Present Scientific Achievements from Lunar Laser Ranging

Abstract

For nearly seven years McDonald Observatory in Texas has measured the round trip time of flight for laser pulses to the retroreflector arrays on the Moon. The typical accuracy, expressed as a one-way distance, is 13 cm and the best accuracy is 8 cm. Several groups have successfully analyzed this data and their results are reviewed here. Typical of these fits is an analysis of over 1700 ranges resulting in an rms residual of 44 cm, the difference from the instrumental accuracy being due to unmodeled effects in the fits. These analyses allow the determination of a number of parameters of the Earth-Moon system. For example the analyses have yielded accuracies of 1 m in the geocentric position of McDonald, 25 m in the three dimensional position of the Moon and the selenocentric coordinates of the retroreflectors, and 2 parts in 10⁷ in the gravitational constant of the Earth-Moon system. The program of range measurements at the Crimean Observatory has permitted the determination of that observatory’s coordinates and the first intercontinental baseline using lunar laser ranging. The laser ranging data is sensitive to the relative orientation of the equatorial, ecliptic, and lunar orbital planes, so that it is possible to set up a dynamical equinox. The laser ranging data, alone and in combination with very long baseline interferometry (VLBI) data on ALSEP transmitters, has been used to measure the lunar physical librations. Such measurements have yielded very accurate values of the lunar fractional moment of inertia parameters β and ϒ and accurate values for several of the third degree harmonics. The coordinates of the retroreflectors, along with those obtained on the ALSEP transmitters using VLBI, are being used as control points in lunar cartography. A test of gravitational theories, the Nordtvedt effect, yields a result consistent with Einstein’s general theory. Values of the rotation of the Earth (UTO) have been determined for more than 300 individual days with typical accuracies of 0.7 ms (30 cm) and best values half this. If the rotation of the Earth is corrected for the above days on which it can be determined from the data then the remaining rms residual for those days is 30 cm. Lunar laser ranging has demonstrated itself to be a powerful tool for astronomical and geophysical measurements in the Earth-Moon system.