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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 retroreflector 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 round-
trip 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 half-
cubes, 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
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 retroreflector 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.
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