Publications (83)422.17 Total impact
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Article: Asteroid 21 Lutetia: low mass, high density.
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ABSTRACT: Asteroid 21 Lutetia was approached by the Rosetta spacecraft on 10 July 2010. The additional Doppler shift of the spacecraft radio signals imposed by 21 Lutetia's gravitational perturbation on the flyby trajectory were used to determine the mass of the asteroid. Calibrating and correcting for all Doppler contributions not associated with Lutetia, a least-squares fit to the residual frequency observations from 4 hours before to 6 hours after closest approach yields a mass of (1.700 ± 0.017) × 10(18) kilograms. Using the volume model of Lutetia determined by the Rosetta Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera, the bulk density, an important parameter for clues to its composition and interior, is (3.4 ± 0.3) × 10(3) kilograms per cubic meter.Science 10/2011; 334(6055):491-2. · 31.20 Impact Factor -
Article: Towards a theory of algorithm‐determined cognitive test construction
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ABSTRACT: A rationale for, and data from, a trial of a theory of item generation by algorithms whose origins are cognitive models of task performance are presented. Since Spearman (1904), intelligence has been operationally defined and assessed in human subjects by administering identical test items whose content and order have been fixed only after empirical iterations. In our approach, intelligence is ostensively defined by theoretically determined algorithms used for item construction and presentation. Knowledge of what cognitive factors limit human performance makes it possible to vary within tightly specified parameters those features of the tasks that contribute to difficulty, which we call radicals, to let those components of the tasks that do not contribute to difficulty vary randomly, and to counterbalance aspects of answer production that might induce biases of response. Empirical data are based on the generation of five different short tests demanding only functional literacy as a prerequisite for their execution. Four parallel forms of each test were administered to young male Army recruits whose scores were collated with their Army Entrance Test results, which were not previously known to us. Results show that the parallel, algorithm-generated item sets are statistically invariant, which item generation theory demands; and that the individual tests differentially predict Army Entrance Test scores. We conclude that IQ test performances are parsimoniously explained by individual differences in encoding, comparison and reconstructive memory processes.British Journal of Psychology. 04/2011; 81(2):173 - 195. -
Article: Odyssey: a solar system mission
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ABSTRACT: The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].Experimental Astronomy 01/2009; 23(2):529-547. · 1.82 Impact Factor -
Article: The Pioneer Anomaly and its Implications
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ABSTRACT: The Pioneer 10/11 spacecraft yielded the most precise navigation in deep space to date. However, their radio-metric tracking data has consistently indicated the presence of a small, anomalous, Doppler frequency drift. The drift is a blue-shift, uniformly changing with a rate of ~6 × 10-9 Hz/s and can be interpreted as a constant sunward acceleration of each particular spacecraft of aP = (8.74 ± 1.33) × 10-10 m/s2. The nature of this anomaly remains unexplained. Here we summarize our current knowledge of the discovered effect and review some of the mechanisms proposed for its explanation. Currently we are preparing for the analysis of the entire set of the available Pioneer 10/11 Doppler data which may shed a new light on the origin of the anomaly. We present a preliminary assessment of such an intriguing possibility.EAS Publications Series 12/2005; 20:243 - 250. -
Article: Saturn's Gravity Field and Interior Structure: First Results From the Cassini Mission
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ABSTRACT: The orbiting Cassini spacecraft can be used as a probe of Saturn's gravity field. In particular, two close approaches are planned for Cassini Radio Science on 2006 September 9 and 2008 May 17 at distances of 2.94 R_S and 3.26 R_S (R_S = 60,330 km), distances comparable with Voyager flyby distances at 3.04 R_S and 2.63 R_S. However, with Doppler tracking at X-Band (~ 8.4 GHz), rather than the Voyager and Pioneer tracking at S-Band (~ 2.3 GHz), and with slower velocities at closest approach, Cassini can improve on results from Voyager I and II and Pioneer 11. The Pioneer flyby distance is the most favorable of all, 1.27 R_S, but at a separation angle of 8° from the Sun, where the S-Band tracking data are compromised by coronal plasma noise. Remarkably, in the process of navigating the craft, the Cassini Navigation Team has determined an improved gravity field from five Saturn periapsis passages between 6 February 