A long lost light reflector that was left on the surface of the moon by the former Soviet Union has been rediscovered by a team of American physicists after nearly 40 years using lasers beamed from Earth.
The French-built laser reflector was sent aboard the unmanned Soviet Luna 17 mission, which landed on ?the moon on Nov. 17, 1970 and released a robotic rover that roamed the lunar surface and carried the sought after laser reflector.
The Soviet lander and its rover, called Lunokhod 1, were last heard from on Sept. 14, 1971.
"No one had seen the reflector since 1971," said Tom Murphy, an associate professor of physics at the University of California San Diego. Murphy leads a team of scientists in a long-term effort to use laser reflectors to measure the shape of the lunar orbit and look for deviations in Einstein's Theory of General Relativity.
"We routinely use the three hardy reflectors placed on the moon by the Apollo 11, 14 and 15 missions, and occasionally the Soviet-landed Lunokhod 2 reflector ? though it does not work well enough to use when illuminated by sunlight," Murphy said. "But we yearned to find Lunokhod 1."
Murphy and his team occasionally looked for the Lunokhod 1 reflector over the last two years, but had no luck finding it until recently.
The breakthrough came last month, when the high-resolution camera on NASA's Lunar Reconnaissance Orbiter (LRO) obtained images of the landing site. The camera team identified the rover as a sunlit speck on the image, which turned out to be miles away from where Murphy and his team had been searching. Until now, the precise location of the rover's reflector had been unknown.
On April 22, Murphy's team sent pulses of laser light from the 3.5 meter telescope at the Apache Point Observatory in New Mexico, zeroing in on the target coordinates provided by the LRO images.
The team was able to find the long lost Lunokhod 1 reflector and pinpointed its distance from Earth to within one centimeter (0.4 inches). They then made a second observation less than 30 minutes later that allowed the team to triangulate the reflector's latitude and longitude on the moon to within 10 meters (0.006 miles).
In the coming months, it will be possible for scientists to establish the reflector's coordinates with even greater precision, Murphy said.
The return signal from the reflector was measured as a collection of photons, or individual particles of laser light.
"We quickly verified the signal to be real and found it to be surprisingly bright: at least five times brighter than the other Soviet reflector, on the Lunokhod 2 rover, to which we routinely send laser pulses," Murphy said. "The best signal we've seen from Lunokhod 2 in several years of effort is 750 return photons, but we got about 2,000 photons from Lunokhod 1 on our first try. It's got a lot to say after almost 40 years of silence."
Prior to this discovery, many scientists had speculated that the Lunokhod 1 rover might have fallen into a crater or parked badly, with its reflector not facing Earth, which would have prevented it from being located by laser pulses.
"Not only now do we know that Lunokhod 1 is there, we also know that it is parked perfectly so that its reflector faces Earth," Murphy explained. "In fact, the signal is so surprisingly strong that the rover could not be in anything but a level parking spot with its commanded roll on the lunar surface deliberately oriented toward the Earth."
The discovery of Lunokhod 1 will greatly improve upon studies of the moon and its composition.
"Lunokhod 1, by virtue of its location, would provide the best leverage for understanding the liquid lunar core, and for producing an accurate estimate of the position of the center of the moon ? which is of paramount importance in mapping out the orbit and putting Einstein's gravity to a test," Murphy stated.
Murphy and his colleagues found in a study published this month that lunar dust may be obscuring the reflectors on the moon. The researchers found that the laser light they bounce off reflectors on the lunar surface is fainter than expected and dims even more whenever the moon is full.
"Near full moon, the strength of the returning light decreases by a factor of ten," Murphy said. "We need to understand what is causing this if we are contemplating putting additional scientific equipment on the moon. Finding the Lunokhod 1 reflector will add important clues to this study."
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