Radio-science investigation on a Mercury Orbiter mission

Abstract Results from Mariner 10 for Mercury's gravity field and results from radar ranging for elevations are reviewed. Implications of improving these results are discussed, 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 h and latitude of periherm at +30 deg, a significant improvement in measurements of Mercury's gravity field and geophysical properties will result. 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. Also, a search for a Hermean ionosphere could be performed during the mission phases featuring Earth occultations. Because of its relatively large eccentricity and proximity to the Sun, Mercury's orbital motion provides one of the best solar system tests of general relativity. Consequently, a number of feasible relativistic gravity tests are described within the context of the parameterized post-Newtonian formalism. Current results on the relativistic precession of Mercury's perihelion are uncertain by 0.5%, and improvements are feasible with a Mercury Orbiter mission. Also, improved limits on a possible time variation in the gravitational constant G as measured in atomic units are feasible. Moreover, 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 10 4 with a USO, and to an accuracy of one part in 10 7 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 the USO's frequency could be measured with an accuracy of one part in 10 6 . Other relativistic effects are discussed 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).