The Last Moon Experiment

(1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible.

(2) The moon probably has a liquid core.

(3) The universal force of gravity is very stable. Newton's gravitational constant G has changed less than 1 part in 100-billion since the laser experiments began.

more Lunar laser ranging at the McDonald Observatory.

Physicists have also used the laser results to check Einstein's theory of gravity, the general theory of relativity. So far, so good: Einstein's equations predict the shape of the moon's orbit as well as laser ranging can measure it. But Einstein, constantly tested, isn't out of the woods yet. Some physicists (Alley is one of them) believe his general theory of relativity is flawed. If there is a flaw, lunar laser ranging might yet find it.

NASA and the National Science Foundation are funding a new facility in New Mexico, the Apache Point Observatory Lunar Laser-ranging Operation or, appropriately, "APOLLO" for short. Using a 3.5-meter telescope with good atmospheric "seeing," researchers there will examine the moon's orbit with millimeter precision, 10 times better than before.

"Who knows what they'll discover?" wonders Alley.

More and better data could reveal strange fluctuations in gravity, amendments to Einstein, the "sloshing" of the moon's core. Time will tell ... and there's plenty of time. Lunar mirrors require no power source. They haven't been covered with moondust or pelted by meteoroids, as early Apollo planners feared. Lunar ranging should continue for decades, perhaps for centuries.

Picture this: Tourists in the Sea of Tranquility, looking up at Earth. Half of the planet is dark, including New Mexico where a pinprick of light appears. A laser.

"Hey, mum," stepping over a footprint, "what's that star?"

Writer's Note: Among many early contributors to the the lunar laser ranging retroreflector array project, Prof. Alley credits Robert Henry Dicke, James Faller, Peter Bender, Douglas Currie and Bendix Corporation. A complete list may be found in Alley's account of the project, "Laser ranging to retro-reflectors on the Moon as a test of theories of gravity," published in Quantum Optics, Experimental Gravitation, and Measurement Theory, Eds. P Meystre and M.O. Scully, Plenum Publishing (1982).

For more information

Video: The Moon [FirstScience presents]
Since its formation 4.5 billion years ago the Moon has been edging away from Earth yet it has influenced our planet in many profound ways.