Apollo Mission Corner Cubes

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The Apollo mission astronauts left corner cube reflectors on the Moon. These corner cubes are used for lunar laser ranging, which provides one of the most precise tests of the universality of free fall. Laser light sent from the Earth is reflected by the corner cubes back to the Earth, allowing scientists to measure the time of flight of the light beam to the Moon and back. From the time of flight, the distance between the Earth and Moon is measured with millimeter accuracy. The top photograph shows the placement of a panel of reflectors on the Moon. The lower-left photograph shows individual reflectors, while the lower-right photograph shows a panel of reflectors as mounted on the surface of the Moon. (Unit: 3)