The internal structure of the Moon influences its movements of rotation and revolution. Fifty years of laser-moon telemetry measurements made possible by Apollo missions have now allowed a new determination of this structure with a precision never before reached for the radius of its core.
The success of the lunar missions, which began 50 years ago with Apollo 11, has led to the scientific and technological revolutions of which we are the heirs and we continue to make the benefits grow. The Apollo program, of course, helped prepare for the electronics and computer revolution of the 1970s and 1980s, and it inspired a generation of teenagers to become later engineers and scientists in the late 20th century. But it also provided valuable data that made possible leaps in comparative planetology.
Indeed, the lunar rocks reported by the Apollo missions have placed constraints on the history of the Solar System by allowing in particular to date the land of our satellite and to link the rate of their cratering to their ages, which made it possible to likewise elsewhere in the Solar System. These rocks also led us to the scenario of the giant impact between the young Earth and Theia, explaining the genesis of the Moon.
There are other legacies of Apollo that are less known but equally exciting. Thus, Apollo 11 missions, 14 and 15 have been an opportunity to drop optical devices on the lunar surface that are capable of returning in the same direction of beam light incidents. The Russians will do the same with the Luna 17 and Luna 21 missions that will drop the Lunokhod 1 and Lunokhod 2 rovers, also equipped with retroreflectors.
Fifty years of laser-moon telemetry with Apollo
The laser revolution of the early 1960s provided precisely what was needed as a light source to perform laser-lunar telemetry (or Lunar Laser Ranging Experiment, LLRE), to measure round-trip times for missions of the lunar retroreflectors to make very precise measurements of the Earth-Moon distance and help measure its movements in orbit around the Earth.
Tens of thousands of rounds of laser beams have been made since 1969 from five stations on Earth, such as those of the Observatory of the Côte d’Azur in France, or that of the Observatory Apache Point, at New Mexico (United States). The results obtained allowed constraints to be put on alternatives to the theory of general relativity, including one involving a violation of the assumptions underlying the relativistic theory of gravitation of Einstein.
But the lessons that can be drawn from the data accumulated over the past 50 years concerning laser-moon telemetry do not stop in search of new physics , as the publication of an article in Geophysical Research Letters once again shows, also available on arXiv. This paper presents the results of a team of researchers from the Paris-PSL Observatory, the Côte d’Azur Observatory, CNRS and Sorbonne University.
A look into the paper
The researchers explain that they have come to a new determination of the shape and especially the size of the Moon’s nucleus. It turns out that the equations describing the rotation of the Moon on itself and its orbital movements are dependent on the internal structure of our satellite. We can, therefore, solve what we call an inverse problem in mathematical physics.
Thus, as early as the 1980s, it was concluded that the Moon had probably been the site of a planetary differentiation, as in the case of the Earth, and that it therefore possessed a nucleus, some of which is liquid. This conclusion was reinforced by the seismic data provided by the instruments, also deposited by the Apollo missions.
There remained, however, an uncertainty of about plus or minus 55 kilometers on the size of this nucleus. Planetologists, however, have just reported that they were able to reduce it to obtain a value of ± 12 kilometers with a radius of the nucleus of the Moon determined to 381 kilometers.