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In order to improve our understanding of the interior structure of Saturn's small moon Enceladus, we reanalyze radiometric tracking and onboard imaging data acquired by the Cassini spacecraft during close encounters with the moon. We compute the global shape, gravity field, and rotational parameters of Enceladus in a reference frame consistent with the International Astronomical Union's definition, where the center of the Salih crater is located at −5° East longitude. We recover a quadrupole gravity field with J3 and a forced libration amplitude of 0.091° ± 0.009° (3‐σ). We also compute a global shape model using a stereo‐photoclinometry technique with a global resolution of 500 m, although some local maps have higher resolutions ranging from 25 to 100 m. While our overall results are generally consistent with previous studies, we infer a thicker 27–33 km mean ice shell, a thinner 21–26 km mean ocean thickness, and a mean core density range of 2,270–2,330 kg/m3. Plain Language Summary: Geodetic data, such as shape, gravity, and rotation, provide important constraints for probing a planetary body's interior structure. We analyze radiometric tracking and onboard imaging data acquired during close encounters of Enceladus by the Cassini spacecraft to compute geodetic products including topographic and gravitational fields in a common reference frame. The recovered Enceladus topography has a global resolution of 500 m, with some local regions having 25–100 m resolution. Our study suggests that Enceladus has a 27–33 km mean ice shell thickness, a 21–26 km mean ocean thickness, and a mean core density range of 2,270–2,330 kg/m3. Key Points: A full quadrupole gravity field with J3 and the forced libration amplitude of 0.091° ± 0.009° are recoveredA 500‐m resolution global topography model was computed, with some local regions having 25−100 m resolutionThe results suggest that Enceladus has a 27–33 km mean ice shell thickness, a 21–26 km ocean thickness, and a mean core density range of 2,270–2,330 kg/m3 [ABSTRACT FROM AUTHOR]

By successively orbiting both 4 Vesta and I Ceres the Dawn mission directly addresses the long-standing goals of understanding the origin and evolution of the solar system. Ceres and Vesta are two complementary terrestrial protoplanets (one apparently "wet" and the other "dry"), whose accretion was probably terminated by the formation of Jupiter. They provide a bridge in our understanding between the rocky bodies of the inner solar system and the icy bodies of the outer solar system. Ceres appears to be undifferentiated while Vesta has experienced significant heating and likely differentiation. Both formed very early in the history of the solar system and while suffering many impacts have remained intact, thereby retaining a record of events and processes from the time of planet formation. Detailed study of the geophysics and geochemistry of these two bodies provides critical benchmarks for early solar system conditions and processes that shaped its subsequent evolution. Dawn provides the missing context for both primitive and evolved meteoritic data, thus playing a central role in understanding terrestrial planet formation and the evolution of the asteroid belt. Dawn is to be launched in May 2006 arriving at Vesta in 2010 and Ceres in 2014, stopping at each to make I I months of orbital measurements. The spacecraft uses solar electric propulsion, both in cruise and in orbit, to make most efficient use of its xenon propellant. The spacecraft carries a framing camera, visible and infrared mapping spectrometer, gamma ray/neutron spectrometer, magnetometer, and radio science. (C) 2003 Elsevier Ltd. All rights reserved.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.12.025 Byline: Alex S. Konopliv, Charles F. Yoder, E. Myles Standish, Dah-Ning Yuan, William L. Sjogren Keywords: Mars; interior; Geophysics; Satellites of Mars; Planetary dynamics; Asteroids Abstract: With the collection of six years of MGS tracking data and three years of Mars Odyssey tracking data, there has been a continual improvement in the JPL Mars gravity field determination. This includes the measurement of the seasonal changes in the gravity coefficients (e.g., J.sub.2, J.sub.3, C.sub.21, S.sub.21, C.sub.31, S.sub.31) caused by the mass exchange between the polar ice caps and atmosphere. This paper describes the latest gravity field MGS95J to degree and order 95. The improvement comes from additional tracking data and the adoption of a more complete Mars orientation model with nutation, instead of the IAU 2000 model. Free wobble of the Mars' spin axis, i.e. polar motion, has been constrained to be less than 10 mas by looking at the temporal history of C.sub.21 and S.sub.21. A strong annual signature is observed in C.sub.21, and this is a mixture of polar motion and ice mass redistribution. The Love number solution with a subset of Odyssey tracking data is consistent with the previous liquid outer core determination from MGS tracking data [Yoder et al., 2003. Science 300, 299-303], giving a combined solution of k.sub.2=0.152[+ or -]0.009 using MGS and Odyssey tracking data. The solutions for the masses of the Mars' moons show consistency between MGS, Odyssey, and Viking data sets; Phobos GM=(7.16[+ or -]0.005)x10.sup.-4 km.sup.3/s.sup.2 and Deimos GM=(0.98[+ or -]0.07)x10.sup.-4 km.sup.3/s.sup.2. Average MGS orbit errors, determined from differences in the overlaps of orbit solutions, have been reduced to 10-cm in the radial direction and 1.5 m along the spacecraft velocity and normal to the orbit plane. Hence, the ranging to the MGS and Odyssey spacecraft has resulted in position measurements of the Mars system center-of-mass relative to the Earth to an accuracy of one meter, greatly reducing the Mars ephemeris errors by several orders of magnitude, and providing mass estimates for Asteroids 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, and 324 Bamberga. Author Affiliation: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Article History: Received 20 May 2005; Revised 24 October 2005

