95 research outputs found
About the various contributions in Venus rotation rate and LOD
% context heading (optional) {Thanks to the Venus Express Mission, new data
on the properties of Venus could be obtained in particular concerning its
rotation.} % aims heading (mandatory) {In view of these upcoming results, the
purpose of this paper is to determine and compare the major physical processes
influencing the rotation of Venus, and more particularly the angular rotation
rate.} % methods heading (mandatory) {Applying models already used for the
Earth, the effect of the triaxiality of a rigid Venus on its period of rotation
are computed. Then the variations of Venus rotation caused by the elasticity,
the atmosphere and the core of the planet are evaluated.} % results heading
(mandatory) {Although the largest irregularities of the rotation rate of the
Earth at short time scales are caused by its atmosphere and elastic
deformations, we show that the Venus ones are dominated by the tidal torque
exerted by the Sun on its solid body. Indeed, as Venus has a slow rotation,
these effects have a large amplitude of 2 minutes of time (mn). These
variations of the rotation rate are larger than the one induced by atmospheric
wind variations that can reach 25-50 seconds of time (s), depending on the
simulation used. The variations due to the core effects which vary with its
size between 3 and 20s are smaller. Compared to these effects, the influence of
the elastic deformation cause by the zonal tidal potential is negligible.} %
conclusions heading (optional), leave it empty if necessary {As the variations
of the rotation of Venus reported here are of the order 3mn peak to peak, they
should influence past, present and future observations providing further
constraints on the planet internal structure and atmosphere.}Comment: 12 pages, 10 figures, Accepted in A&
Anomalies of Density, Stresses, and the Gravitational Field in the Interior of Mars
We determined the possible compensation depths for relief harmonics of
different degrees and orders. The relief is shown to be completely compensated
within the depth range of 0 to 1400 km. The lateral distributions of
compensation masses are determined at these depths and the maps are
constructed. The possible nonisostatic vertical stresses in the crust and
mantle of Mars are estimated to be 64 MPa in compression and 20 MPa in tension.
The relief anomalies of the Tharsis volcanic plateau and symmetric feature in
the eastern hemisphere could have arisen and been maintained dynamically due to
two plumes in the mantle substance that are enriched with fluids. The plumes
that originate at the core of Mars can arise and be maintained by the anomalies
of the inner gravitational field achieving +800 mGal in the region of plume
formation, - 1200 mGal above the lower mantle-core transition layer, and -1400
mGal at the crust.Comment: 9 pages, 5 figure
On the Lense-Thirring test with the Mars Global Surveyor in the gravitational field of Mars
I discuss some aspects of the recent test of frame-dragging performed by me
by exploiting the Root-Mean-Square (RMS) orbit overlap differences of the
out-of-plane component N of the orbit of the Mars Global Surveyor (MGS)
spacecraft in the gravitational field of Mars. A linear fit of the full time
series of the entire MGS data (4 February 1999-14 January 2005) yields a
normalized slope 1.03 +/- 0.41 (with 95% confidence bounds). Other linear fits
to different data sets confirm the agreement with general relativity. The huge
systematic effects induced by the mismodeling in the martian gravitational
field claimed by some authors are absent in the MGS out-of-plane record. The
non-gravitational forces affect at the same level of the gravitomagnetic one
the in-plane orbital components of MGS, not the out-of-plane one. Moreover,
they experience high-frequency variations which does not matter in the present
case in which secular effects are relevant.Comment: LaTex2e, 8 pages, no figures, no tables, 17 references. It refers to
K. Krogh, Class. Quantum Grav., 24, 5709-5715, 2007 based on
astro-ph/0701653. Final version to appear in CEJP (Central European Journal
of Physics
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
Constraining Ceres' interior from its Rotational Motion
Context. Ceres is the most massive body of the asteroid belt and contains
about 25 wt.% (weight percent) of water. Understanding its thermal evolution
and assessing its current state are major goals of the Dawn Mission.
Constraints on internal structure can be inferred from various observations.
Especially, detailed knowledge of the rotational motion can help constrain the
mass distribution inside the body, which in turn can lead to information on its
geophysical history. Aims. We investigate the signature of the interior on the
rotational motion of Ceres and discuss possible future measurements performed
by the spacecraft Dawn that will help to constrain Ceres' internal structure.
