Interplanetary ranging can achieve millimeter-level measurement accuracy.
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Peer-reviewed literature demonstrates that active laser ranging techniques can achieve sub-millimeter to millimeter-level accuracy across interplanetary distances.
Precision laser ranging between planetary bodies will allow advances in the study of fundamental physics and solar system dynamics. Current precision ranging techniques based on retro-reflectors are limited to the Earth-Moon distance. We present a method of active laser ranging over interplanetary distances with asynchronous two-way ranging. The method is validated in real time laboratory experiments and field tests. Sub-millimeter accuracy has been achieved in real-time active laser ranging for interplanetary distances, providing precision improvement well above three orders of magnitude over the current RF techniques. An instrument developed with the approach and deployed in future planetary missions will significantly advance planetary sciences and fundamental physics. For example, implemented in a future Mars lander mission our approach will clarify the Mars interior (liquid or solid), which is still an open question due to limitation of RF ranging precision although extensive efforts have been made over years with numerous missions since 1970s.
Existing capabilities of laser ranging, optical interferometry, and metrology, in combination with precision frequency standards, atom-based quantum sensors, and drag-free technologies, are critical for space-based tests of fundamental physics; as a result of the recent progress in these disciplines, the entire area is poised for major advances. Thus, accurate ranging to the Moon and Mars will provide significant improvements in several gravity tests, namely the equivalence principle, geodetic precession, PPN parameters β and γ, and possible variation of the gravitational constant G. Other tests will become possible with the development of an optical architecture that allows one to proceed from meter to centimeter to millimeter range accuracies on interplanetary distances. Motivated by anticipated accuracy gains, we discuss the recent renaissance in lunar laser ranging and consider future relativistic gravity experiments with precision laser ranging over interplanetary distances.
Lunar laser ranging (LLR) has been a workhorse for testing general relativity over the past four decades. The three retroreflector arrays put on the Moon by the Apollo astronauts and the French built arrays on the Soviet Lunokhod rovers continue to be useful targets, and have provided the most stringent tests of the Strong Equivalence Principle and the time variation of Newton's gravitational constant. The relatively new ranging system at the Apache Point 3.5 meter telescope now routinely makes millimeter level range measurements. Incredibly, it has taken 40 years for ground station technology to advance to the point where characteristics of the lunar retroreflectors are limiting the precision of the range measurements. In this article, we review the gravitational science and technology of lunar laser ranging and discuss prospects for the future.
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