The Big Think

March 5, 2010

Still Doing Science

Filed under: Space/Astronomy — jasony @ 8:55 am

During the heyday of the Apollo missions, astronauts put smallish reflectors on the surface of the moon.


About the size of a large pizza box and covered in prisms, these reflectors have been used by Earthbound scientists for scientific study of the moon ever since. These scientists fire very precise lasers at the reflectors and measure the amount of time it takes for the reflections to return. Since they know the speed of light with extreme accuracy (and have very accurate clocks), they can measure the time it takes the reflection to return and thus determine how far away the moon is.

Did you know that the moon is moving away from us? True thing. Our celestial neighbor is spiraling away at a rate of about 38mm per year, or approximately the same rate as the continents drift or your thumbnail grows. Neat, huh? Or how about this for accuracy: the laser that scientists shoot at the moon is powerful, but the reflectors on the surface are so far away that they don’t catch all of the light that hits it on the rebound. Of the 10,000,000,000,000,000 photons (10 quadrillion) that leave the laser, only one will return, and that’s on a good day. The detectors have to be very accurate to pick up the reflection.

A big mystery that scientists are trying to solve about the reflectors is just why, after over three decades, are the mirrors becoming so much less efficient? Their reflectivity has dropped by an order of magnitude (10x), and, during a full moon, they drop an additional order of magnitude. The reflectors are as much as 100x less reflective today than they were when they were placed on the surface. In addition, during a total lunar eclipse (full moon going into the shadow of the Earth), the reflectors’ efficiency returns to normal levels (which eliminates any explanation of lunar dust on the lenses). Strangely, this effect didn’t happen when the reflectors were first insalled. Why? We don’t know, but there are theories.

It’s a bizarre mystery that tells us there are still many things left for us to lear about “that boring place we’ve already been”.

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