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Anderson, J. D.

Publications and source records attributed to Anderson, J. D..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Distribution of rock, metals, and ices in Callisto

Radio Doppler data from a single encounter (C3) of the Galileo spacecraft with Callisto, the outermost Galilean moon of Jupiter, indicated that Callisto was probably undifferentiated. Now, similar data from a second encounter (C9) corroborate this conclusion, but more accurate data from a third encounter (C10) indicate that the rock and ice within Callisto have partially, but not completely, separated. Callisto may be differentiated into a rock-metal core less than 25 percent of Callisto's radius, an outer layer of clean ice less than 350 km thick, and a middle layer of mixed rock and ice. Models in which ice and rock are mixed all the way to the center of Callisto are also consistent with the data.

Galileo Project↗
NASA NTRS1998

Distribution of Silicates and Ices in Callisto

Using radio Doppler data from a single encounter (C3) of the Galileo spacecraft with Callisto, we have reported previously that this outermost Galilean moon might be undifferentiated.

Callisto Silicates Ice Galileo↗
NASA NTRS1997

A spacecraft going behind the Sun will support SOHO

The problems that can be solved by combining the Solar and Heliospheric Observatory (SOHO) and the magnetic structures on and around the sun (MagSonas) observations are discussed. A magneto-Doppler imager and X and Ka band linearly polarized radio signals sent to the other side of the sun can support extended SOHO mission. This is the purpose of the MagSonas mission. The MagSonas radio system, designed to serve as spacecraft communications and a sounding coronal magnetic field, is described.

Ruzmaikin, A.↗
NASA NTRS1997

Europa's Differentiated Internal Structure: Inferences from Two Galileo Encounters

Doppler data generated with the Galileo spacecraft's radio carrier wave during two Europa encounters on 19 December 1996 (E4) and 20 February 1997 (E6) were used to measure Euroapa's external gravitational field. The measurements indicate that Europa has a predominantly water ice-liquid outer shell about 100 to 200 klilometers thick and a deep interior with a density in excess of about 4000 kilograms per cubic meter.

Galileo↗
NASA NTRS1997

Europa's differentiated internal structure: inferences from two Galileo encounters

Doppler data generated with the Galileo spacecraft's radio carrier wave during two Europa encounters on 19 December 1996 (E4) and 20 February 1997 (E6) were used to measure Europa's external gravitational field. The measurements indicate that Europa has a predominantly water ice-liquid outer shell about 100 to 200 kilometers thick and a deep interior with a density in excess of about 4000 kilograms per cubic meter. The deep interior could be a mixture of metal and rock or it could consist of a metal core with a radius about 40 percent of Europa's radius surrounded by a rock mantle with a density of 3000 to 3500 kilograms per cubic meter. The metallic core is favored if Europa has a magnetic field.

Flight Experiment↗
NASA NTRS1997

Gravitational evidence for an undifferentiated Callisto

Before the arrival of the Galileo spacecraft at Jupiter, models for the interior structure of the four galilean satellites--Io, Europa, Ganymede and Callisto-ranged from uniform mixtures of rock and ice (that is, undifferentiated objects) or rocky cores surrounded by a mantle of water ice. Now it appears that Io has a large metallic core and that Ganymede is strongly differentiated, most probably into a three-layer structure consisting of a metallic core, a silicate mantle and a deep outer layer of ice. Direct information on the interior structure of Callisto determined from previous spacecraft fly-bys was essentially limited to an estimate of the mean density being intermediate between pure ice and pure rock. Here we report measurements of Callisto's gravitational field which reveal that, in contrast to Io and Ganymede, this galilean satellite is most probably a homogeneous object consisting of a solar mixture of 40% compressed ice and 60% rock (including iron and iron sulphide). Callisto's undifferentiated state is consistent with the apparent lack of an intrinsic magnetic field, and indicates that the outermost galilean satellite has not experienced a heating phase sufficiently high to separate its rock and metal components from the lighter ices.

unmanned↗
NASA NTRS1997

Cassini Titan Radio Science

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Radio Frequency Instrument Subsystem (RFIS) Ultras↗
NASA NTRS1996

