page_title: Retroreflector
section_title: On the Moon
hierarchical_section_title: Retroreflector / Applications / In space / On the Moon
caption_attribution_description: Lunar Laser Ranging Experiment with the stereo camera in the background (NASA image number AS11-40-5952). This Retroreflector was left on the Moon by astronauts on the Apollo 11 mission. Astronomers all over the world have reflected laser light off the reflectors to measure precisely the Earth-Moon distance.
context_page_description: A retroreflector is a device or surface that reflects radiation back to its source with minimum scattering. This works at a wide range of angle of incidence, unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence. Being directed, the retroflector's reflection is brighter than that of a diffuse reflector. Corner reflectors, and Cat’s eye reflectors are the most used kinds.
context_section_description: Astronauts on the Apollo 11, 14, and 15 missions left retroreflectors on the Moon as part of the Lunar Laser Ranging Experiment. The Soviet Lunokhod 1 and Lunokhod 2 rovers also carried smaller arrays. Reflected signals were initially received from Lunokhod 1, but no return signals were detected from 1971 until 2010, at least in part due to some uncertainty in its location on the Moon.  In 2010, it was found in Lunar Reconnaissance Orbiter photographs and the retroreflectors have been used again.  Lunokhod 2's array continues to return signals to Earth. Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally occurring photons challenging.
caption: The Apollo 11 Lunar Laser Ranging Experiment
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