Geophysical Exploration of Vesta

Dawn’s year-long stay at Vesta allows comprehensive mapping of the shape, topography, geology, mineralogy, elemental abundances, and gravity field using it’s three instruments and highprecision spacecraft navigation. In the current Low Altitude Mapping Orbit (LAMO), tracking data is being acquired to develop a gravity field expected to be accurate to degree and order ~20 [1, 2]. Multi-angle imaging in the Survey and High Altitude Mapping Orbit (HAMO) has provided adequate stereo coverage to develop a shape model accurate to ~10 m at 100 m horizontal spatial resolution. Accurate mass determination combined with the shape yields a more precise value of bulk density, albeit with some uncertainty resulting from the unmeasured seasonally-dark north polar region. The shape and gravity of Vesta can be used to infer the interior density structure and investigate the nature of the crust, informing models for Vesta’s formation and evolution

The Dawn Gravity Investigation at Vesta and Ceres

The objective of the Dawn gravity investigation is to use high precision X-band Doppler tracking and landmark tracking from optical images to measure the gravity fields of Vesta and Ceres to a half-wavelength surface resolution better than 90-km and 300-km, respectively. Depending on the Doppler tracking assumptions, the gravity field will be determined to somewhere between harmonic degrees 15 and 25 for Vesta and about degree 10 for Ceres. The gravity fields together with shape models determined from Dawn's framing camera constrain models of the interior from the core to the crust. The gravity field is determined jointly with the spin pole location. The second degree harmonics together with assumptions on obliquity or hydrostatic equilibrium may determine the moments of inertia

Correlation of pulse wave velocity with left ventricular mass in patients with hypertension once blood pressure has been normalized

Vascular stiffness has been proposed as a simple method to assess arterial loading conditions of the heart which induce left ventricular hypertrophy (LVH). There is some controversy as to whether the relationship of vascular stiffness to LVH is independent of blood pressure, and which measurement of arterial stiffness, augmentation index (AI) or pulse wave velocity (PWV) is best. Carotid pulse wave contor and pulse wave velocity of patients (n=20) with hypertension whose blood pressure (BP) was under control (<140/90 mmHg) with antihypertensive drug treatment medications, and without valvular heart disease, were measured. Left ventricular mass, calculated from 2D echocardiogram, was adjusted for body size using two different methods: body surface area and height. There was a significant (P<0.05) linear correlation between LV mass index and pulse wave velocity. This was not explained by BP level or lower LV mass in women, as there was no significant difference in PWV according to gender (1140.1+67.8 vs 1110.6+57.7 cm/s). In contrast to PWV, there was no significant correlation between LV mass and AI. In summary, these data suggest that aortic vascular stiffness is an indicator of LV mass even when blood pressure is controlled to less than 140/90 mmHg in hypertensive patients. The data further suggest that PWV is a better proxy or surrogate marker for LV mass than AI and the measurement of PWV may be useful as a rapid and less expensive assessment of the presence of LVH in this patient population

Radio Science Investigation on a Mercury Orbiter Mission

We review the results from {\it 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 {\it 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) {\it Mercury Orbiter} assessment study 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 {\it Mercury Orbiter}. We discuss the importance of {\it Mercury Orbiter} for setting limits on a possible time variation in theComment: 23 pages, LaTeX, no figure

Testing General Relativity with Atomic Clocks

We discuss perspectives for new tests of general relativity which are based on recent technological developments as well as new ideas. We focus our attention on tests performed with atomic clocks and do not repeat arguments present in the other contributions to the present volume. In particular, we present the scientific motivations of the space projects ACES and SAGAS.Comment: Contribution for "The Nature of Gravity" (eds. F. Everitt et al

MORE: an advanced tracking experiment for the exploration of Mercury with the mission BepiColombo

