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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

The Thermal Evolution and Internal Structure of Saturn's Midsize Icy Satellites

The Cassini-Huygens mission is returning new geophysical data for the midsize, icy satellites of Saturn (i.e., satellites with radii between 100 and 1,000 km). These data have enabled a new generation of geophysical model studies for Phoebe, Iapetus, Rhea, Mimas, Tethys, Dione, as well as Enceladus (see Spencer et al. 2009). Here we consider the new model studies that have reported significant results elucidating the evolutionary histories and internal structures of these satellites. Those results have included their age, the development of their internal structures and mineralogies, which for greatest fidelity must be done concomitantly with coupled dynamical evolutions. Surface areas, volumes, bulk densities, spin rates, orbit inclinations, eccentricities, and distance from Saturn have changed as the satellites have aged. Heat is required to power the satellites evolution, but is not overly abundant for the midsized satellites. All sources of heat must be evaluated and taken into account. This includes their intensities and when they occur and are available to facilitate evolution, both internal and dynamical. The mechanisms of heat transport must also be included. However, to model these to high fidelity the material properties of the satellite interiors must be accurately known. This is not the case. Thus, we discuss what is known about these properties and how the uncertainties affect the estimation of heat sources, transport processes, and the consequential changes in composition and evolution. Phoebe has an oblate shape that may be in equilibrium with its spin period of ~9.3 h. Its orbital properties suggest that it is not one of the regular satellites, but is a captured body. Its density is higher than that of the other satellites, consistent with formation in the solar nebula rather than from material around Saturn. Oblate shape and high density are unusual for objects in this size range, and may indicate that Phoebe was heated by Al-26 decay soon after its formation, which is consistent with some models of the origin of Kuiper belt objects. Iapetus has the shape of a hydrostatic body with a rotation period near 16 h. It subsequently despun to its current synchronous rotation state, ~79 day period. These observations are sufficient to constrain the required heating in Iapetus early history, suggesting that it formed several My after CAI condensation. Because Saturn had to be present for Iapetus to form, this date also constrains the age of Saturn and how long it took to form. Both shape and gravitational data are available for Rhea. Gravity data were obtained from the single Cassini flyby during the prime mission and within the uncertainties cannot distinguish between hydrostatic and non-hydrostatic gravitational fields. Both Dione and Tethys display evidence of smooth terrains, with Dione's appearing considerably younger. Both are conceivably linked to tidal heating in the past, but the low rock abundance within Tethys and the lack of eccentricity excitation of Tethys orbit today make explaining this satellite's geology challenging.

Matson, Dennis L.↗
DOE OSTI2022

Loss of a satellite could explain Saturn’s obliquity and young rings

The origin of Saturn’s ~26.7° obliquity and ~100-million-year-old rings is unknown. The observed rapid outward migration of Saturn’s largest satellite, Titan, could have raised Saturn’s obliquity through a spin-orbit precession resonance with Neptune. We use Cassini data to refine estimates of Saturn’s moment of inertia, finding that it is just outside the range required for the resonance. We propose that Saturn previously had an additional satellite, which we name Chrysalis, that caused Saturn’s obliquity to increase through the Neptune resonance. Destabilization of Chrysalis’s orbit ~100 million years ago can then explain the proximity of the system to the resonance and the formation of the rings through a grazing encounter with Saturn.

Science & Technology - Other Topics↗
NASA NTRS2017

Resolution of Orbit Determination Prediction Instabilities at Titan During Cassini's Solstice Mission

The Cassini spacecraft has been in orbit about Saturn since 2004. Exploration of the Saturn system is driven by gravitational flybys of the moon Titan which alter the spacecraft trajectory. The Cassini Navigation Team receives regular updates to the Saturn satellites ephemeris from JPL's Solar System Dynamics group. The difference between subsequent ephemeris deliveries can be hundreds of meters in the position of Titan at the time of a flyby. Errors in Titan's position propagate downstream to the next flyby through the estimated spacecraft trajectory. Prior to 2013, the Cassini Orbit Determination Team estimated the Saturn satellite ephemeris parameters and used the a posteriori states and covariance of an operations arc as a priori inputs to subsequent estimation arcs. Since 2013, the OD Team has only been considering errors in the ephemeris and not estimating a correction to the satellite positions. The T119 Titan flyby exhibited a 3D miss distance of 2.44 km and the following T120 flyby yielded a smaller miss of 1.06 km at the 2.9s error level. These discrepancies between pre-flyby prediction and post-flyby trajectory reconstruction were due to errors in the Titan ephemeris. In order to improve the targeting of Titan in future flybys, the team restarted the satellite ephemeris estimation process for orbit determination solutions. Subsequent flybys had target misses of less than 1 km at the sub-3 error level. This paper describes the method of scaling the a priori satellite ephemeris covariance in the orbit determination process to allow larger corrections to the satellite system and improve the prediction of the spacecraft’s Titan-relative position at the time of encounters.

