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NMSU's Apache Point Observatory uses 3.5-meter telescope to find long-lost laser reflector

SUNSPOT, N.M. - New Mexico State University's Apache Point Observatory has been instrumental in solving a nearly 40-year-old lunar mystery.

This is what the Lunokhod rover released on the moon looked like in 1970. Nearly 40 years later, scientists have been able to locate the rover and its retroreflector, with the help of a telescope at Apache Point Observatory. (Courtesy of NASA)

On Nov. 10, 1970, the Russian unmanned Luna 17 mission landed on the moon, releasing a robotic rover, called Lunokhod 1, to roam the lunar surface. The rover carried an optical retroreflector, a device that reflects incident light precisely back in the direction from which it comes. The rover and its retroreflector were last heard from on Sept. 14, 1971.

"The scientists were not sure physically where the laser reflector was on the moon," said Kurt Anderson, director of Apache Point. "The device was effectively lost."

In March, NASA's Lunar Reconnaissance Orbiter (LRO) took new pictures of the moon's surface as part of a mission.

"The high-resolution pictures of the moon showed what looked like the rover," Anderson said. "It was actually miles away from where people thought it might be."

Now, physicists, led by Tom Murphy, an associate professor at the University of California San Diego, have detected the lost reflector using Apache Point's 3.5-meter telescope.

The 3.5-meter telescope at Apache Point is a general-purpose instrument that studies a variety of astronomical objects from planets in our own solar system to more distant galaxies. It is used with a number of spectrographs and imaging devices to make observations at optical and infrared wavelengths.

The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) is a special project, which is using the telescope to test Albert Einstein's General Theory of Relativity, Anderson said. General relativity predicts how mass and energy give rise to gravitation and how objects should move under the influence of gravity. For the APOLLO project, physicists are using the telescope to bounce laser pulses off lunar reflectors on the moon to measure the round-trip travel time to within a few picoseconds and the instantaneous distance of the moon to within about a millimeter. With this information, scientists can determine the relative accelerations of the Earth and moon toward the sun. This will provide a test of the equivalence principle upon which Einstein's General Theory of Relativity rests.

Anderson said that once the scientists had a better position from the LRO images to hit their target with laser pulses, they were able to pinpoint its exact location to within 10 meters.

As for the fate of the decades-old reflector and rover, Anderson said there are no plans to retrieve the device. Since its position is now known, it can now also be used to test Einstein's theory of gravity.

"This was a serendipitous discovery," Anderson said. "It enables us to help the APOLLO project be more successful."