James Faller: Science on the Moon

Most people remember President Kennedy’s urging the nation to commit itself “to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth.” But his inspiring address to a joint session of Congress on May 25, 1961 was more expansive and ambitious.

Kennedy believed it was an opportunity for science and discovery, such as expanding and improving satellite technology for telecommunications and world-wide weather observation.

The Apollo program would provide the opportunity to conduct lunar science. As the competition began within the astronaut corps to determine who might be selected to be part of the first moon mission, a similar competition began within the scientific community: what will be the first scientific experiments chosen for the moon?

Answering the most basic question: What’s the distance?

One potential experiment was of longtime interest in the astronomy community: how far, exactly, was the distance between the Earth and the Moon? The question fascinated scientists for hundreds of years, and even played a major role in Newton’s theory of gravitation when it was published in 1687.

Now, nearly 275 years later, as Apollo was becoming a reality, so was the possibility of finally and accurately answering that eternal question. A lunar ranging experiment could turn the Moon into a “satellite” and, with the right technology, the Earth and the Moon could be able to “talk to one another.”

Coming up with the lunar laser experiment

In the late 1950’s, Dr. James Faller was a graduate student at Princeton University. He was part of a research group led by Professor Robert Henry Dicke, an American physicist who made important contributions to the fields of astrophysics, atomic physics, cosmology and gravity. The students and scientists were studying the use of artificial earth satellites for precision tracking of the star field.

As a result of those discussions, Dr. Faller explored the concept of a lunar laser experiment that could be set up on the moon. He wrote a paper, “A Proposed Lunar Package: A Corner Reflector on the Moon,” envisioning a durable, yet lightweight reflector weighing only two to three pounds that would be deployed on the Moon. A laser beam from Earth would be targeted at the reflector; the instrument would reflect the light back to Earth. The time would be calculated and “permit a precise earth-moon distance measurement to be made.”

When he turned in his paper, Faller hand-wrote at the very top: “Professor Dicke, would you see if this makes any sense?”

Less than a decade later, the world would know how right Dr. Faller’s proposal was.

Why the lunar ranging initially failed

In the weeks leading up to the launch, Dr. Faller was at the Lick Observatory, an astronomical observatory operated by the University of California, just east of San Jose, California. “We were getting ready to start laser ranging as soon as we were given a precise landing location, which was slow coming,” Faller said.

July 20th and the days that followed were frustrating for everyone at the Lick Observatory. While the landing and deployment were personally exciting for Faller and the ranging teams, the ability to begin doing the actual science faced a lunar obstacle: the moon was too low in the sky to allow ranging without going through a fair amount of the Earth’s atmosphere.

The decision was made to shut down and check and re-check every detail, but no problems were discovered. The ranging resumed on August 1st when the Moon was positioned more favorably in the sky. Under these more optimal conditions, the laser was fired 162 times before any returns from the lunar array were recognized. The final series of 120 laser beam shots, after some adjustments, yielded 80 detected returns.

Small leaps of progress – up to a precision of a few millimeters

Every laser pulse transmitted enabled the data to become more precise. On August 1st Faller and the ranging team, with a timing precision of 0.1 microseconds (just one ten-millionth of a second!) –, had established the distance to the Moon with a single shot accuracy of 8 meters; two days later, they improved accuracy to 6 meters.

Fifty years later, the Lunar Laser Retro-Reflector’s durability far exceeded NASA’s design requirement of having a ten-year life in the lunar environment. Some expected degradation has occurred, as a small layer of lunar dust now covers the array’s fused-silica prism. To compensate, scientists increased the amount of laser power is used.

The information gathered from the reflector has helped answer important questions about gravity, the Earth, the Moon and the Earth-Moon system. Dr. Faller adds, “We made dramatic progress in our ability to measure the distance between the Earth and the Moon. Initially the accuracy came within centimeters. Today, it has an accuracy of millimeters.”