Precise microwave tracking of interplanetary spacecraft has been a crucial tool in solar system exploration. Range and range rate measurements, the main observable quantities in spacecraft orbit determination and navigation, have been widely used to refine the dynamical model of the solar system and to probe planetary interiors. Thanks to the use of Ka-band and multifrequency radio links, a significant improvement in microwave tracking systems has been demonstrated by the radio science experiments of the Cassini mission to Saturn. The Cassini radio system has been used to carry out the most accurate test of General Relativity to date. Further developments in the radio instrumentation have been recently started for the MORE experiment, selected for the ESA mission to Mercury, BepiColombo. MORE addresses the mission’s scientific goals in geodesy, geophysics and fundamental physics. In addition, MORE will carry out a navigation experiment, aiming to a precise assessment of the orbit determination accuracies attainable with the use of the novel instrumentation. The key instrument is a Ka/Ka band digital transponder enabling a high phase coherence between uplink and downlink carriers and supporting a wideband ranging tone. The onboard instrumentation is complemented by a ground system based upon the simultaneous transmission and reception of multiple frequencies at X and Ka-band. The new wideband ranging system is designed for an end-to-end accuracy of 20 cm using integration times of a few seconds. Two-way range rate measurements are expected to be accurate to 3 micron/s, thanks to nearly complete cancellation or calibration of the propagation noise from interplanetary plasma and troposphere. We review the experimental configuration of the experiment and outline its scientific goals and expected results

The Tracking System of the Mercury Orbiter Radioscience Experiment

Precise microwave tracking of interplanetary spacecraft has been a crucial tool in solar system exploration. Not only range and range rate measurements are the main observable quantities in spacecraft orbit determination and navigation, but they have been widely used to refine the dynamical model of the solar system and to probe planetary interiors. Thanks to the use of Ka-band and multifrequency radio links, a significant improvement in microwave tracking systems has been demonstrated by the radio science experiments of the Cassini mission to Saturn. The Cassini radio system has been used to carry out the most accurate test of General Relativity to date. Further developments in the radio instrumentation have been recently started for the Mercury Orbiter Radioscience Experiment (MORE), selected for the ESA mission to Mercury, BepiColombo. MORE addresses the mission's scientific goals in geodesy, geophysics and fundamental physics. In addition, MORE will carry out a navigation experiment, aiming to a precise assessment of the orbit determination accuracies attainable with the use of the novel instrumentation. The key instrument is a Ka/Ka band digital transponder enabling a high phase coherence (to a level of 10^-15 over 1000 s integration time) between uplink and downlink carriers and supporting an accurate ranging system designed for an end-to-end accuracy of 20 cm. The onboard instrumentation must be complemented by a ground system, capable of simultaneous transmission and reception of multiple frequencies at X and Ka-band. Ground support for the MORE experiment is currently foreseen from the DSN station DSS25 (Goldstone, California). DSS 25 was specifically upgraded in support of the Cassini Radio Science experiment and, at present, is the only deep space antenna with Ka-band uplink and multifrequency capabilities. On the ESA side, the upgrade to Ka-band of the 35-m Cebreros antenna in Spain is also being considered with the goal of increasing the science return of the experiment and to provide Europe with state-of-the-art tracking systems. In this paper we review the experimental configuration of MORE and outline a plan for the engineering development of the ground segment which is currently under consideration in ESA in support of the BepiColombo radio science experiment

The Cassini solar Faraday rotation experiment

The Cassini Faraday rotation experiment improves the current understanding of the coronal magnetic field by making the first measurements of the magnetic field within two solar radii of the south pole and by allowing the separation of the changing electron density from the magnetic field during transient crossings of the line of sight. Simultaneous ranging data to Cassini also contributes to the growing body of empirical electron density models by providing electron density data at solar maximum at varying latitudes including over the poles. Faraday rotation data in the solar corona were collected in 2002 and 2003 using the Cassini spacecraft in cruise to Saturn. Although Cassini primarily transmits in right-hand polarization, enough left-hand is produced to enable polarization measurements. We show that during solar conjunction Faraday rotation is measurable with Cassini. In the X- and Ka-bands both datasets currently show an asymmetric diurnal pattern in polarization not associated with the ionosphere. The Ka-band is much more sensitive to pointing accuracy and experienced periods of power drop outs which caused the left-circularly polarized signal to drop into the noise on occasion during the 2003 conjunction. Interpretation of the dataset is deferred to future papers. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved