The Laser Ranging Retroreflector
experiment is a setup on the Moon
very accurately measuring the distance
between Earth and the Moon. The
setup was installed during the Apollo 11
, and 15
missions (1969-1971). After 30 years, the Laser Ranging
Retroreflector is still being used for data collection.
The setup of the Laser Ranging Retroreflector consists of four arrays
of reflecting mirrors, designed to reflect an incoming light beam in
the direction it came from. The reflectors are illuminated by
lasers, aimed through telescopes on Earth. The
reflected laser beam is then observed with the telescope. The distance
between Earth and the Moon can be calculated by measuring the time
it takes for the laser light pulses to make the round trip.
The accuracy of this method is phenomenal; the relative
accuracy is better than one part in 10 billion. Visualize this as
measuring the distance between Los Angeles and New York to 0.01"
(0.25 mm). The average distance between the centers of the
Earth and the Moon was measured to be 239 000 miles (385 000
kilometers). Measurements over the last 30 years have shown
that the Moon is receding from the Earth at about 1.5" (3.8
cm) per year. The Laser Ranging Retroreflector has also been
used to measure changes in the shape of the Earth, to monitor changes
in the length of an Earth day, to accurately back-calculate the
occurrence of solar eclipses to 1400 B.C., and to
confirm Einstein's theory of relativity.
Although highly accurate, the measurement technique is very
challenging. Due to the gravitational pull of the Earth, the Sun,
the planets and even some asteroids, the reflectors appear
as moving targets. Also, the reflectors are too small to observe
directly from Earth. Compare this to rifle shooting at a moving dime,
two miles away. Another complication is the divergence of the laser
beam; on the Moon's surface, the laser beam is approximately one mile
wide. The laser beam further diverges on its way back to
Earth. Hence, the reflected signal is very weak; on the order of one
photon per second.
Several telescopes around the world (Texas, Hawaii,
France, Germany and Australia) are involved with Laser Ranging