RADIO-SCIENCE INVESTIGATION ON A MERCURY ORBITER MISSION

Results from Mariner 10 for Mercury's gravity field and results from r adar ranging for elevations are reviewed. Implications of improving th ese results are discussed, as well as the opportunity to perform relat ivist gravity tests with a future Mercury Orbiter. With a spacecraft p laced 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 impr ovement in measurements of Mercury's gravity field and geophysical pro perties will result. The 2000 Plus mission that evolved during the Eur opean Space Agency (ESA) Mercury Orbiter assessment study (Hechler, 19 94) can provide a global gravity field complete through the 25th degre e and order in spherical harmonics. If after completion of the main mi ssion, the periherm could be lowered to 200 km altitude, the gravity f ield could be extended to 50th degree and order. Also, a search for a Hermean ionosphere could be performed during the mission phases featur ing Earth occultations. Because of its relatively large eccentricity a nd 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 contex t of the parameterized post-Newtonian formalism. Current results on th e 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 con stant G as measured in atomic units are feasible. Moreover, by includi ng a space-borne ultrastable crystal oscillator (USO) or an atomic clo ck in the Mercury Orbiter payload, a new test of the solar gravitation al 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 standar d. With an atomic clock and additional hardware for a multi-link Doppl er system, including Doppler extraction on the spacecraft, the effect of Mercury's gravity field on the USO's frequency could be measured wi th an accuracy of one part in 10(6). Other relativistic effects are di scussed including the geodetic precession of the orbiter's orbital pla ne about Mercury, a planetary test of the Equivalence Principle (Nordt vedt effect), and a solar conjunction experiment to measure the relati vistic time delay (Shapiro effect). (C) 1997 Elsevier Science Ltd.