Methods. We compute the polar motion, precession-nutation, and length-of-day
variations. We estimate the amplitudes of the rigid and non-rigid response for
these various motions for models of Ceres interior constrained by recent shape
data and surface properties. Results. As a general result, the amplitudes of
oscillations in the rotation appear to be small, and their determination from
spaceborne techniques will be challenging. For example, the amplitudes of the
semi-annual and annual nutations are around ~364 and ~140 milli-arcseconds, and
they show little variation within the parametric space of interior models
envisioned for Ceres. This, combined with the very long-period of the
precession motion, requires very precise measurements. We also estimate the
timescale for Ceres' orientation to relax to a generalized Cassini State, and
we find that the tidal dissipation within that object was probably too small to
drive any significant damping of its obliquity since formation. However,
combining the shape and gravity observations by Dawn offers the prospect to
identify departures of non-hydrostaticity at the global and regional scale,
which will be instrumental in constraining Ceres' past and current thermal
state. We also discuss the existence of a possible Chandler mode in the
rotational motion of Ceres, whose potential excitation by endogenic and/or
exogenic processes may help detect the presence of liquid reservoirs within the
asteroid.Comment: submitted to Astronomy and Astrophysic
Solar system constraints on f(T) gravity
We use recent observations from solar system orbital motions in order to
constrain f(T) gravity. In particular, imposing a quadratic f(T) correction to
the linear-in-T form, which is a good approximation for every realistic case,
we extract the spherical solutions of the theory. Using these spherical
solutions to describe the Sun's gravitational field, we use recently determined
supplementary advances of planetary perihelia, to infer upper bounds on the
allowed f(T) corrections. We find that the maximal allowed divergence of the
gravitational potential in f(T) gravity from that in the teleparallel
equivalent of General Relativity is of the order of 6.2 \times 10^{-10}, in the
applicability region of our analysis. This is much smaller than the
corresponding (significantly small too) divergence that is predicted from
cosmological observations, as expected. Such a tiny allowed divergence from the
linear form should be taken into account in f(T) model building.Comment: 7 pages, no figures, version published in Mon.Not.Roy.Astron.So
Advancing Tests of Relativistic Gravity via Laser Ranging to Phobos
Phobos Laser Ranging (PLR) is a concept for a space mission designed to
advance tests of relativistic gravity in the solar system. PLR's primary
objective is to measure the curvature of space around the Sun, represented by
the Eddington parameter , with an accuracy of two parts in ,
thereby improving today's best result by two orders of magnitude. Other mission
goals include measurements of the time-rate-of-change of the gravitational
constant, and of the gravitational inverse square law at 1.5 AU
distances--with up to two orders-of-magnitude improvement for each. The science
parameters will be estimated using laser ranging measurements of the distance
between an Earth station and an active laser transponder on Phobos capable of
reaching mm-level range resolution. A transponder on Phobos sending 0.25 mJ, 10
ps pulses at 1 kHz, and receiving asynchronous 1 kHz pulses from earth via a 12
cm aperture will permit links that even at maximum range will exceed a photon
per second. A total measurement precision of 50 ps demands a few hundred
photons to average to 1 mm (3.3 ps) range precision. Existing satellite laser
ranging (SLR) facilities--with appropriate augmentation--may be able to
participate in PLR. Since Phobos' orbital period is about 8 hours, each
observatory is guaranteed visibility of the Phobos instrument every Earth day.
Given the current technology readiness level, PLR could be started in 2011 for
launch in 2016 for 3 years of science operations. We discuss the PLR's science
objectives, instrument, and mission design. We also present the details of
science simulations performed to support the mission's primary objectives.Comment: 25 pages, 10 figures, 9 table
Phenomenology of the Lense-Thirring effect in the Solar System
Recent years have seen increasing efforts to directly measure some aspects of
the general relativistic gravitomagnetic interaction in several astronomical
scenarios in the solar system. After briefly overviewing the concept of
gravitomagnetism from a theoretical point of view, we review the performed or
proposed attempts to detect the Lense-Thirring effect affecting the orbital
motions of natural and artificial bodies in the gravitational fields of the
Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of
the impact of several sources of systematic uncertainties of dynamical origin
to realistically elucidate the present and future perspectives in directly
measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in
Astrophysics and Space Science (ApSS). Some uncited references in the text
now correctly quoted. One reference added. A footnote adde
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
Gravity, Geodesy and Fundamental Physics with BepiColomboâs MORE Investigation
open40siThe Mercury Orbiter Radio Science Experiment (MORE) of the ESA mission BepiColombo will provide an accurate estimation of Mercuryâs gravity field and rotational state, improved tests of general relativity, and a novel deep space navigation system. The key experimental setup entails a highly stable, multi-frequency radio link in X and Ka band, enabling two-way range rate measurements of 3 micron/s at nearly all solar elongation angles. In addition, a high chip rate, pseudo-noise ranging system has already been tested at 1-2 cm accuracy. The tracking data will be used together with the measurements of the Italian Spring Accelerometer to provide a pseudo drag free environment for the data analysis. We summarize the existing literature published over the past years and report on the overall configuration of the experiment, its operations in cruise and at Mercury, and the expected scientific results.openIess L.; Asmar S.W.; Cappuccio P.; Cascioli G.; De Marchi F.; di Stefano I.; Genova A.; Ashby N.; Barriot J.P.; Bender P.; Benedetto C.; Border J.S.; Budnik F.; Ciarcia S.; Damour T.; Dehant V.; Di Achille G.; Di Ruscio A.; Fienga A.; Formaro R.; Klioner S.; Konopliv A.; Lemaitre A.; Longo F.; Mercolino M.; Mitri G.; Notaro V.; Olivieri A.; Paik M.; Palli A.; Schettino G.; Serra D.; Simone L.; Tommei G.; Tortora P.; Van Hoolst T.; Vokrouhlicky D.; Watkins M.; Wu X.; Zannoni M.Iess L.; Asmar S.W.; Cappuccio P.; Cascioli G.; De Marchi F.; di Stefano I.; Genova A.; Ashby N.; Barriot J.P.; Bender P.; Benedetto C.; Border J.S.; Budnik F.; Ciarcia S.; Damour T.; Dehant V.; Di Achille G.; Di Ruscio A.; Fienga A.; Formaro R.; Klioner S.; Konopliv A.; Lemaitre A.; Longo F.; Mercolino M.; Mitri G.; Notaro V.; Olivieri A.; Paik M.; Palli A.; Schettino G.; Serra D.; Simone L.; Tommei G.; Tortora P.; Van Hoolst T.; Vokrouhlicky D.; Watkins M.; Wu X.; Zannoni M
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