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Apollo 11 Experiment Still Going Strong after 35 Years
http://www.jpl.nasa.gov/news/features.cfm?feature=605
Apollo 11 Experiment Still Going Strong after 35 Years Jet Propulsion Laboratory Media contact: Charli Schuler (818) 393-5467 July 20, 2004 It was the summer of '69. Director John Schlesinger's "Midnight Cowboy" had won the Oscar for Best Picture; the Rolling Stones' newly released "Honky Tonk Women" was climbing the charts; 400,000 people were gearing up to attend Woodstock?and America landed on the Moon, making "one giant leap for mankind." On the afternoon of July 20, 1969, Apollo 11 astronauts Neil Armstrong and Edwin "Buzz" Aldrin explored the surface of the Moon for two and a half hours, collecting samples and taking photographs while Michael Collins orbited in the command module Columbia. On July 21, about an hour before the end of their final moonwalk, they left an experiment on the lunar surface which, after 35 years, continues to work as well as it did the day it got there. Called the lunar laser ranging experiment, it studies the Earth-Moon system and returns data to scientific centers around the world, including NASA's Jet Propulsion Laboratory. "An accurate knowledge of the Moon's orbit and orientation is needed for future robotic and manned missions to our satellite," said Dr. James G. Williams, one of four JPL scientists who analyze the data from the Lunar Laser Ranging Experiment. "Scientists have been able to use the data they received through lunar laser ranging to study the Earth, the Moon and the character of gravity." Scientists from various institutions who analyze the data from the lunar laser ranging experiment have observed, among other things, that the Moon is moving away from the Earth and has a fluid core, and that Einstein's Theory of Relativity is accurate. The experiment consists of an instrument called the lunar laser ranging reflector, designed to reflect pulses of laser light fired from the Earth. The idea was to determine the round-trip travel time of a laser pulse from the Earth to the Moon and back again, thereby calculating the distance between the two bodies to unprecedented accuracy. Unlike the other scientific experiments left on the Moon, this reflector requires no power and is still functioning perfectly after 35 years. The Apollo 11 laser reflector consists of 100 fused silica half cubes, called corner cubes, mounted in a 46-centimeter (18-inch) square aluminum panel. Each corner cube is 3.8 centimeters (1.5 inches) in diameter. Corner cubes reflect a beam of light directly back toward its point of origin; it is this fact that also makes them so useful in Earth surveying. Three more reflectors have since been left on the Moon, including two by later Apollo 14 and 15 missions and one (built by the French) on the unmanned Soviet Lunokhod 2 rover. Each of the reflectors rests on the lunar surface in such a way that its flat face points toward the Earth. The McDonald Observatory in Western Texas and a second observatory near the city of Grasse in southern France regularly send a laser beam through an optical telescope to hit one of the reflectors. The reflectors are too small to be seen from Earth, so even when the beam is correctly aligned in the telescope, actually hitting a lunar reflector is quite challenging. At the Moon's surface, the beam is a few kilometers or miles wide and scientists liken the task of properly aiming the beam to using a rifle to hit a moving dime 3.2 kilometers (two miles) away. Once the laser beam hits a reflector, scientists at the observatories use sensitive filtering and amplification equipment to detect any return signal. The reflected light is too weak to be seen with the human eye, but under good conditions, one photon -- the fundamental particle of light -- will be received every few seconds. The lunar laser ranging experiment is the only lunar investigation continuously operating since the Apollo project. Improvements in lasers and electronics over the years have led to measurements currently accurate to about 2 centimeters (less than one inch). Scientists know the average distance between the centers of the Earth and the Moon is 385,000 kilometers (239,000 miles), implying that the modern lunar ranges have relative accuracies of better than one part in 10 billion. This level of accuracy represents one of the most precise distance measurements ever made and is equivalent to determining the distance between Los Angeles and New York to one hundredth of an inch. "Technical improvements at the observatories rejuvenate the lunar laser ranging effort," Williams said. "When the range accuracy improves, it is like getting a new experiment on the Moon." To this end, a new lunar ranging instrument with significantly improved accuracy is being constructed at Apache Point Observatory in New Mexico by the University of California at San Diego and the University of Washington. "The usefulness of continued improvements in range determinations for further advancing our understanding of the Earth-Moon system will keep the lunar reflectors in service for years to come," Williams said. |
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I have a couple of small question about the laser reflectors.
We know abouyt the corner prisms, and the reflectors are postioned to point at the earth. Was there any need for design tolerances so that the actual angle of the whole reflector assembly was optimised to point at the eart, or was this simply unneccessary? Did the astronauts simply position them in a "that looks close enough" way, or was there some method to the procedure? |
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Wally Anglesea wrote:
I have a couple of small question about the laser reflectors. We know abouyt the corner prisms, and the reflectors are postioned to point at the earth. Was there any need for design tolerances so that the actual angle of the whole reflector assembly was optimised to point at the eart, or was this simply unneccessary? Did the astronauts simply position them in a "that looks close enough" way, or was there some method to the procedure? The whole reason they used "Corner Cubes" is that "Looks close enough" works with them, any light falling onto them from a reasonable angle (margin of 20-30 degrees if I remember corrrectly). Besides, if they had to point extremely accurately, the experiment couldn't have kept on working, as the moon is continually wobbling, so much so that in a telescope you can easily see the perspective changing (see http://www.stargazing.net/david/moon/moonlibration.html ) which would turn the reflector a considerable ways away from where it was originally pointing. Bart |
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In article ,
Wally Anglesea wrote: Was there any need for design tolerances so that the actual angle of the whole reflector assembly was optimised to point at the eart, or was this simply unneccessary? Did the astronauts simply position them in a "that looks close enough" way, or was there some method to the procedure? There was a systematic procedure for getting the thing pointed roughly correctly (and likewise for the ALSEP radio antenna, which was also moderately directional), but no requirement that it be exact. Corner-cube reflectors reflect light back in the direction it came from, independent of which direction that is (within limits). -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
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"Ron" wrote in message om... At the Moon's surface, the beam is a few kilometers or miles wide and scientists liken the task of properly aiming the beam to using a rifle to hit a moving dime 3.2 kilometers (two miles) away. If your rifle has a duty cycle approaching that of a beam of light, you'll hit that dime soon enough |
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In sci.space.history Peter Munn wrote:
Yes, I thought diffraction would have such an effect. At the distance of the moon and with visible wavelengths, wouldn't you need a near- perfect reflector 10m to 20m across if you wanted a near-perfect return beam? (10-20m measured in wavelengths being approximately the square root of the total distance in wavelengths). And, correspondingly, isn't it misleading to think of an individual photon having been reflected by a particular corner-cube? Don't the wave characteristics of light mean that the places photons end up are a feature of an interference pattern from reflections off all the corner- cubes? "If it's not weird, it's not quantum". With wavefunction collapse, you could identify individual photons at the reflector, but only if you (say) detected some, thereby destroying them. Fortunately for those of us who get confused when both wave and particle guises play a role, reflection from flat surfaces gives the same geometric- optics results when worked out either way. Bill Keel |
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