conditions, only one photon -- the fundamental particle of light -- will be received every few seconds.
The range accuracy of these reflectors has been improved over the lifetime of the lunar laser ranging experiments, the team noted in Science. While the earliest ranges had accuracies of several meters (or several yards), continuing improvements in the lasers and the detection electronics have led to recent measurements that are accurate to about 3 centimeters (about 1 inch).
From the ranging experiments, scientists know that the average distance between the centers of the Earth and the moon is 385,000 kilometers (239,000 miles), showing that 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," Dickey said. "The degree of accuracy is equivalent to determining the distance between Los Angeles and New York to one fiftieth of an inch."
Laser ranging has also made possible a wealth of new information about the dynamics and structure of the moon. Among many new observations, scientists now believe that the moon may harbor a liquid core. The theory has been proposed from data on the moon's rate of rotation and very slight bobbing motions caused by gravitational forces from the sun and Earth.
Other recent findings from the laser ranging experiments include:
-- Verification of Einstein's theory of relativity, which states that all bodies fall with the same acceleration regardless of their mass.
-- The length of an Earth day has distinct small-scale variations, changing by about one thousandth of a second over the course of a year. These changes are caused by the atmosphere, tides and the Earth's core.
-- Precise positions of the laser ranging observatories on Earth are slowly drifting as the crustal plates on Earth drift. The observatory on Maui is seen to be drifting away from the observatory in Texas.
-- Ocean tides on Earth have a direct influence on the moon's orbit. Measurements show that the moon is receding from Earth at a rate of about 3.8 centimeters (1.5 inches) per year.
-- Lunar ranging has greatly improved scientists' knowledge of the moon's orbit, enough to permit accurate analyses of solar eclipses as far back as 1400 BC.
Continued improvements in range determinations and the need for monitoring the details of the Earth's rotation will keep the lunar reflector experiments in service for years to come, Dickey stated in her article.
"For the immediate future, we have under way the implementation of dramatically increased station computing power, offset guiding capability and hands-off auto guiding," she reported. "The benefits from these improvements will not only be an increased number of normal points spread over significantly more of the lunar phase, but also a significantly increased number of photons within a given normal range.
"Farther down the road, we foresee the availability of more precise and more efficient photon detectors, such as micro-channel plates, significantly improved timing systems and shorter-pulse, more powerful lasers," she added. "This will increase data, provide higher accuracy ranging and improve sensitivity to lunar signatures, or conditions brought about by the phases of the moon."
At JPL the lunar ranging analysis is carried out by JPL scientists Drs. Jean Dickey, James G. Williams, X X Newhall and Charles F. Yoder. The work is sponsored jointly by the Astrophysics Division of NASA's Office of Space Science and the Solid Earth Science Branch of NASA's Mission to Planet Earth Office, Washington, D.C.
Additional work is done at the Joint Institute for Laboratory Astrophysics at the University of Colorado at Boulder; at the University of Texas in Austin; and at Observatoire de la Cote d'Azur, Grasse, France.