Lunar Laser Ranging: A Continuing Legacy of the Apollo Program

Abstract

On 21 July 1969, during the first manned lunar mission, Apollo 11, the first retroreflector array was placed on the moon, enabling highly accurate measurements of the Earthmoon separation by means of laser ranging. Lunar laser ranging (LLR) turns the Earthmoon system into a laboratory for a broad range of investigations, including astronomy, lunar science, gravitational physics, geodesy, and geodynamics. Contributions from LLR include the three-orders-of-magnitude improvement in accuracy in the lunar ephemeris, a several-orders-of-magnitude improvement in the measurement of the variations in the moon's rotation, and the verification of the principle of equivalence for massive bodies with unprecedented accuracy. Lunar laser ranging analysis has provided measurements of the Earth's precession, the moon's tidal acceleration, and lunar rotational dissipation. These scientific results, current technological developments, and prospects for the future are discussed here.

Precambrian length of day and the validity of tidal rhythmite paleotidal values

“…Indeed, both interpretations of the Weeli Wolli rhythmites avert a close approach since 4.5 Ga. The present high rate of lunar retreat of 3.82 + 0.07 cm/yr [Dickey et al, 1994] may imply a near-resonance of the oceans [Brosche, 1984]. …”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…Lunar Laser Ranging, in which the timing of laser pulses bounced off the Moon is measured, has provided very precise measurements of the rate of the Moon's recession from Earth (Dickey et al, 1994). If this recession is due only to tidal dissipation within Earth, then Earth's tidal quality factor is Q Earth = 12 (Yoder, 1995).…”

“…The difficulty lies in the complexity of the energy dissipation processes and the very long timescales associated with tidal evolution. The Earth-Moon system is the most accurately studied, with lunar laser ranging providing a precise measurement of the recession of the Moon due to tides of *38 mm/yr (Dickey et al, 1994), as well as direct measurements of the locations of dissipation in the ocean (Egbert and Ray, 2000). However, using the currently estimated tidal dissipation parameters to extrapolate backward in time predicts that the Moon was at Earth's surface about *2 Gyr ago (MacDonald, 1964), which contradicts the standard impact model for the lunar origin [note that numerous issues remain such as the origin of Earth's and the Moon's obliquities (Touma and Wisdom, 1994) and the perturbations from other planets (Ć uk, 2007)].…”

Relativistic Effects in the Motion of the Moon

“…For instance, the post-fit residuals in the Earth-Moon distance are at the centimeter level [2,3]. Simple theoretical estimates lead to the conclusion that the main relativistic effects in the lunar theory are due to the spin-orbit coupling of the Earth-Moon system in the gravitational field of the Sun.…”

“…The lunar laser ranging experiment has opened up the possibility of measuring relativistic effects in the motion of the Moon; indeed, the agreement between the standard general relativistic model that contains over a hundred model parameters and the ranging data accumulated over the past three decades is excellent [2,3]. For instance, the post-fit residuals in the Earth-Moon distance are at the centimeter level [2,3].…”