Catching Photons Coming from the Moon 146
Roland Piquepaille writes "In 'Shooting the moon,' the San Diego Union-Tribune describes how and why physicists from UCSD are using lasers to send light pulses in direction of an array of reflectors installed on our moon in 1969 by Neil Armstrong and Buzz Aldrin. One of the goals of these experiments is to check the validity of Einstein's theory of general relativity. Another one is to measure the distance between the Earth and moon with a precision of one millimeter by catching photons after their round trip to the moon. But it is amazing to realize how difficult it is to capture photons after such a trip. I also have up a summary, which contains additional details and pictures, if you just want to learn how difficult it is to capture photons back from the moon."
Keep experiments running (Score:4, Interesting)
The LR^3 retroreflector featured here was part of the ALSEP station on several of the apollo missions. In the years since these missions the ALSEP stations have been shut down. The reflectors are passive devices and don't have an off switch, which is why they are still working.
In particular the seismonitors which were a part of each system could still be operating today, and delivering new scientific results.
I think this article is an example of why experiments should not be shut down before they really stop working.
Re:Mirrors? On the moon! (Score:4, Interesting)
The Hatter idea is that no living thing can escape the atmosphere and survive (due to radiation [wikipedia.org] or whatever reason the Hatters claim). And no, the Space Station and the sattelites are technically inside the atmosphere, well below the Van Allen belt.
The reflector delivery and the soil sample return could be done by a robotic probe, which in fact is what the Russians did with their Luna 16 [wikipedia.org] mission about a year after the alleged Apollo 11.
Since the Russkies got the first sattelite (Sputnik), the first man in orbit, the first suit walk, the first docking, etc., the thinking was that we could sound-stage their glorious defeat, end the darn space-race, then go spend the money on something more profitable [wikipedia.org].
The credible conspiracy theory: Send the humans into orbit, camp in orbit while the robot [nasa.gov] fetches the samples, reunite humans with the 'bot, then land as heroes.
Wouldn't... (Score:1, Interesting)
Quite far from difficult, can be done by almanyone (Score:4, Interesting)
What's difficult is doing it with nanosecond resolution. That requires very wide bandwith antennas and receivers, which also let in a lot of wide band background noise.
NASA laser ranging history (Score:2, Interesting)
In 1980 I visited the MOBLAS-5 Yarragadee [nasa.gov] station in the Western Australia outback, which was custom-built for this purpose. MOBLAS meant MOBile LASer, and as you can see from the picture it is built in a trailer. But the equipment does not move. The site has clear night skies and no geological or human interference. And results from one location over time are valuable. It was beautiful and eerie to see the green laser beam. Even though the air was clear you could see it when standing around the pad. It seemed to last longer than the tiny fraction of a second it pulsed. At the time the operators would get a visual feedback from their instruments on who big the return signal was. And they would tweak the telescope tracking as required to get good returns. Tapes of the results were then sent back to NASA. I assume this is all automated now. Those guys looked extremely bored.
BTW, notice how big the empty tarmac is around the trailer. I was told NASA wanted a 100 by 100 foot pad and they mistakenly got a 100 by 100 metre pad, making it about 10 times bigger than they needed!