Engineering topics

Murray, C. D.

Publications and source records attributed to Murray, C. D..

15 records

DOE OSTI2016

ORIGIN OF THE CHAOTIC MOTION OF THE SATURNIAN SATELLITE ATLAS

We revisit the dynamics of Atlas. Using Cassini ISS astrometric observations spanning 2004 February to 2013 August, Cooper et al. found evidence that Atlas is currently perturbed by both a 54:53 corotation eccentricity resonance (CER) and a 54:53 Lindblad eccentricity resonance (LER) with Prometheus. They demonstrated that the orbit of Atlas is chaotic, with a Lyapunov time of order 10 years, as a direct consequence of the coupled resonant interaction (CER/LER) with Prometheus. Here we investigate the interactions between the two resonances using the CoraLin analytical model, showing that the chaotic zone fills almost all the corotation sites occupied by the satellite's orbit. Four 70:67 apse-type mean motion resonances with Pandora are also overlapping, but these resonances have a much weaker effect. Frequency analysis allows us to highlight the coupling between the 54:53 resonances, and confirms that a simplified system including the perturbations due to Prometheus and Saturn's oblateness only captures the essential features of the dynamics.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

Saturn's inner satellites: Orbits, masses, and the chaotic motion of atlas from new Cassini imaging observations

We present numerically derived orbits and mass estimates for the inner Saturnian satellites, Atlas, Prometheus, Pandora, Janus, and Epimetheus from a fit to 2580 new Cassini Imaging Science Subsystem astrometric observations spanning 2004 February to 2013 August. The observations are provided as machine-readable and Virtual Observatory tables. We estimate GM{sub Atlas} = (0.384 ± 0.001) × 10{sup −3} km{sup 3} s{sup −2}, a value 13% smaller than the previously published estimate but with an order of magnitude reduction in the uncertainty. We also find GM{sub Prometheus} = (10.677 ± 0.006) × 10{sup −3} km{sup 3} s{sup −2}, GM{sub Pandora} = (9.133 ± 0.009) × 10{sup −3} km{sup 3} s{sup −2}, GM{sub Janus} = (126.51 ± 0.03) × 10{sup −3} km{sup 3} s{sup −2}, and GM{sub Epimetheus} = (35.110 ± 0.009) × 10{sup −3} km{sup 3} s{sup −2}, consistent with previously published values, but also with significant reductions in uncertainties. We show that Atlas is currently librating in both the 54:53 co-rotation-eccentricity resonance (CER) and the 54:53 inner Lindblad (ILR) resonance with Prometheus, making it the latest example of a coupled CER-ILR system, in common with the Saturnian satellites Anthe, Aegaeon, and Methone, and possibly Neptune's ring arcs. We further demonstrate that Atlas's orbit is chaotic, with a Lyapunov time of ∼10 years, and show that its chaotic behavior is a direct consequence of the coupled resonant interaction with Prometheus, rather than being an indirect effect of the known chaotic interaction between Prometheus and Pandora. We provide an updated analysis of the second-order resonant perturbations involving Prometheus, Pandora, and Epimetheus based on the new observations, showing that these resonant arguments are librating only when Epimetheus is the innermost of the co-orbital pair, Janus and Epimetheus. We also find evidence that the known chaotic changes in the orbits of Prometheus and Pandora are not confined to times of apse anti-alignment.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2014

Scientific Rationale for Saturn's in Situ Exploration

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases׳ abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere may help constraining its bulk O/H ratio. We compare predictions of Jupiter and Saturn's bulk compositions from different formation scenarios, and highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations, all based on an entry probe that would descend through Saturn's stratosphere and troposphere under parachute down to a minimum of 10 bar of atmospheric pressure. We finally discuss the science payload required on a Saturn probe to match the measurement requirements.

Galileo↗

DOE OSTI2013

Local variability in the orbit of Saturn's F ring

We present an analysis of the orbit of Saturn's F ring using images recorded by the Imaging Science Subsystem of the Cassini spacecraft. A total of 9805 observations have been made from 10 image sequences obtained between 2006 November 23 and 2009 July 28. Each sequence of up to 240 images spans a single orbit of the F ring, allowing 10 independent high-precision estimates of the ring orbit to be made over this ∼3 year period. The ring has been modeled as an inclined uniformly precessing ellipse. The results show a variability in the orbital elements with, for example, the semi-major axis scattered between 140211.2 ± 0.1 km and 140232.9 ± 0.4 km and the fitted periapses locked to the value obtained from a combined fit using the entire three-year span of observations. We show that the observed scatter between the individual estimates of the ring orbit reflect the differing past histories of the particular segments of ring being fitted and that the values are scattered within the limits expected from a single gravitational encounter with the nearby moon, Prometheus. In the combined fit, the scatter averages out to reveal a small systematic bias with respect to the results of Bosh et al. and Albers et al. We believe this is a consequence of the proximity of Prometheus to the ring in the image sequences chosen for this analysis. Finally, we note a close empirical commensurability between the apsidal precession rate, ϖ-dot , of the F ring and the synodic period between Prometheus and the F ring, such that n {sub Prom} – n {sub Fring} ≈ 2 ϖ-dot {sub Fring}, where n {sub Prom} and n {sub Fring} are the mean motions, and discuss its implications.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2007

Revised Orbits of Saturn's Small Inner Satellites

We have updated the orbits of the small inner Saturnian satellites using additional Cassini imaging observations through 2007 March. Statistically significant changes from previously published values appear in the eccentricities and inclinations of Pan and Daphnis, but only small changes have been found in the estimated orbits of the other satellites. We have also improved our knowledge of the masses of Janus and Epimetheus as a result of their close encounter observed in early 2006.

celestial mechanics↗

NASA NTRS2007

Cassini Observes the Active South Pole of Enceladus

Cassini has identified a geologically active province a the south pole of Saturn's moon Enceladus. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Porco, C. C.↗

NASA NTRS2001

Saturn's Rings: Pre-Cassini Status and Mission Goals

Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized.