2004 and 10 March 2005, two of which are closer than the Voyager flybys. They report a Saturn GM of 37931123 ± 36 km3/s2, and zonal harmonic coefficients J2 = 16291.4 ± 0.9, J4 = -931 ± 5, and J6 = 91 ± 13, all in units of 10-6, and for a reference radius R_S, the one-bar equatorial radius. With the Gundlach and Merkowitz G of (6.674215 ± 0.000092) × 10-11 m3/kg/s2, the total mass M is (5.683234 ± 0.000078) × 1026 kg. The mean radius R is 58,265,253 m for a mean density of 685.9297 ± 0.0094 kg/m3. We use this improved gravity field, along with the Zharkov-Trubitsyn theory of figures, to infer properties of Saturn's interior, similar to the inferences drawn from the gravity fields of the Galilean satellites during the Galileo mission at Jupiter. We use a solid-body rotation period from magnetic-field data of 10h 39m 23.0s, in close agreement with the IAU value. The associated smallness parameter ω2R3/GM is 0.139883.AGU Fall Meeting Abstracts. 11/2005; -1:01. -
Article: Mass Anomalies on Ganymede
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ABSTRACT: The discovery of mass anomalies on Ganymede was reported this summer (Anderson et al., Science 305, 989 (2004)). We report here on a more detailed characterization of the source of the anomalies. In order to reduce the noise evident in the Doppler residuals previously used, we applied a variable-width Gaussian filter to the time series. The filter width in the time domain increases with the spacecraft altitude, reducing the noise before and after closest approach. The smoothed Doppler data were numerically differentiated and the resulting accelerations along the line of sight were fit with a multiple point-mass model. The variable-width filter reveals a previously obscured positive acceleration feature about 300 s before closest approach. Rather than two or three mass points as reported in Anderson et al., we find that five mass points provide a much improved fit to the data, including the new acceleration feature before closest approach. Two of the five masses are near the previous masses from the two-point fit, and are in good agreement with their mass values. There is a positive anomaly at about 60o north latitude and a negative anomaly at about 24o north latitude. We conclude that the two-point fit reveals two major anomalies on Ganymede, but misses three more revealed by the five-point fit. Further, the mass anomalies can be divided into two groups. Three of the five masses could indicate a single broad anomaly under the outgoing flyby trajectory centered roughly at 45o north latitude and 18o west longitude. The other two masses could indicate a single extended anomaly under the incoming trajectory centered roughly at 20o north latitude and 173o west longitude. We also include results on placing the five masses at different depths from zero to 1450 km below the surface. A good fit is obtained at any depth from surface to rock-ice interface at about 800 km depth, but the fit deteriorates at greater depth. It is highly unlikely mass anomalies exist within Ganymede's ice shell. We prefer either the near surface or the rock-ice interface. The rock-ice interface is attractive based on rigidity arguments, and the suggestion of two major extended anomalies is even more striking at greater depth. In order to fit the acceleration data, the anomalies must be six or seven times more massive at the 800 km depth than at the surface.AGU Fall Meeting Abstracts. 11/2004; -1:0967. -
Article: Gravity Inversion Considerations for Radio Doppler Data from the JUNO Jupiter Polar Orbiter
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ABSTRACT: Gravity science is a major component of the JUNO Mission, one of two selected by NASA for detailed study as a candidate for the next mission in the New Frontiers Program. The plan for JUNO is to insert a spinning spacecraft into an eccentric orbit with period of about 11 days and a periapse only a few thousand kilometers above Jupiter's surface. On each of a possible 32 orbits, the Jovian gravitational field is accessible for an observing interval of plus and minus six hours from closest approach. On each observing run, multi-link radio Doppler data are generated by the Deep Space Network (DSN) in the X-Band ( ˜8.4 GHz) and Ka-Band ( ˜34.3 GHz). This produces Doppler velocity measurements to an accuracy of about 0.005 mm/s at a sample interval of 60 s. We propose a gravity inversion technique that can yield an accuracy of 10-9 for lower degree normalized zonal harmonics through degree six, providing information on Jupiter's core, and higher degree harmonics, especially degree 12 through 30, can be measured to an accuracy of about 10-8. The requirement on the gravity measurements for a discrimination between solid-body rotation and rotation in deep zonal flows has been discussed by Hubbard (Icarus 137 357). The proposed inversion technique meets this requirement. In addition, over the total time interval of 32 orbits, or about one year, the polar precession rate can possibly be measured, and tides raised by the five Jovian satellites JI to JV, as reflected in the gravity data, can yield information on Jupiter's Love numbers kn at five different depths in the atmosphere. The JPL contribution to this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. G.S. and J.L.P acknowledge support by grants from NASA through the Planetary Geology and Geophysics program.10/2004; 36:1094. -
