Advancing Tests of Relativistic Gravity via Laser Ranging to Phobos - General Relativity and Quantum CosmologyReport as inadecuate

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Abstract: Phobos Laser Ranging PLR is a concept for a space mission designed toadvance tests of relativistic gravity in the solar system. PLR-s primaryobjective is to measure the curvature of space around the Sun, represented bythe Eddington parameter $\gamma$, with an accuracy of two parts in $10^7$,thereby improving today-s best result by two orders of magnitude. Other missiongoals include measurements of the time-rate-of-change of the gravitationalconstant, $G$ and of the gravitational inverse square law at 1.5 AUdistances-with up to two orders-of-magnitude improvement for each. The scienceparameters will be estimated using laser ranging measurements of the distancebetween an Earth station and an active laser transponder on Phobos capable ofreaching mm-level range resolution. A transponder on Phobos sending 0.25 mJ, 10ps pulses at 1 kHz, and receiving asynchronous 1 kHz pulses from earth via a 12cm aperture will permit links that even at maximum range will exceed a photonper second. A total measurement precision of 50 ps demands a few hundredphotons to average to 1 mm 3.3 ps range precision. Existing satellite laserranging SLR facilities-with appropriate augmentation-may be able toparticipate in PLR. Since Phobos- orbital period is about 8 hours, eachobservatory is guaranteed visibility of the Phobos instrument every Earth day.Given the current technology readiness level, PLR could be started in 2011 forlaunch in 2016 for 3 years of science operations. We discuss the PLR-s scienceobjectives, instrument, and mission design. We also present the details ofscience simulations performed to support the mission-s primary objectives.

Author: Slava G. Turyshev, William Farr, William M. Folkner, Andre R. Girerd, Hamid Hemmati, Thomas W. Murphy, Jr., James G. Williams, Jo


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