Radio science investigation on a Mercury Orbiter mission

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Title: Radio science investigation on a Mercury Orbiter mission
Author: Anderson, John; Turyshev, Slava; Asmar, W.; Bird, M.; Konopliv, A.; Krisher, T.; Lau, Eunice; Schubert, G.; Sjogren, W.
Abstract: We review the results from Mariner 10 regarding Mercury’s gravity field and the results from radar ranging regarding topography. We discuss the implications of improving these results, including a determination of the polar component, as well as the opportunity to perform relativistic gravity tests with a future Mercury Orbiter. With a spacecraft placed in orbit with periherm at 400 km altitude, apherm at 16,800 km, period 13.45 hr and latitude of periherm at +30 deg, one can expect a significant improvement in our knowledge of Mercury’s gravity field and geophysical properties. The 2000 Plus mission that evolved during the European Space Agency (ESA) Mercury Orbiter assessment study (Hechler, 1994) can provide a global gravity field complete through the 25th degree and order in spherical harmonics. If after completion of the main mission, the periherm could be lowered to 200 km altitude, the gravity field could be extended to 50th degree and order. We discuss the possibility that a search for a Hermean ionosphere could be performed during the mission phases featuring Earth occultations. Because of its relatively large eccentricity and close proximity to the Sun, Mercury’s orbital motion provides one of the best solar-system tests of general relativity. Consequently, we emphasize the number of feasible relativistic gravity tests that can be performed within the context of the parameterized post-Newtonian formalism - a useful framework for testing modern gravitational theories. We pointed out that current results on relativistic precession of Mercury’s perihelion are uncertain by 0.5 %, and we discuss the expected improvement using Mercury Orbiter. We discuss the importance of Mercury Orbiter for setting limits on a possible time variation in the gravitational constant G as measured in atomic units. Moreover, we mention that by including a space-borne ultrastable crystal oscillator (USO) or an atomic clock in the Mercury Orbiter payload, a new test of the solar gravitational redshift would be possible to an accuracy of one part in 104 with a USO, and to an accuracy of one part in 107 with an atomic standard. With an atomic clock and additional hardware for a multi-link Doppler system, including Doppler extraction on the spacecraft, the effect of Mercury’s gravity field on USO’s frequency could be measured with an accuracy of one part in 106. We discuss other relativistic effects including the geodetic precession of the orbiter’s orbital plane about Mercury, a planetary test of the Equivalence Principle (Nordtvedt effect), and a solar conjunction experiment to measure the relativistic time delay (Shapiro effect).
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Date: 1997-01

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