Increasingly the trend at LLR stations has been toward narrower laser pulses and greater accuracy (Fig. The fourth filter was mathematical to neutralize noise in the photodetector.'

The third filter was spectral - we fired red light from the ruby laser or green from neodymium and looked for those wavelengths coming back in. Second, we used a pinhole aperture only a few arc seconds in diameter to look at a very small part of the surface of the moon. 'First, we opened the detector to the sky just before we thought the photons were due back-about 2.5 seconds later. 'We devised a four-filter system,' says Peter Shelus, senior research scientist at UT-Austin's Center for Space Research and member of the MLRS team. 3) and only about 10 or so returned to the photodetector, so being able to verify them was absolutely essential. MLRS would fire 10 14 or 10 15 photons to the moon (Fig. In the early days one of the biggest challenges was discerning returning photons from 'stray' photons. Lunar Reconnaissance Orbiter image of Apollo 11 landing site, including LLR. After 15 years of operation the 2.7-m system was replaced by a dedicated 0.76-m system, built around a frequency-doubled Nd:YAG laser that produces LLR data approaching 1 cm normal point accuracy.įigure 2. The 2.7-m system, using a Korad ruby laser, routinely produced normal point data with an accuracy of 10-15 cm. McDonald Observatory was the premier LLR station during the 1970s and early 1980s. (After 40 years of continuous lunar laser ranging, MLRS lost its annual NSF funding of $125,000, the agency's reasons being the older equipment and the existence of more modern LLR stations that carry out the same work with greater accuracy-namely Apache Point in New Mexico and Observatoire de la Côte d'Azur in France.) Much of this work has been done at McDonald Laser Ranging Station (MLRS) at McDonald Observatory, operated by the University of Texas-Austin. The speed of rotation of the earth and moon, as well as their axial and orbital variations, can also be determined. The average distance from the centers of the earth and the moon is 238,897 miles (384,467 km) - measured with a precision equivalent to determining the distance between Los Angeles and New York to 1/100 of an inch. Some calculations using this data, such as the distance between the earth and moon, are straightforward. Laser Ranging Retroreflector Experiment (LRRR), part of the ALSEP (Apollo Lunar Surface Experiment Package), left by Apollo 15. The arrays left on the moon were used continuously in the lunar laser ranging, or LLR experiment.įigure 1. Corner mirrors are important scientific instruments because, when struck precisely by a laser beam, they reflect the beam in a parallel path back to the source of the laser.

Five 'corner mirrors' or 'retro-reflector arrays' were taken to the moon in 1969 and the early 1970s (Figures 1 and 2). Lunar reflectors left by Apollo astronauts have been used continuously since 1969 to study the earth-moon system and test the validity of the laws of physics.

Reflector on moon