Boone, Dylan↗
NASA NTRS1976

Surface compositions of the satellites of Saturn from infrared photometry

JHKL photometry (1.2-3.5 microns) of Rhea, Dione, Tethys, and the high-albedo side of Iapetus reveals broadband spectral signatures essentially identical to those of the water-ice-covered surfaces of Ganymede and the rings of Saturn, although significantly different from either the surface of Europa or laboratory measurements of pure water ice. The dark side of Iapetus has more neutral infrared colors, indicative of a nonicy surface.

Morrison, D.↗
NASA NTRS1978

Mutual phenomena of Saturn's satellites in 1979-1980

Using two sets of orbital elements and the radii of the Saturnian satellites 1 (Mimas) through 7 (Hyperion), we find that from October 1979 until August 1980 nearly 300 mutual eclipses and occultations involving these bodies will occur. To allow for the expected errors in the satellite ephemerides, we repeat these calculations in order to obtain the additional events that occur when all satellite radii (save Titan's) are increased by 1000 km. A third listing predicts eclipses of satellites by (the shadow of) the ring. Photometric observations of a large number of these events will add much precise information to our knowledge of the Saturnian system at a critical time.

Aksnes, K.↗
NASA NTRS1985

Chronology of surface units on the icy satellites of Saturn

Because the impact flux history in the Saturnian system has been similar in its general form to that for the inner solar system and Jupiter, it is possible to develop an absolute calibration model for the relative ages derived from the density of impact craters. This has been done by identifying the density at which the crater populations change characteristics from that during post-accretion heavy bombardment to a later population of low but continuous flux. The transition between these two populations occurred at a known time throughout the solar system, 3.9 by, and provides an absolute calibration point on which to construct a calibration curve. Results based on calibrating relative age data to this absolute flux model suggest that most surfaces are ancient, greater than 3.9 by (or within several 100 million years). Only Enceladus and Dione have had prolonged activity that has resulted in resurfacing. The activity on Enceladus was probably driven by tidal forces, whereas the mechanism for driving resurfacing on Dione has yet to be identified.

Boyce, J. M.↗
NASA NTRS1987

Planetary geodesy

New geodetic data obtained during the years of 1983-1986 on terrestrial planets are presented. New or improved data on rotation, topography, and gravity are reported for Venus (from Pioneer Venus Orbiter observations), Jupiter (Pioneer 10 and 11 and Voyager 1 and 2 data) and its satellites, Saturn (from Voyager data) and its satellites, Uranus (from Voyager 2) and its satellites, and Neptune and Pluto (from various indirect observations). There was relatively little new to report on moon and Mars. The physical significance of the information is discussed.

Bills, Bruce G.↗
NASA NTRS1983

Saturn's small satellites - Voyager imaging results

Voyagers 1 and 2 provided images of sufficient resolution for morphologic and photometric studies of Saturn's small satellites. These objects, all very difficult to observe from earth, orbit Saturn at distances of 2.3 to 6.3 Saturn radii (just outside the A ring to the orbit of Dione) and range in mean diameter from 22 to 188 km. All are irregularly shaped (long/short axis ratios range from 1.4 to 2.0) and probably heavily cratered. While impacts have apparently been important in shaping these objects, observed crater densities suggest that the present forms may have survived for about 4 billion years. Geometric albedos vary from 0.4 to at least 0.8. These albedos and the few color data available are similar to those of larger Saturn satellites which are known to have surfaces made predominantly of water ice. The range of observed albedos could be explained by minor variations in the amount of dark, opaque contaminants.

Thomas, P.↗
NASA NTRS1980

Tidal dissipation, orbital evolution, and the nature of Saturn's inner satellites

Estimates of tidal damping times of the orbital eccentricities of Saturn's inner satellites place constraints on some satellite rigidities and dissipation functions Q. These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturn's inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite.