Article: Stardust Dynamic Science at Wild 2: First Look
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ABSTRACT: The Dynamic Science investigation on the STARDUST mission has been described previously. The data delivered by the STARDUST Project is multifold, but basically it consists of radio Doppler data from the Deep Space Network (DSN) and attitude control data (ACS) from the spacecraft. Doppler data were successfully recorded by JPL's Navigation System (closed-loop data) and also by its Radio Science System (open-loop data) at DSN stations DSS43 near Canberra Australia and at DSS14 at Goldstone California. Attitude control data were also successfully delivered to the Dynamic Science Team. Here we describe a preliminary analysis of the data. Beyond a closest approach distance of 150 km, a Doppler detection of a the Wild 2 nucleus mass was not expected. The current best estimate of the closest approach distance is 236.4 km, and as expected, any mass signal in the Doppler data is hopelessly buried in the noise. We have attempted to fit the data to a mass model with no success. However, analysis of the Doppler data and the ACS data for particle impacts on the spacecraft's Whipple shields is in progress, and will be reported at the meeting. The DSS43 closed-loop Doppler residuals are plotted as a function of time from the current best estimate of the time of Wild 2 closest approach, 2 January 2004, 19:43:11.7 UTC, Earth-receive time at the station.02/2004; -
Article: Mass Anomalies on Ganymede
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ABSTRACT: Radio Doppler data from two Ganymede encounters (G1 and G2) on the first two orbits in the Galileo mission have been analyzed previously for gravity information . For a satellite in hydrostatic equilibrium, its gravitational field can be modeled adequately by a truncated spherical harmonic series of degree two. However, a fourth degree field is required in order to fit the second Galileo flyby (G2). This need for a higher degree field strongly suggests that Ganymede s gravitational field is perturbed by a gravity anomaly near the G2 closest approach point (79.29 latitude, 123.68 west longitude). In fact, a plot of the Doppler residuals , after removal of the best-fit model for the zero degree term (GM) and the second degree moments (J2 and C22), suggests that if an anomaly exists, it is located downtrack of the closest approach point, closer to the equator.02/2004; -
Article: Geodesy of Amalthea and the Galilean Satellites of Jupiter
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ABSTRACT: An important scientific legacy of the Galileo mission is the determination of the masses and quadrupole components of the gravitational fields of the Galilean satellites. A final report of the mission results is given including values of GM (G is the universal gravitational constant, M is satellite mass), the gravitational coefficients J2 and C22, and the correlation coefficient μ between J2 and C22. The values of J2 and C22 are deduced using the a priori assumption J2 = (10/3)C22. The least squares method for fitting the Doppler residuals does not fix this ratio, but allows J2 and C22 to vary independently and determines the correlation between them. The a priori assumption is consistent with the hydrostatic equilibrium of a satellite, but it does not require hydrostaticity. Values of μ show that J2 and C22 are independently determined only for Io; the ratio of J2 and C22 is consistent with a hydrostatic Io. J2 and C22 are not independently determined for Ganymede even though there are both equatorial and polar flybys of the satellite. A quadrupole field is insufficient to fit the Ganymede data to the noise level. The additional signal is interpreted in terms of mascon anomalies at the surface of Ganymede. The gravitational coefficients, together with the assumption that the degree~2 gravitational fields of the satellites derive from their hydrostatic distortions to rotation and the Jovian tidal force, are used to infer the moments of inertia of the satellites and their internal structures. The mass and closest approach distance for Amalthea can be determined from Doppler data from the Galileo encounter of 5~November 2002. The final results indicate a density that is significantly smaller than the approximate 1000~kg\ m-3 density of water ice. The quadrupole components of Amalthea's gravitational field are undetectable in the encounter Doppler data.AGU Fall Meeting Abstracts. 11/2003; -1:01. -
Conference Proceeding: The Cassini solar conjunction experiment: a new test of general relativity
