Testing Gravity via Lunar Laser Ranging: APOLLO Analysis and Acquisition

University Of California-San Diego, La Jolla CA

Investigators

Abstract

The recently built Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) has become the premier lunar laser ranging (LLR) facility in the world, delivering one-millimeter range precision between Earth and Moon as a way to test General Relativity. General Relativity makes specific predictions about the shape of the lunar orbit, so that we may use the Moon as a laboratory, made possible by the reflectors left on the Moon by the Apollo astronauts and unmanned Soviet rovers. The goal of this work is to extend high-quality LLR observations over an additional three-year period, simultaneously collecting calibrated measurements of local gravity well enough to determine site displacements at the millimeter level. In tandem, an intensive effort will be initiated to advance the solar system model to a state that can take advantage of millimeter-quality data. It is by comparison to a complete and sophisticated model that questions relating to gravity, geophysics, and lunar physics can be explored. Though not the main focus, knowledge of the lunar interior stands to gain the most from APOLLO, as the experiment routinely acquires 4 to 5 reflectors in one session, providing exquisite measurements of the lunar orientation and distortion. But this refined knowledge of the moon will also facilitate understanding of how the moon moves in its orbit, as such information is crucial to determining the path of the Moon's center of mass through space. LLR tests many fundamental aspects of gravity, like the equivalence principle, the constancy of gravity, gravitomagnetism, and the inverse-square law. Improving our knowledge of gravity therefore informs a diverse range of cosmologists, astrophysicists, particle physicists, and string theorists. This effort will also contribute to Earth and planetary science, especially via the inclusion of the superconducting gravimeter data. The open-source solar-system analysis code emerging from the project will provide an interesting platform for analyzing a wide range of data sets, including radar ranging in the solar system, Doppler radar to spacecraft around the solar system, pulsar timing experiments, etc. The same platform can form the basis of covariant studies for future proposed space missions that aim to test fundamental physics. Additionally, graduate student and postdoctoral training is a valuable component of the total effort. Finally, APOLLO will continue its strong tradition of public outreach, including featured appearances on popular television shows.