Scientists believe lunar dust heated by the sun is degrading the performance of the Apollo reflector arrays and could explain a strange phenomenon that occurs during a full moon.
The findings may have implications for future missions to the moon, including lunar-based telescopes.
The reflectors were placed on the lunar surface during the Apollo 11, 14 and 15 missions, as well as the Soviet Lunakhod 1 and 2 robotic missions.
Since 1969 astronomers have been firing lasers at these reflectors, timing how long it takes the photons to get back.
Of the 100 thousand million million photons fired out in each laser pulse, on average only one makes it back — and then only if clouds and other airborne particles aren't in the way.
These laser pulses have provided a wealth of data, including confirmation that the moon is spiralling away from Earth at 38 millimetres per year.
But during the past four decades, harsh conditions on the lunar surface have affected the performance of the reflector arrays.
Originally, the Lunakhod reflectors were 25 per cent stronger than the best of the Apollo reflectors. Today they are 10 times worse, with Lunakhod 1 not reflecting at all.
But the biggest mystery for researchers is why does their reflectivity drop by a factor of 10 during a full moon.
In a paper published on the electronic preprint website arXiv and accepted for publication in the journal Icarus, scientists led by Tom Murphy from the University of California, San Diego, say material settling on the reflectors is reducing their efficiency.
"Dust is perhaps the most likely candidate for the observed degradation," they write.
The researchers also identified micrometeorite damage and the breakdown of Teflon mountings, which may have left deposits on the back of the reflector.
The clue to solving the full Moon mystery came out of observations taken during total lunar eclipses.
They found that within 15 minutes of an eclipse occurring, the efficiency of the reflectors returned to its normal levels. When the eclipse ended, the efficiency immediately dropped.
Murphy and colleagues believe dust on the reflectors heats up during a full moon, causing unintended thermal effects, which distort their shape.
They say a four-degree temperature difference across the reflector is enough to reduce its efficiency by a factor of 10.
The researchers say the findings are important for future missions to the moon.
"The evidence for substantially worsened performance of the lunar reflectors over time makes it important to consider the long-term usefulness of next-generation devices proposed for the lunar surface," they write.
"The results could impact the designs of a wide variety of space hardware — especially next-generation laser ranging reflectors, telescopes, optical communication devices, or equipment dependent on passive thermal control."