A joint Fermilab/SLAC publication

A long-lost object on the Moon will help test general relativity


In 1971, a Soviet moon lander called Lunokhod 1 sent its last signal back to Earth. Since that time, scientists have been keeping an eye out for it but not had any luck. Now, says a press release from the University of California, San Diego, the lander has been found, and a simple but important piece of cargo on it is intact. Different teams contributed to the process with an Arizona State University team analyzing images from NASA Lunar Reconnaissance Orbiter. The San Diego team went the next step to find a key object.

The object sought is a corner reflector. It's just three mirrors at right angles to each other forming a corner. The beauty of this device is that it reflects any light that hits it back along the path it came from, no matter which direction it came from. That means you can just put one in place, not worry too much about its orientation, and know that it will reflect back to you. I've made them myself using a few foot-square mirror tiles from IKEA that cost about $1 each, and it works really well and is definitely worth playing with for the unusual sight of yourself in a mirror the way other people see you, not seeming flipped left to right. The ones on the Moon aren't actually three mirrors mounted that way but a transparent prism, with the surfaces well polished and acting as mirrors.

There are a few sets of corner reflectors on the Moon. Three were left by Apollo missions, and there is another on Lunokhod 2. Scientists routinely send laser pulses to these corner reflectors knowing that they will come back to the source. By timing the pulses, they can work out the distance to the Moon with astonishing precision--potentially to the nearest millimeter. By using three corner reflectors, scientists can work out the orientation of the Moon precisely, and a fourth can give information on the tidal distortions of the Moon.

By piecing together all this information, physicists such as Tom Murphy and his colleagues from UCSD, who found the missing Soviet reflector, can test Einstein's General Theory of Relativity. They can compare the detailed orbit of the Moon with what theory predicts and see if it all checks out. It's a relatively cheap and simple way to do an experiment that gives nice results. Along the way, lunar geologists can learn more about the structure and changes of the Moon.

Finding the additional corner reflector has extra importance at this time, as the same physicists who found this old lander had concluded a few weeks ago that the known corner reflectors were started to be covered in moon dust, degrading their usefulness.

Spend a few minutes to read the intriguing and fun story of how they found the lander.

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