The Apollo Landing Sites Pose a Danger to NASA's New Orbiter
While conspiracy nuts debate the reality of the Apollo landings, scientists must deal with some practical consequences of what astronauts put on the Moon. For the new Lunar Orbiter Laser Altimeter (LOLA), this means dodging the retro reflective mirrors mounted by astronauts at some of the landing sites.
The Apollo astronauts installed arrays of prisms at some landing sites, designed to bounce laser light directly back in the direction it came from. They reflect a signal, proportional to 1/R**2 instead of 1/R**4, where R is distance. These devices are still in use today, to monitor exact motion of the Moon and test physical theories.
The LRO carries a highly accurate laser altimeter, similar to the one installed on recent American Mars orbiters. It occured to me that this instrument might also verify the existance of the Apollo landing sites (as the LRO cameras have already done), but then it also occured to me that it might return a laser signal so strong that it could damage the instrument.
I wrote to David E. Smith at Goddard, the principal investigator for LOLA, to ask about this. He replied that this was most definitely a problem, and the LOLA instrument switches off when the orbiter passes over Apollo sites. If by small chance, the beam did strike the retro reflector, the light bounced back would be 1000 times the detector damage threshold!
The Russians have also been helpful in giving the LOLA team the best known locations for the two Lunokhod rovers, which also have laser retro reflectors mounted on them. Lunokhod-2 has been located precisely and is routinely probed by lasers from Earth. Lunokhod-1 has never been found by laser, and it is not known for certain if its reflector is deployed. Seen in the photo above, we can see the relfector (extended to the left) bouncing back the light from the camera's flash.
The Apollo astronauts installed arrays of prisms at some landing sites, designed to bounce laser light directly back in the direction it came from. They reflect a signal, proportional to 1/R**2 instead of 1/R**4, where R is distance. These devices are still in use today, to monitor exact motion of the Moon and test physical theories.
The LRO carries a highly accurate laser altimeter, similar to the one installed on recent American Mars orbiters. It occured to me that this instrument might also verify the existance of the Apollo landing sites (as the LRO cameras have already done), but then it also occured to me that it might return a laser signal so strong that it could damage the instrument.
I wrote to David E. Smith at Goddard, the principal investigator for LOLA, to ask about this. He replied that this was most definitely a problem, and the LOLA instrument switches off when the orbiter passes over Apollo sites. If by small chance, the beam did strike the retro reflector, the light bounced back would be 1000 times the detector damage threshold!
The Russians have also been helpful in giving the LOLA team the best known locations for the two Lunokhod rovers, which also have laser retro reflectors mounted on them. Lunokhod-2 has been located precisely and is routinely probed by lasers from Earth. Lunokhod-1 has never been found by laser, and it is not known for certain if its reflector is deployed. Seen in the photo above, we can see the relfector (extended to the left) bouncing back the light from the camera's flash.
posted by DonPMitchell at 11:23 PM
7 Comments:
This hadn't occurred to me, but now that you mention it first I can see that it would indeed be a problem. I'm glad they thought of it before the fact!
Does the laser altimeter always point straight down? The A11, 14 and 15 sites are close enough to the center of the near face of the moon to be near horizontal. But if the altimeter were to look at an oblique angle beyond the limits of the reflectors, the returns might be at a safe amplitude.
What about specular reflections from other materials at the Apollo sites? Sheet metal and metallized mylar, especially OSR (optical solar reflector) could be a problem. The ALSEP central stations look like they were covered with it.
Specular materials are not a problem, they still return a signal that is order of 1/R**4 in power -- you get a 1/R**2 loss going from the orbiter to the surface, and the reflection is a spherical wave that falls off by another factor of 1/R**2 returning to the orbiter.
The prisms are bad news because they bounce all the light back in the same direction, like a flat mirror pointed perfectly directly at the spacecraft, so you get a signal more like 1/(2R)**2.
I'm not sure I agree with the corner reflector turning it into inverse square. You still have inverse square going to the reflector and inverse square coming back. The greater the distance, the smaller the fraction of outgoing light that's captured by the corner reflector, and the smaller the fraction of the returned light that's captured by the sender. The corner reflector just increases the coefficient on the inverse r^4 equation.
It's equivalent more or less to a flat mirror aimed directly at the orbiter. Which means the return light is coming from a virtual point source at a distance of 2R, making the return strength propotional to 1/(2R)**2.
Even if it were a flat mirror, the loss would still be O(1/r^4), not O(1/r^2). A flat mirror of a fixed size will pick up less of the total radiated power as distance increases, and a smaller fraction of the power it does reflect will be picked up back at the source.
All the corner reflector does is greatly increase the reflection coefficient. (It also allows the reflection to be coherent).
Corner reflectors are commonly used on boats and aircraft to help reflect radars. I've never heard of them changing the standard 1/r^4 radar equation. They just create a very large radar cross section with a small device.
Since you initially wrote this, Lunokhod 1 has been located (interestingly, by looking at LRO pictures) and laser signals have been detected on earth after being bounced off its laser reflector, so its reflector is most definitely deployed and aimed toward earth. In fact, it seems to be in the best shape of any of the reflectors on the moon, as it reflects more of the received signal. See http://www.astronomy.com/asy/default.aspx?c=a&id=9804.
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