Cuzzi, Jeffrey N.↗

NASA NTRS1989

Secular perturbations of the Uranian satellites - Theory and practice

A simple revised secular perturbation theory which incorporates the averaged secular effect of first-order near-resonances is derived. By including the effects of these near-resonances, the largest error in the secular frequencies is reduced from 16 percent to less than 3 percent. It is concluded that the revised secular perturbation theory is adequate for the quantitative modeling of the long-term perturbations in the Uranian satellite system. If incorporated within the general theory of Laskar (1986), this theory would lead to completely analytic theory.

Malhotra, R.↗

NASA NTRS1985

Variation of the UBV colors of S-class asteroids with semimajor axis and diameter

A large fraction of the several thousand asteroids which exist mainly between the orbits of Mars and Jupiter have now been separated into a small number of more or less distinct classes. The most populous classes are S (moderate albedo and red UBV color) and C (low albedo and neutral UBV color), while other less populous classes include M, E, R, D, A, F, and P. Asteroids in the S and C classes are considered to be the most likely sources of the abundant stony-iron and carbonaceous chondritic meteorites. An identification of the source asteroid of a particular meteorite, could lead to the use of the meteorite as a sample of that asteroid. Such a sample could then be employed to determine the bulk composition and the evolution of the asteroid. There are, however, factors which may make the interpretation of the observed albedos, colors, and spectra difficult. In the present investigation, it is shown that the mean UBV color of S-class asteroids varies markedly with distance from the sun and may vary with diameter. Interpretations of these findings are discussed.

Dermott, S. F.↗

NASA NTRS1984

Distribution and evolution of asteroid rotation rates

Data on the rotational characteristics of more than 300 asteroids are currently available, and it is now clear that the distribution of the rotation rates is nonrandom. A plot of rotation rate against asteroid diameter shows large dispersion but is distinctly V-shaped. The minimum of this curve at about 120 km may separate primordial asteroids from their collision products. There is also evidence that rotation rate depends on type classification, and weak evidence that it may also depend on family membership. Recent bias-free observations suggest that the marked rise of rotation rate with decreasing diameter D for those asteroids with D less than 120 km cannot be completely accounted for by observational-selection effects. A significantly large subset of the small asteroids have exceptionally long rotation periods suggestive of either a different nature and origin or a peculiar history. Models that have been proposed to account for these results are discussed.

Dermott, S. F.↗

NASA NTRS1984

Asteroid rotation rates

A trend of increasing mean rotational frequency with increasing diameter is noted in asteroids with diameters greater than 120 km, irrespective of M-, S-, and C-type asteroid subset and family or nonfamily membership. This trend cannot be accounted for by observational selection. For asteroids with diameters smaller than 120 km mean rotational frequency increases with decreasing diameter, but within this group there is a subset with exceptionally long rotational periods. This marked change in the distribution at 120-km diameter could separate primordial asteroids from their collision products. It is also noted that, for asteroids of a given diameter, M asteroids rotate faster than S asteroids, which in turn rotate faster than C asteroids. For all types, family members rotate faster than nonfamily members.

Dermott, S. F.↗

NASA NTRS1983

Nature of the Kirkwood gaps in the asteroid belt

It is demonstrated that the Kirkwood gaps are not merely regions of low asteroidal number density, but are regions in a-e-sin 1/2 I space where libration of some argument is possible. It is argued that neither the statistical nor the cosmogonic hypothesis of gap formation can account for these new observations. It is shown that the present distribution of asteroidal semimajor axes can be used to deduce the present semimajor axis of Jupiter to an accuracy of one part in five thousand. Thus, there has been very little change in the orbital period of Jupiter since the time of formation of the present gaps. This observation eliminates the possibility that the observed gaps were formed by resonance sweeping at the time of the dispersal of the accretion disk. It is concluded that the gaps have been formed by the gravitational action of Jupiter on individual asteroids and that gap formation has probably continued throughout the lifetime of the solar system.

Dermott, S. F.↗

NASA NTRS1982

The asteroid ring

From a statistical analysis of asteroid orbital period data, it is shown that the present distribution of asteroids is strongly correlated with the present orbital period of Jupiter (to 1 part in 5000). By analyzing the distribution of orbital eccentricities and inclinations it is shown that the resonant structure of the belt was formed after the asteroids dispersed from the near-coplanar disk in which they accreted.

Dermott, S. F.↗

NASA NTRS1980

The narrow rings of Jupiter, Saturn and Uranus

The origin of the newly discovered narrow ring systems around Jupiter, Saturn and Uranus is considered. It is pointed out that both the Uranian and Jovian ring systems have mean orbital radii of 1.8 planetary radii and lie within the Roche zones of their respective planets, and it is suggested that the Jovian ring is the product of the disintegration of a satellite that entered the Roche zone, and that large numbers of small particles are now in horseshoe orbits about the Lagrangian equilibrium points of the remnant chunks. Analysis of the path of a ring particle in a horseshoe orbit is shown to result in ring structures in agreement with those observed for the circular rings of Jupiter and the highly eccentric ring of Uranus. The stability of these ring systems is then considered, and it is suggested that the F ring of Saturn, which lies outside the Roche zone, represents primordial matter not yet accreted by small satellites just inside the Mimas first-order resonances.

Dermott, S. F.↗

NASA NTRS1971

Glass tube splitting tool

Tool accurately splits glass tubing so cuts are aligned 180 deg apart and reassembled tube forms low pressure, gastight enclosure. Device should interest industries using cylindrical closed glass containers.

Klein, J. A.↗