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Monday, April 26, 2010

Dimming moon reflectors

Forty years ago, Apollo astronauts Neil Armstrong and Buzz Aldrin left ‘lunar laser ranging retroreflector arrays’ in precise locations on the moon. The arrays contain glass prisms called ‘corner-cube reflectors’, a type of mirror that always reflects incoming light straight back to the original source. The prisms have been used for a variety of purposes, most notably to give us an accurate measurement of the distance from the Earth to the moon.

Caption: Apollo astronauts left arrays of corner-cube prisms, like the one shown here, on the moon 40 years ago. Now light bouncing back from these reflectors appears to have dimmed. Scientist think dust is to blame.

Credit: UC San Diego

In recent years, the amount of light coming back from the reflectors has decreased significantly. According to Tom Murphy of the University of California San Diego, we’re now receiving only one tenth as much light as we should. The possible culprit? Moon dust.

Under the best conditions, astronomers can only expect to intercept one photon out of every 100 million billion sent to the moon from earthbound telescopes. Much of the outgoing light never hits the lunar reflectors, having been scattered by Earth’s atmosphere on the way out, and the photons that do arrive on target are dispersed over a wide area on the way back. Still that small amount of light has yielded some valuable information. For example, it turns out that moon is receding away form the earth by just under four centimeters per year.

The 2.7 meter Harlan J. Smith Telescope of the McDonal Observatory (USA) is used to point a laser beam to a mirror stationed on the surface of the moon.

The astronomers aren’t sure exactly what is causing the low return problem, but they suspect that dust is involved. Not so much because the surface of the reflectors are dirty, but because the dust causes uneven heating of the glass prisms, affecting the returning laser pulses. How is the dust accumulating on an atmosphereless, windless moon? Small amounts of dust could be attracted to the prisms by electrostatic forces. Alternatively, micrometeorites landing nearby could be kicking up dust.

If the problem is one of uneven heating, that should be made clear by making observations during a lunar eclipse. While the moon is in Earth’s shadow, the reflectors won’t be heated by the sun. The astronomers are waiting for a fair weather eclipse to test their theories.