Lunar Laser Ranging: A Continuing Legacy of the Apollo Program (Dickey et al., 1994)
- Original: J. O. Dickey, P. L. Bender, J. E. Faller, X X Newhall, R. L. Ricklefs, J. G. Ries, P. J. Shelus, C. Veillet, A. L. Whipple, J. R. Wiant, J. G. Williams, and C. F. Yoder, “Lunar Laser Ranging: A Continuing Legacy of the Apollo Program,” Science 265 (5171), 482–490, July 22, 1994. DOI 10.1126/science.265.5171.482.
- Rights: © AAAS — not redistributed in this library; cited by DOI (above). See the copyright note in the Bender 1973 note.
Summary
Section titled “Summary”The 25-year retrospective on lunar laser ranging (LLR), opening from the moment “on 21 July 1969 … the first retroreflector array was placed on the moon” by Apollo 11. It is the citable statement of what the LR³ and its successors delivered: LLR “turns the Earth-moon system into a laboratory” for astronomy, lunar science, gravitational physics, geodesy, and geodynamics — with the lunar ephemeris improved by three orders of magnitude, the equivalence principle verified for massive bodies, and the Moon’s tidal recession measured directly.
Key takeaways
Section titled “Key takeaways”- The data set: ~8,300 normal-point ranges, 1969 – December 1993, from McDonald Observatory (the 2.7-m, later the dedicated MLRS), Haleakalā (Hawaii), and CERGA/Grasse (France); modern stations use Nd:YAG lasers with ~100–200 ps pulses, and accuracies progressed from a few meters to the 2–3 cm level.
- The Moon is receding at 3.82 ± 0.07 cm/yr. Tidal dissipation in the Earth drives a secular acceleration of the lunar longitude of −25.88 ± 0.5 arcsec/century², measured by LLR as an increase in the semimajor axis of 3.82 ± 0.07 cm per year — the headline geophysical number.
- Gravitational physics: ranging verifies the equivalence principle for massive bodies (Earth and Moon fall alike toward the Sun; Nordtvedt test) with the strong-equivalence parameter β_G = 0.9999 ± 0.0006, and constrains changes in the gravitational constant.
- Precision constants: Earth+Moon/Moon mass ratio 328 900.560 ± 0.002; GM(Earth+Moon) = 398 600.443 ± 0.004 km³/s²; lunar Love number and dissipation measurements hint at a small liquid lunar core.
- Geodesy/geodynamics: Earth-rotation variations, precession-nutation corrections, station motions consistent with plate tectonics — the experiment doubles as an Earth-science instrument.
- Why it endures: the reflectors are passive and remain usable decades after deployment — the durable, citable basis for calling the Apollo 11 array a permanent geophysical benchmark (still ranged today by successors such as APOLLO at Apache Point).