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ABSTRACT: During the Cassini solar conjunction experiment, radio-metric data have been used to measure the space components of the sun's metric tensor. In the parameterized post-newtonian formalism, this quantity is controlled by a parameter (γ) which is unity in General Relativity but may assume slightly different values in other metric theories of gravity. The measurement of γ with Cassini is based upon a new observable quantity: the frequency shift induced on a radio wave when the spacecraft is near solar conjunction. The calibration of the large frequency fluctuations induced by the solar corona, necessary in order to guarantee the required signal stability, is performed using the novel Cassini radio frequency system, based on a multifrequency link. In this paper the operations performed and the results obtained from the preliminary analysis of the June-July 2002 Cassini solar conjunction data are described. The multifrequency link plasma calibration scheme is also illustrated along with the techniques for the reduction of the non-dispersive, tropospheric effects. The high quality of the calibrated Doppler data indicates that the experiment provides a high accuracy test of general relativity.Aerospace Conference, 2003. Proceedings. 2003 IEEE; 02/2003 -
Article: Radio Science Concepts and Approaches for Jupiter Icy Moons Orbiter
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ABSTRACT: Radio Science experiments have been conducted on most deep space missions leading to numerous scientific discoveries. A set of concepts and approaches are proposed for the Jupiter Icy Moons Orbiter (JIMO) to apply Radio Science tools to investigate the interior structures of the Galilean Satellites and address key questions on their thermal and dynamical evolution. Measurements are identified that utilize the spacecraft's telecommunication system. Additional instruments can augment these measurements in order to leverage observational synergies. Experiments are also offered for the purpose of investigating the atmospheres and surfaces of the satellites.02/2003; -
Article: Gravity Field, Topography, and Interior Structure of Amalthea
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ABSTRACT: A close Galileo flyby of Jupiter's inner moon Amalthea (JV) occurred on 5 November 2002. The final aimpoint was selected by the Galileo Radio Science Team on 5 July 2002. The closest approach distance for the selected aimpoint was 221 km from the center of mass, the latitude was - 45.23 Deg and the west longitude was 266.41 Deg (IAU/IAG/COSPAR cartographic coordinate system). In order to achieve an acceptable impact probability (0.15%), and yet fly close to Amalthea, the trajectory was selected from a class of trajectories running parallel to Amalthea's long axis. The Deep Space Network (DSN) had the capability to generate continuous coherent radio Doppler data during the flyby. Such data can be inverted to obtain information on Amalthea's gravity field. Amalthea is irregular and neither a triaxial ellipsoid nor an equilibrium body. It has a volume of about 2.4 x 106 km3, and its best-fit ellipsoid has dimensions 131x73x67 km. Its mass can be determined from the 2002 flyby, and in combination with the volume, a density can be obtained accurate to about 5%, where the error is dominated by the volume uncertainty. Similarly, gravity coefficients (Cnm Snm) can be detected up to fourth degree and order, and the second degree field (quadrupole) can be measured. Topography data are available from Voyager imaging and from images taken with Galileo's solid state imaging system at various times between February and June 1997. By combining the gravity and topography data, new information can be obtained on Amalthea's interior. For example if the gravity coefficients agree with those calculated from the topography, assuming constant density, we can conclude that Amalthea is homogeneous. On the other hand, if the gravity coefficients are smaller than predicted from topography, we can conclude that there is a concentration of mass toward Amalthea's center. We are presenting preliminary pre-publication results at the Fall meeting. This work was sponsored by the Galileo Project and was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. G.S., P.C.T., and W.B.M. acknowledge support by grants from NASA under the Planetary Geology and Geophysics program. G.W. is a visiting PhD student at JPL, May 2002 - May 2003, and acknowledges support from the Austrian Ministry for Technology and a Zonta - Amelia Earhart fellowship.AGU Fall Meeting Abstracts. 11/2002; -1:13. -
Article: Effect of the Pioneer Anomaly on Long-Period Comet Orbits
