60 research outputs found
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
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
Indirect evaluation of Mars Gravity Model 2011 using a replication experiment on Earth
Curtin University’s Mars Gravity Model 2011 (MGM2011) is a high-resolution composite set of gravity field functionals that uses topography-implied gravity effects at medium- and short-scales (~125 km to ~3 km) to augment the space-collected MRO110B2 gravity model. Ground-truth gravity observations that could be used for direct validation of MGM2011 are not available on Mars’s surface. To indirectly evaluate MGM2011 and its modelling principles, an as-close-as-possible replication of the MGM2011 modelling approach was performed on Earth as the planetary body with most detailed gravity field knowledge available. Comparisons among six ground-truth data sets (gravity disturbances, quasigeoid undulations and vertical deflections) and the MGM2011-replication over Europe and North America show unanimously that topography-implied gravity information improves upon space-collected gravity models over areas with rugged terrain. The improvements are ~55% and ~67% for gravity disturbances, ~12% and ~47% for quasigeoid undulations, and ~30% to ~50% for vertical deflections. Given that the correlation between space-collected gravity and topography is higher for Mars than Earth at spatial scales of a few 100 km, topography-implied gravity effects are more dominant on Mars. It is therefore reasonable to infer that the MGM2011 modelling approach is suitable, offering an improvement over space-collected Martian gravity field models
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
Exploring the smallest terrestrial planet: Dawn at Vesta
Dawn maps the surface of Vesta and Ceres and probes their internal density distributions, during one year of orbital operations at each body. Dawn carries a framing camera, a visible and infrared spectrometer (VIR), a gamma ray and neutron spectrometer (GRaND), and determines their gravity fields. The camera maps the surface in color, and obtains stereo data to derive global topography models. VIR determines the mineral composition of the surface and GRaND determines the elemental composition. Dawn maps from three science orbits at altitudes of ~2700 km, ~700 km, and ~200 km. Thus far, we have surveyed for moons around Vesta, accurately determined Vesta’s mass, refined the rotation axis, and have preliminary information on surface features and composition. The existence of abundant meteorites on Earth that came from Vesta or Vesta-derived material provides deep insight into geochemistry of Vesta, albeit without geologic context. We expect our Vesta data to provide that missing geologic context for the Howardite, Eucrite and Diogenite (HED) meteorites. According to the ages of the HED meteorites, Vesta formed in the solar system’s first few million years. Vesta likely accreted close to the time of a supernova explosion that provided short-half-life radionuclides supplying sufficient heat to melt Vesta, drive off the water and allow differentiation and formation of an iron core. Vesta is the second most massive asteroid in the main belt. It is thought to have a large impact structure surrounding its south pole. Initial observations with Dawn are not inconsistent with this hypothesis, but the terrain is unlike other impact basins. Outside this structure, the surface is heavily cratered. Vesta’s albedo is higher than most other asteroids, the Moon and Mercury. Vesta’s compositional diversity is more similar to the Moon and Mercury than other asteroids and it possesses a global 1-micron spectral feature due to ferrous iron absorption, which appears only locally on other airless bodies, and in stark contrast to Mercury where it is absent. In this and the following talks, we discuss current understanding of this complex body and what it teaches us about the earliest days of the solar system
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