1,721,226 research outputs found
The detection of Jupiter normal modes with gravity measurements of the mission Juno
Full text linkArriving at Jupiter on July 4, 2016, NASA’s Juno mission will complete 37 orbits (14-days period) around the planet, revealing details of the interior structure and composition, a crucial aspect to understand the origin and evolution of Jupiter. A radio science experiment will help to select and validate the existing models of Jupiter internal composition, in particular the mass of the silicate core.
Recently it has been proposed to exploit the Doppler data for the determination of Jupiter’s acoustic normal modes. Jupiter is a gaseous giant and its masses are subject to oscillations (normal modes) due to internal pressure waves, which cause potentially detectable disturbances in the gravity field. By displacing large masses, Jupiter’s normal modes can therefore perturb the spacecraft motion to levels that can be measured by Juno’s extremely accurate Doppler system. Theoretical models that explain these phenomena have been proposed in the past and experimental works looking for these oscillations have been carried out recently with ground-based optical telescopes. But the frequencies and the amplitudes of normal modes can in principle be modeled and estimated by means of orbit determination codes
Advanced radio science instrumentation for the mission BepiColombo to Mercury
No full textRadio science experiments of BepiColombo will provide a detailed mapping of Mercury's gravity field and important information about its deep internal structure. The global orbital solutions, obtained from precise radio metric data, entail also very accurate tests of General Relativity and other metric theories of gravity. The classical tests of the solar gravitational deflection and the precession of perihelion could improve the measurement of the post-Newtonian parameters beta and gamma by 2-3 orders of magnitude, to a value in the range 10(-6)-10(-5). At these levels, violations of General Relativity due to scalar fields, remnant of the inflation age, could occur. In order to achieve the scientific objectives in geophysics and fundamental physics, a suitable radio frequency instrumentation both for onboard and ground equipment is needed. The target two-way accuracy is 20-30 cm. for range and 3 x 10(-4) cm/s for range rate (at 1000-10,000 s integration time). This precision requires the capability of transmitting and receiving at multiple frequencies (to reduce plasma noise) and larger modulation bandwidths for improved ranging performances. We propose an architecture of the onboard and ground radio frequency subsystems which combines minimization of mass and power, technological feasibility, and adequate phase stability and ranging accuracy. (C) 2001 Published by Elsevier Science Ltd
Determination of the planetary rotation by imaging from orbit
No full textThe knowledge of the rotational state of planetary bodies provides crucial information on their interior structure. Evolution models of the orbital dynamics use the obliquity as a constraint, together with the eccentricity. When the quadrupole gravity field is known, the obliquity may provide also the moment of inertia, one of the most important quantities to constrain the body's density profile. The spin rate and the physical librations provides a strong indication on the internal differentiation of a body, as well as information on the possible orbital resonances. Here we present a technique for the estimation of the rotational state of a body from orbit, with applications to Titan and Mercury. For Titan we have used existing SAR images from the Cassini mission, while for Mercury we relied on simulations of the optical observations from ESA's BepiColombo high resolution camera. Georeferenced images of the same area, taken at different times, are compared by pattern matching algorithms in order to determine the registration error. Different pattern matching procedures can be applied, as such as cross-correlation, mutual information technique, and SIFT/SURF algorithms. The mismatching is mainly due to errors in the rotational model, with smaller contributions from the spacecraft ephemerides and attitude, camera or radar calibration, and image processing. The image correlation is followed by a weighted least-squares fit to update the rotational model and minimize the mismatch between the features. The apparent misregistration of tiepoints is used to estimate the rotational parameters, such as the spin pole location, the spin rate and precession and nutation coefficients. We report on the results and the methods obtained for Cassini and BepiColombo, providing new estimates of the obliquity and spin rate of Titan and expected accuracies the obliquity and physical libration amplitude of Mercury. We report on the applied methods, the error budgets relative to each experiment and the obtained results. Copyright © (2012) by the International Astronautical Federation
INTERPLANETARY PLASMA TURBULENCE AND THE DOPPLER DETECTION OF A GRAVITATIONAL-WAVE BACKGROUND
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Stability and control of electrodynamic tethers for de-orbiting applications
No full textElectrodynamic tethers provide a very promising propulsion system for de-orbiting of spent upper stages or LEO satellites. In this application, the Lorentz force generated by the interaction between the current in the wire and the geomagnetic field produces an electrodynamic drag leading to a fast orbital decay. The attractiveness of tether system lies especially in their capability to operate with uncontrollable satellites and in the modest mass requirement. The need for significant along-track forces leads however to the onset of an undesirable torque which, if not controlled, may drive the system into a dangerous instability. The electrodynamic torque determines in-plane and out-of-plane librations whose amplitude depends upon the current in the wire, mass distribution and system dimensions. Even more important, this torque is modulated along the orbit due to the changing magnetic field and ionospheric plasma density, giving rise to forced oscillations. The counteracting (and stabilizing) gravity grad ient torque is generally to small to ensure stability in typical, strongly non-symmetrical mass distributions, where a massive satellite or upper stage is attached at the lower end and a light electron collecting device (or passive ballast mass) is deployed a few kilometers above. Reducing the electron current or increasing the mass at the upper end are both unattractive solutions. In this paper we show how the electrodynamic torque pumps energy into the system (finally leading to large librations angles) and indicate that many proposed configurations are intrinsically unstable. Our results point out the need for a control strategy. Fortunately, the librations amplitudes can be bruited by acting on the current flowing in the wire Our model of a rigid, conductive tether shows that a control based upon nmely current switch-off, using energy criteria, is indeed effective and simple to implement. The resultant dutycycles are satisfactory and affect only marginally the deorbiting times. © 2001 International A stronautical Federation. Published by Elsevier Science Ltd
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