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ABSTRACT: Radio tracking data from Pioneer 10 and 11 show an anomalous, constant, Doppler frequency drift that can be interpreted as an acceleration directed towards the Sun of aP = (8.74 +/- 1.33) x 10-8 cm/s2, at heliocentric distances ~ 20-70 AU (Anderson et al., Phys. Rev. Lett. 81, 2858; Anderson et al., Phys. Rev. D 65, 082004). In the absence of any physical theory that predicts such an acceleration, the primary candidate remains systematic error generated by spacecraft systems. However, neither we nor anyone else has been able to find a viable spacecraft systematic that is both large enough and constant enough to explain the anomaly. We show that the Pioneer anomaly, interpreted as a physical effect external to the spacecraft, can have profound implications for LP comet orbits. If the anomaly behaves as an ever-present constant force, it shifts the 364 1/aorig values for higher quality LP orbits (Marsden and Williams, Catalogue of Cometary Orbits 2001, 14th ed., SAO) to higher values, where a is the osculating semi-major axis in the Kepler ellipse. In addition, it eliminates all hyperbolic orbits. The Oort cloud becomes a narrow shell of comets at about 2500 AU with a thickness of about 400 AU. Two drag models also seem viable. In the first, the drag acceleration is proportional to the orbital speed V and in the second it is proportional to V2. In these models, comets entering the solar system on interstellar orbits become nearly parabolic on their first pass, as observed, and then undergo further evolution by drag and planetary perturbations. Large effects on short period comets such as Halley and Encke are avoided by assuming a hole in the resisting medium inside 10 or 20 AU, consistent with limited Pioneer data analysis in this region. The work of J.D.A. and S.G.T was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. M.M.N. acknowledges support by the U.S. DOE. J.D.A. acknowledges partial support as a 2002 Visiting Scientist at Monash University, Australia, under the sponsorship of Dr. A. J. R. Prentice, Department of Mathematics.11/2002; 34:1172. -
Article: Radio Science Investigations with the Deep Space Network
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ABSTRACT: Scientists utilize the telecommunication links between spacecraft and Earth to examine changes in the phase/frequency, amplitude, line-width, or polarization, as well as round-trip light time, of radio signals to investigate: planetary atmospheres and ionospheres, planetary rings, planetary surface characteristics, shapes, gravitational fields, orbital motion and dynamics of solar system bodies, magnetic fields of the Sun and planets, the solar wind and corona, cometary atmospheres, gravitational waves, gravitational redshift, relativistic time-delay, and other phenomena. Recommendations are made to maintain and expand the existing strong science capabilities in the network, carry them into the next generation DSN, continue advancing the technology to improve sensitivity, and to enable future experiments via an efficient multi-mission system that reduces cost via remote and automated operations. This report is dedicated to the memory of George M. Resch, who passed away on November 22, 2001.07/2002; 272:355-360. -
Article: Recent Results for Solar-System Tests of General Relativity
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ABSTRACT: We use the current JPL archive of planetary positional data, along with the data-analysis implementation described by Newhall, Standish, and Williams, to solve for all the conventional parameters included in the JPL planetary ephemerides, plus four more parameters specific to tests of General Relativity. Spacecraft ranging data generated by the Deep Space Network (DSN) for NASA missions, and radar ranging measurements to Mercury and Venus are of primary importance. Venus radar ranging has been calibrated for topography by means of altimetry data from the Pioneer 12 Venus orbiter. Martian ranging data consists of orbiter data from Mariner 9 (1971 to 1972) and Mars Global Surveyor (1998 to 2000), as well as lander data from Viking (1976 to 1982) and Pathfinder (1997). In fitting the data, we weight the separate data sets, except for Mars, such that the assumed standard error for each data set is equal to the RMS residual for that particular set after the fit. For Mars, we use a standard error equal to 5 times the RMS residual for each of the four data sets. This is done to compensate for the systematic error from asteroid perturbations. We interpret the resulting parameter errors after the fit as realistic errors, as opposed to formal errors that would result from setting the Mars standard errors equal to their RMS values. The new values of the PPN parameters β and γ , and the solar gravitational quadrupole moment J2, are: β - 1 = -0.0010 +/- 0.0012, γ - 1 = -0.0015 +/- 0.0021, J2 = (2.3 +/- 5.2) x 10-7. The fourth parameter represents a possible time variation ˙ {G} in the gravitational constant. It is unchanged from what we reported last year at the Pasadena meeting. We conclude that the orbits of the inner planets are consistent with the Newtonian inverse-square law of attraction and with General Relativity at the post-Newtonian order O(v2/c2). This work was sponsored by NASA's Space Astrophysics Research and Analysis (SARA) program and was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.04/2002; 34:660. -
Article: Recent Results on Io's Gravity Field and Interior Structure