Gravitational Constraints on the Internal Structure of Ganymede

Before the arrival of the Galileo spacecraft in the jovian system, there was little information on the interior structure of jupiter's largest moon, Ganymede. Its mean density (1,940 kg/cu m), determined by the Pioneer and Voyager spacecraft, implies a composition that is roughly 60% rock and 40% ice, which could be uniformly mixed or differentiated into a rocky core and icy mantle. Here we report measurements by the Galileo spacecraft of Ganymede's overall density and the spherical harmonics, J(sub 2) and C(sub 22) of its gravitational field. These data show clearly that Ganymede has differentiated into a core and mantle. Combined with the recent discovery of an intrinsic magnetic field, our gravity results suggest that Ganymede has a metallic core of radius 400-1,300 km surrounded by a silicate mantle, which is in turn enclosed by an ice she approx.800 km thick. Depending on whether the core is pure iron or an alloy of iron and iron sulphide, it could account for as little as 1.4% or as much as one-third of the total mass. If the ice were stripped away, Ganymede could look much like lo' in terms of its size and internal mass distribution.

Anderson, J. D.↗
NASA NTRS1996

Shape and Orientation of Mercury from Radar Ranging Data

If Mercury's radius is expanded in Legendre functions to the second degree and order, the systematic error in radar ranging can be reduced significantly. We interpret the expansion coefficients in terms of a best-fit ellipsoid displaced with respect to the center of mass in the equatorial plane. The ellipsoid's principal axes are rotated in the equatorial plane such that the long axis is aligned with cartographic longitude 15.3 deg +/- 2.9 deg (west). The pole location is consistent with the IAU pole, normal to Mercury's orbital plane. There is a significant equatorial ellipticity (a - b)/a = (540 +/- 54) x 10(exp -6). The center of figure is offset from the center of mass (C.F.-C.M.) by 640 +/- 78 m in the equatorial plane in the direction of cartographic longitude 319.5 deg +/- 6.9 deg. The magnitude of the 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 coefficient C(sub 22) and assuming Airy isostatic compensation, we conclude that Mercury's crustal thickness is in the range from 100 to 300 km.

Anderson, J. D.↗
NASA NTRS1996

Initial Galileo Gravity Results and the Internal Structure of IO

This article contains results from the Galileo spacecraft's arrival at Jupiter on Decomber 7, 1995. We report the discovery of a tri-axial gravity field for the innermost Galilean satellite Io. Based on this discovery, we conclude that Io has a large metallic core.

Io Galileo Galileo Gravity↗
NASA NTRS1996

Rosetta Radio Science Investigations

The Rosetta Radio Science Investigations (RSI) experiment was selected by the European Space Agency to be included in the International Rosetta Mission to comet P/Wirtanen (launch in 2003, arrival and operational phase at the comet 2011-2013). The RSI science objectives address fundamental aspects of cometary physics such as the mass and bulk density of the nucleus, the gravity field, non-gravitational forces, the size and shape, the internal structure, the composition and roughness of the nucleus surface, the abundance of large dust grains and the plasma content in the coma and the combined dust and gas mass flux on the orbiter. RSI will make use of the radio system of the Rosetta spacecraft.

P/Wirtanen radio science cometary physics cometary↗
NASA NTRS1994

Gravitational coefficients and internal structures of the icy Galilean satellites: An assessment of the Galileo Orbiter mission

Simulated radio tracking data from one flyby of Europa, two flybys of Ganymede, and two flybys of Callisto by the Galileo Orbiter yield estimates of the standard errors in the gravitational coefficient J(sub 2) of 44, 8.5, and 12 (in units of 10(exp -6)) for Europa, Ganymede, and Callisto, respectively; errors in C(sub 22) (in units of 10(exp -6)) are 12, 1.7, and 2.1. These errors are sufficiently small that the values of J(sub 2) and C(sub 22) to be measured by Galileo should suffice to determine if the ice and rock in the satellite interiors are uniformly mixed or separated, so long as the bodies are in hydrostatic equilibrium.

Schubert, G.↗