As part of the NEAR Radio Science investigation, a global solution that includes both spherical and ellipsoidal harmonic gravity fields of Eros, Eros pole and rotation rate, Eros ephemeris, and landmark positions from the optical data was generated. This solution uses the entire one-year in orbit collection of X-band radiometric tracking (Doppler and range) from the Deep Space Network and landmark tracking observations generated from the NEAR spacecraft images of Eros. When compared to a constant density shape model, the gravity field shows a nearly homogeneous Eros. The Eros landmark solutions are in good agreement with the Eros shape model, and they reduce the center-of-mass and center-of-figure offset in the z direction to 13 m. Most of the NEAR spacecraft orbits are determined in all directions to an accuracy of several meters. The solution for the ephemeris of Eros constrains the mass of Vesta to 18.2 [+ or -] 0.4 [km.sup.3]/[s.sup.2] and reduces the uncertainty in the Earth-Moon mass ratio. Key Words: asteroids, Eros; asteroids, Vesta; asteroids, rotation; surfaces, asteroids.

The gravity field of the moon has been investigated since 1966 when the Russian Luna l0 was placed in orbit around the moon and provided dynamical proof that the oblateness […]

A 60th degree and order spherical harmonic gravity field for the planet Venus is obtained from Magellan and Pioneer Venus Orbiter radiometric tracking data. The gravity field obtained is accurate since the Magellan and Pioneer data includes the high-precision X-band gravity tracking provided by the various instruments in Magellan and Pioneer spacecraft. Models for the gravitational field in Venus are developed using Kaula's power rule for Venus in combination with radiometric tracking data.

For the first time for any planetary body other than the Earth, the free wobble of the pole called the Chandler wobble has been detected for Mars with a period of 206.9 ± 0.5 days and amplitude of 10 cm from radio tracking observations of Mars Odyssey, Mars Reconnaissance Orbiter (MRO), and Mars Global Surveyor (MGS), in order of decreasing sensitivity. The motion of the rotation pole location on the surface of Mars, or polar motion, is observed using two different approaches: (1) joint global estimates of Mars' orientation and its gravity field and (2) time series solutions of C21 and S21. For Mars interior models, the Chandler wobble period is combined with other measurements including the moments of inertia from our estimated precession rate ψ̇=−7603.9±1.3mas/year and tidal Love number k2 = 0.169 ± 0.006. The Chandler wobble period constrains the rheology of the Martian mantle and in particular its long‐term frequency dependence. Plain Language Summary: For the first time for any solar system body other than the Earth, one component of the motion of the Mars spin axis on the surface of Mars, called the Chandler wobble, has been detected. The motion has a period of 206.9 ± 0.5 days, an amplitude of 10 cm on the surface, and is in a nearly circular counterclockwise direction as viewed from the North Pole. The pole motion is determined from radio tracking observations of Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Global Surveyor, in order of decreasing sensitivity. The detection of the Chandler wobble improves our understanding of energy dissipation in the mantle for time intervals near the wobble period. Key Points: For the first time, the Chandler wobble has been detected for a solar system body other than the EarthThe Chandler wobble is visible in the tracking data from two Mars orbiting spacecraftThe Chandler wobble significantly improves our understanding of the long period behavior of the mantle rheology [ABSTRACT FROM AUTHOR]

