APOLLO measures Moon distance to 1 millimeter; also helps with gravity

William Atkins
Tuesday, 23 October 2007 20:31

Science - Space

The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) instrument has measured the distance to the Moon to an accuracy of one millimeter. Such accuracies will also help with measurements involving the force of gravity.

This is the most exacting measurement ever performed on the Moon-Earth distance.

In the 1970s, during the time of the NASA Apollo missions, the average distance from the center of the Moon to the center of the Earth was thought to be about 238,700 miles (384,200 kilometers)—with an accuracy of about 25 centimeters (10 inches). In the 1980s, during the early space shuttle missions, that distance accuracy was cut to about 2 centimeters (less than one inch).

Now, it has been reduced to about 1 millimeter, about the thickness of a paperclip.

The researchers who made this measurement are Thomas “Tom” W. Murphy, Jr., Eric G. Adelberger (University of Washington, Seattle), J.B.R. Battat (Harvard-Smithsonian Center of Astrophysics), L.N. Carey (UW, Astronomy Department), C.D. Hoyle (Humboldt State University), P. LeBlanc (UC, San Diego), E.L. Michelsen (UC, San Diego), K. Nordtvedt (Northwest Analysis, Bozeman), A.E. Orin (UC, San Diego), Jana D. Strasburg (Pacific National Laboratory, Richland), Christopher W. Stubbs (Harvard University), H.W. Swanson (UW, Seattle), and E. Williams (UC, San Diego).

Their paper is titled “APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections”. Murphy, the head of the study, is a University of California, San Diego researcher.

The APOLLO is considered a high-tech, next-generation lunar laser ranging apparatus. It is hooked up to the 3.5-meter telescope at the Apache Point Observatory in southern New Mexico, near Sunspot, New Mexico. According to the abstract to their paper, the APOLLO “has achieved one-millimeter range precision to the moon which should lead to approximately one-order-of-magnitude improvements in the precision of several tests of fundamental properties of gravity.”

The APOLLO instrument measures the round-trip travel time of laser pulses bounced off devices called lunar retroreflectors, which are positioned on the Moon’s surface. The time measurement of these pulses are within a few picoseconds (where one picosecond is equal to one-trillionth of a second), which corresponds to about one millimeter of distance between the Earth and the Moon.

In the experiment, the APOLLO will generate a pulse of light, using one billion watts of power, which will be sent to the Moon. The researchers will calculate the distance that the light pulse travels as it goes to the Moon, hits one of the lunar retroreflectors on the Moon’s surface, and then returns to Earth. The researchers state that, on average, only, 0.25 photon out of 30 million will actually be retrieved from its journey to the Moon. This number was achieved with the Apollo 15 retroreflector, which is larger in size than the other ones on the Moon.

The lunar retroreflectors were placed on the Moon by the Apollo 11, 14, and 15 astronauts. The Soviets also left behind lunar retroreflectors from their robotic missions called Lunokhod 1 and Lunokhod 2.

The Moon is approximately 239,000 miles (384,600 kilometers) away from the Earth—about 225,600 miles (363,104 kilometers) at its closest point (perigee) and 252,100 miles (405,696 kilometers) at its furthest point (apogee).

The actual measurements of the study, down to the nearest millimeter, were not released. (That is, I could not find them.)

Such accuracy is predicted to help test for basic properties of gravity under Albert Einstein’s theory of general relativity. One such test involves Einstein’s equivalence principle (equivalence of gravity and inertial mass), which states that bodies with different physical compositions will accelerate at the same rate in a gravity field. Another test to be conducted based on this study is whether or not the force of gravity becomes less as the universe expands. Results of such tests could profoundly effect our understanding of the universe and our small little volume within it.