Peale, S. J.↗
NASA NTRS1980

Pioneer Saturn celestial mechanics experiment

The paper investigates a continuous round-trip radio link at S band (2.2 GHz) that was maintained during the Pioneer Saturn encounter between stations of the Deep Space Network and the spacecraft. From an analysis of the Doppler shift in the radio carrier frequency, a number of gravitational effects on the trajectory were determined. Gravitational moments for Saturn were found from a preliminary analysis, as well as mass values for the Saturn satellites Rhea, Iapetus, and Titan. It was determined that the densities of all three satellites are low and consistent with the compositions of ices. Theoretical calculations for the Saturn interior are described which use the latest observational data, including Pioneer Saturn and state-of-the-art physics for the internal composition.

Anderson, J. D.↗
NASA NTRS1977

Voyager imaging experiment

The Voyager imaging experiment, which involves two independently operated television systems (a narrow- and a wide-angle camera), is designed to conduct investigations of the atmospheres and surface properties of Jupiter, Saturn, their satellites and Saturn's rings. Objects of the investigations include the horizontal and vertical structure of visible clouds, the vertical structure of high, optically thin scattering layers on Jupiter and Saturn, the Great Red Spot, the South Equatorial Belt, chromophores on Io and Titan, the geology of several satellites, the masses, spin axes and periods of rotation of several satellites, the radial distribution of material in Saturn's rings, and the optical scattering properties of the primaries, rings, and satellites at a variety of wavelengths and phase angles.

Smith, B. A.↗
NASA NTRS1978

Saturn's ring and nearby faint satellites

Observations of Saturn's rings during passage of the Earth through the ring plane, coupled with those of others, suggest a ring thickness of 1.3 plus or minus 0.3 km. The wide disparity in the optical depth of Cassini's division found by other investigators is resolved, and for conservative isotropic single scattering, a normal optical depth for Cassini's division of 0.060 plus or minus 0.006 is obtained. We find the mean normal optical depth of ring C to be 0.074 plus or minus 0.007. Analysis of all available observations of faint objects near Saturn indicates the presence of at least one previously undiscovered satellite of Saturn. The orbit for Janus determined by Dollfus is supported. These satellites may be major members of an extended ring.

Fountain, J. W.↗
NASA NTRS1978

Organic chemistry on Titan

Observations of nonequilibrium phenomena on the Saturn satellite Titan indicate the occurrence of organic chemical evolution. Greenhouse and thermal inversion models of Titan's atmosphere provide environmental constraints within which various pathways for organic chemical synthesis are assessed. Experimental results and theoretical modeling studies suggest that the organic chemistry of the satellite may be dominated by two atmospheric processes: energetic-particle bombardment and photochemistry. Reactions initiated in various levels of the atmosphere by cosmic ray, Saturn wind, and solar wind particle bombardment of a CH4 - N2 atmospheric mixture can account for the C2-hydrocarbons, the UV-visible-absorbing stratospheric haze, and the reddish color of the satellite. Photochemical reactions of CH4 can also account for the presence of C2-hydrocarbons. In the lower Titan atmosphere, photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. Hot H-atom reactions initiated by photo-dissociation of NH3 can couple the chemical reactions of NH3 and CH4 and produce organic matter.

Chang, S.↗
NASA NTRS1973

New kind of ring around Saturn.

It is believed that barring extraordinary circumstances the Saturn satellite Titan must lose its atmosphere at a prodigious rate. The velocity of the atmospheric particles, however, although sufficient to escape from the satellite, does not quite reach Saturnian escape velocity. Consequently, atoms and molecules lost by Titan are forced by the planet's gravitational field to orbit Saturn until ionized or until they are recaptured by Titan, forming a gaseous torus encompassing Titan's orbit.

Mcdonough, T. R.↗
NASA NTRS1982

Astrometric observations of the satellites of the outer planets. V - The oppositions of 1978-1979, 1980, and 1981

Accurate photographic positions obtained during the 1978-1979, 1980 and 1981 oppositions are presented for the positions of the Galilean and the Saturn satellites. Spherical-equatorial coordinates are presented for a total of 1032 pairs, for the equator and equinox of 1950.0 and 1316 intersatellite positions. The data were obtained by the Leander McCormick Observatory's 67-cm refractor telescope, and answers the need for more refined orbital element data on these satellites required by the Galileo Jupiter orbiter vehicle.

Rohde, J. R.↗