The lunar gravity field is determined from the tracking data of previous missions to the Moon with the 1998-1999 Lunar Prospector (LP) mission being the major contributor. LP provided the first measurement of the gravity field in a low polar circular orbit giving complete coverage at high resolution for the entire lunar nearside. However, since there is no direct measurement of the lunar farside from LP or any other mission, gravity details for the farside gravity are greatly limited. Even so, it has become apparent that there are mascons on the farside of the Moon together with the newly identified mascons in the LP data for the lunar nearside. The extended mission low-altitude data (at times less than 10 km above the surface) has gravity information for the nearside to nearly degree and order 180. The 100th-degree lunar gravity models (LP100J and LP100K) extract most of the information from the nominal 100-km altitude. A 165th degree model LP165P attempts to model the extended mission data with some but limited success. This model provides a smooth solution without aliasing when evaluated up to degree 110 allowing for resolution of numerous craters. In addition, a preliminary solution for the lunar Love number is [k.sub.2] = 0.026 [+ or -] 0.003. [C] 2001 Academic Press Key Words: lunar, gravity; spherical harmonics; Doppler.

Abstract: With 2years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k 2 =0.164±0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGMSun /dt <1.6×10−13 GMSun year−1). [ABSTRACT FROM AUTHOR]

Prior to the Near Earth Asteroid Rendezvous (NEAR) mission, little was known about Eros except for its orbit, spin rate, and pole orientation, which could be determined from ground-based telescope observations. Radar bounce data provided a rough estimate of the shape of Eros. On December 23, 1998, after an engine misfire, the NEAR-Shoemaker spacecraft flew by Eros on a high-velocity trajectory that provided a brief glimpse of Eros and allowed for an estimate of the asteroid''s pole, prime meridian, and mass. This new information, when combined with the ground-based observations, provided good a priori estimates for processing data in the orbit phase.After a one-year delay, NEAR orbit operations began when the spacecraft was successfully inserted into a 320×360 km orbit about Eros on February 14, 2000. Since that time, the NEAR spacecraft was in many different types of orbits where radiometric tracking data, optical images, and NEAR laser rangefinder (NLR) data allowed a determination of the shape, gravity, and rotational state of Eros. The NLR data, collected predominantly from the 50-km orbit, together with landmark tracking from the optical data, have been processed to determine a 24th degree and order shape model. Radiometric tracking data and optical landmark data were used in a separate orbit determination process. As part of this latter process, the spherical harmonic gravity field of Eros was primarily determined from the 10 days in the 35-km orbit. Estimates for the gravity field of Eros were made as high as degree and order 15, but the coefficients are determined relative to their uncertainty only up to degree and order 10. The differences between the measured gravity field and one determined from a constant density shape model are detected relative to their uncertainty only to degree and order 6. The offset between the center of figure and the center of mass is only about 30 m, indicating that Eros has a very uniform density (1% variation) on a large scale (35 km). Variations to degree and order 6 (about 6 km) may be partly explained by the existence of a 100-m, regolith or by small internal density variations. The best estimates for the J2000 right ascension and declination of the pole of Eros are α=11.3692±0.003° and δ=17.2273±0.006°. The rotation rate of Eros is 1639.38922±0.00015°/day, which gives a rotation period of 5.27025547 h. No wobble greater than 0.02° has been detected. Solar gravity gradient torques would introduce a wobble of at most 0.001°. [Copyright &y& Elsevier]

This paper presents the results of simulations of a high-resolution and temporal lunar gravity field expected from the Gravity Recovery and Interior Laboratory mission. An overall mission capability is presented based on detailed error analysis of spacecraft dynamics and kinematics models, realistic ground-based and interspacecraft tracking measurement uncertainties, and length of data arcs. The largest source of dynamics unmodeled error comes from the spacecraft thermal radiation force and, in order to characterize its error contribution, an a priori error constraint model is derived based on orbit geometry and expected force magnitude. The result shows that estimating a lunar gravity field is robust against both dynamics and kinematics errors, and a nominal field of degree 300 or better can be determined assuming a 2.5 x 10-4/n² power law. The resolution of the gravity field is most sensitive to the interspacecraft Ka-band tracking accuracy. The core signature, however, is more sensitive to dynamic modeling errors, and satisfying the latter science requirements depends on how accurately the spacecraft dynamics can be modeled. [ABSTRACT FROM AUTHOR]