arXiv subjects

Thomas W. Murphy Jr

Publications and source records attributed to Thomas W. Murphy Jr.

4 recordsLinked to original sources

arXiv2024

A clear case for dust obscuration of the lunar retroreflectors

The passive retroreflector arrays placed on the moon by Apollo 11, 14 and 15 astronauts continue to produce valuable Earth-Moon range measurements that enable high-precision tests of gravitational physics, as well as studies of geo- and selenophysics. The optical throughput of these retroreflectors has declined since their deployment, with an additional signal loss at full moon when the reflectors experience direct solar illumination. We show that the loss in return rate can be attributed to the accumulation of a thin layer of lunar dust on the surfaces of the corner cube retroreflectors. First, a careful analysis of the optical link budget for the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) experiment reveals that the lunar return rate is 15--20 times smaller than predicted, a deficit that can be explained by a reflector dust covering fraction of ${\sim} 50$\%. Second, range measurements taken during a lunar eclipse indicate that the solar illumination of the retroreflectors degrades their throughput by an additional factor of ${\sim}15$. Finally, a numerical simulation of heat transfer in dust-coated reflectors is able to model the resulting thermal lensing effect, in which thermal gradients in the retroreflectors degrade their far-field diffraction pattern. A comparison of this simulation to eclipse measurements finds a dust coverage fraction of ${\sim}50$%. Taken together, the link analysis, eclipse observations and thermal modeling support the claim that the retroreflectors are obscured by lunar dust, with both link budget and simulation independently finding the dust fraction to be $\sim$50%.

astro-ph.IM↗

arXiv2004

Laser Ranging to the Moon, Mars and Beyond

Current and future optical technologies will aid exploration of the Moon and Mars while advancing fundamental physics research in the solar system. Technologies and possible improvements in the laser-enabled tests of various physical phenomena are considered along with a space architecture that could be the cornerstone for robotic and human exploration of the solar system. In particular, accurate ranging to the Moon and Mars would not only lead to construction of a new space communication infrastructure enabling an improved navigational accuracy, but will also provide a significant improvement in several tests of gravitational theory: the equivalence principle, geodetic precession, PPN parameters $β$ and $γ$, and possible variation of the gravitational constant $G$. Other tests would become possible with an optical architecture that would allow proceeding from meter to centimeter to millimeter range accuracies on interplanetary distances. This paper discusses the current state and the future improvements in the tests of relativistic gravity with Lunar Laser Ranging (LLR). We also consider precision gravitational tests with the future laser ranging to Mars and discuss optical design of the proposed Laser Astrometric Test of Relativity (LATOR) mission. We emphasize that already existing capabilities can offer significant improvements not only in the tests of fundamental physics, but may also establish the infrastructure for space exploration in the near future. Looking to future exploration, what characteristics are desired for the next generation of ranging devices, what is the optimal architecture that would benefit both space exploration and fundamental physics, and what fundamental questions can be investigated? We try to answer these questions.

gr-qc↗

arXiv2003

Improving LLR Tests of Gravitational Theory

Accurate analysis of precision ranges to the Moon has provided several tests of gravitational theory including the Equivalence Principle, geodetic precession, parameterized post-Newtonian (PPN) parameters $γ$ and $β$, and the constancy of the gravitational constant {\it G}. Since the beginning of the experiment in 1969, the uncertainties of these tests have decreased considerably as data accuracies have improved and data time span has lengthened. We are exploring the modeling improvements necessary to proceed from cm to mm range accuracies enabled by the new Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) currently under development in New Mexico. This facility will be able to make a significant contribution to the solar system tests of fundamental and gravitational physics. In particular, the Weak and Strong Equivalence Principle tests would have a sensitivity approaching 10$^{-14}$, yielding sensitivity for the SEP violation parameter $η$ of $\sim 3\times 10^{-5}$, $v^2/c^2$ general relativistic effects would be tested to better than 0.1%, and measurements of the relative change in the gravitational constant, $\dot{G}/G$, would be $\sim0.1$% the inverse age of the universe. Having this expected accuracy in mind, we discusses the current techniques, methods and existing physical models used to process the LLR data. We also identify the challenges for modeling and data analysis that the LLR community faces today in order to take full advantage of the new APOLLO ranging station.

gr-qc↗

arXiv2003

Astrophysical constraints on hypothetical variability of fundamental constants

(Abridged) Many-multiplet (MM) method applied to three inhomogeneous samples of Keck/HIRES quasar absorption spectra gives a shift in the value of the fine-structure constant of Delta alpha/alpha = (-5.4+/-1.2) 10^{-6} in the redshift range 0.2 < z < 3.7 (Murphy et al. 2003).The 1sigma error is, however, much too small and cannot be maintained by current observations of quasars. We present a modified MM method to set an upper limit on Delta alpha/alpha from a homogeneous sample of FeII lines identified in the up-to-date best quality VLT/UVES spectrum of HE 0515-4414. Our result is Delta alpha/alpha = (1.1+/-1.1) 10^{-5} at z = 1.149. Theoretical models of the fundamental physical interactions predict that the proton-to-electron mass ratio (mu = m_p/m_e) may relate to the shift in Delta alpha/alpha as Delta mu/mu = R Delta alpha/alpha. We use VLT/UVES high-resolution observations of molecular hydrogen H2 ultraviolet absorption lines at z = 3.025 toward Q 0347-3818 to bound the value of R. The obtained constraints on Delta ln mu = (2.1+/-3.6) 10^{-5} and on Delta ln alpha = (1.1+/-1.1) 10^{-5} rule out vary large values of |R| > 6.

astro-ph↗