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ABSTRACT: During the last close flyby of Io on 17 January 2002, the Galileo spacecraft automatically shut down the science sequence in response to the detection of a possible fault. However, because shutting down the telecommunications system could be suicidal, Doppler tracking with the S-band radio carrier wave proceeded as scheduled. As a result, coherent Doppler data are available for four close Io flybys. Although a fifth close approach before JOI produced one-way Doppler data referenced to the spacecraft oscillator, and although it was responsible for the discovery of a large metallic core in Io and remains a part of the data set (Anderson et al., Science 272, 709-712, 1996), it adds little gravity information to the four later flybys. The closest of the five flybys suitable for gravity analysis is the last on the 33rd orbital revolution at a 102 km altitude, followed by a flyby on the 27th orbital revolution at a 198 km altitude and a polar flyby on the 25th orbital revolution at a 300 km altitude. A combination of data from this polar pass with the other four equatorial passes can be used to obtain an independent determination of rotational and tidal terms in the gravitational potential. We find that adding data from the last 102 km flyby reduces the error on the gravity coefficient C22 by about 30% over what we previously reported from four flybys (Anderson et al., Jour. Geophys. Res. 106, 32,963-32,969, 2001), and it also reduces the correlation between J2 and C22 from 0.752 to 0.472. The determination of the two coefficients is still in progress, but so far the new values are consistent with previous results, and they satisfy the equilibrium constraint that J2 = 10/3 C22. Under the assumption that Io's interior is in hydrostatic equilibrium, interior models can be constructed that satisfy the constraints of mean radius R = 1821.6 +/- 0.5 km (Thomas et al., Icarus 135, 175-180, 1998), mean density (3527.8 +/- 2.9 kg/m3), and normalized axial moment of inertia C/MR2 = 0.37685 +/- 0.00035. Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe-FeS core or between 350 and 650 km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the gravity data. This work was sponsored by the Galileo Project and was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.AGU Spring Meeting Abstracts. 04/2002; -1:01. -
Article: Mercury's Global Topography from Radar Ranging Data
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ABSTRACT: When Mercury's radius is expanded in Legendre functions to the second degree and order, the systematic error in radar ranging data is reduced substantially. Previously, data spanning an observing interval from 1966 to 1990 were used to infer an equatorial ellipticity (a - b)/a = (540 +/- 54) X 10(exp -6) and a center-of-figure minus center-of-mass offset of (640 +/- 78) m. The magnitude of this equatorial center of figure offset implies an excess crustal thickness of 12 km or less, comparable to the Moon's excess. By comparing the equatorial ellipticity with the Mariner 10 gravity field, and assuming Airy isostatic compensation, bounds on crustal thickness can be derived. Mercury's crustal thickness is in the range from 100 to 300 km. The Mercury radar ranging observing interval has been extended from 1966 to the present. In addition, improvements in data reduction techniques have resulted in a set of Mercury ranging data less affected by systematic error, in particular the biases introduced by local topographic variations. We use this new set of reduced ranging data to improve Mercury's global topography and center-of-figure minus center-of-mass offset. New results on crustal thickness are derived, and prospects for further improvement with Mercury Orbiter data are discussed.02/2001; -
Article: The gravity field and interior structure of Callisto.
08/1999; 31:1181. -
Article: Europa's differentiated internal structure: inferences from four Galileo encounters.
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ABSTRACT: Radio Doppler data from four encounters of the Galileo spacecraft with the jovian moon Europa have been used to refine models of Europa's interior. Europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice-liquid outer shell, but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice-liquid shell. The size of a metallic core is uncertain because of its unknown composition, but it could be as large as about 50 percent of Europa's radius. The thickness of Europa's outer shell of water ice-liquid must lie in the range of about 80 to 170 kilometers.Science 10/1998; 281(5385):2019-22. · 31.20 Impact Factor
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2005
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CSU Mentor
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Jet Propulsion Laboratory
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1997–2005
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California Institute of Technology
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Pasadena, CA, USA
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