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At least 235 records · Page 13
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NASA NTRS1975

Cosmological considerations regarding Uranus

The cosmogony of Uranus is discussed within the context of a picture in which solid condensed materials accumulate to form a large body, which then acquires significant amounts of gas from the primitive solar nebula. Of prime cosmogonical importance is the tilt of the equatorial plane of the planet and of the plane of the satellite orbits by 98 deg with respect to the plane of the planetary orbit. The tilt of the planet can easily occur as a result of a major collision during the formation process; it seems most likely that the tilt of the satellite orbits requires that they were formed from a gaseous disk rotating about the planet after the tilt of the planetary rotational axis had occurred. Possible methods for tilting this gaseous disk are discussed. A strong early magnetic field may have helped in this and may have played an essential role in slowing down the spin of the planet to the present observed value. These processes may have produced significant compositional differences between the satellites of Uranus and those of Jupiter and Saturn.

Cameron, A. G. W.↗
NASA NTRS2021

Neptune Odyssey: A Flagship Concept for the Exploration of the Neptune–Triton System

The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune–Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft—in a class equivalent to the NASA/ESA/ASI Cassini spacecraft—would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moonʼs gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ∼37 minutes to the 10 bar pressure level in Neptune’s atmosphere just before Odysseyʼs orbit-insertion engine burn. Odysseyʼs mission would end by conducting a Cassini-like “Grand Finale,” passing inside the rings and ultimately taking a final great plunge into Neptuneʼs atmosphere.

Abigail M. Rymer↗
NASA NTRS1972

Nonthermal radio observations of the major planets

Seventeen years of observations have shown that Jupiter is a complex and intriguing source of emission over the entire radio spectrum accessible from the ground. The observations indicate the presence of intense radiation belts and a strong magnetic field inclined to the axis of rotation. At decametric wavelengths, several remarkably powerful sources are correlated with the rotation of the planet and the orbital motion of the satellite Io. Longer-term variations of these sources may be associated with the sunspot cycle, with Jupiter's orbital motion, or with changes in the internal structure of the planet. There is at most marginal evidence of nonthermal emission from Saturn, and as yet no evidence for such radiation from Uranus or Neptune.

Smith, A. G.↗
NASA NTRS1976

Astronomical imaging applications for CCDs

Preliminary testing of a back illuminated, buried channel CCD camera for astronomical telescopic imaging applications was demonstrated. Images of several planets and both galactic and extragalactic ojects were obtained using a 400 x 400 element TI CCD in a JPL developed camera on a 154 cm telescope on Mt. Lemmon. The high quantum efficiency and relatively low noise of this CCD, along with the inherently high photometric and geometric stability, opens up new opportunities for astronomical research, especially in the spectral range from 0.8 to 1.0 micrometers. For example, images of Uranus obtained in the 0.89 micrometers absorption band of methane clearly reveal for the first time markings on the disk of this remote planet. Applications for both ground based and near earth orbiting telescopes are discussed.

Smith, B. A.↗
NASA NTRS1982

Sharp edges of planetary rings

The ring systems of Saturn and Uranus exhibit several sharp edges, across which the optical depth drops from order unity to essentially zero. At least two and perhaps all of these features are associated with the location of orbital resonances between a satellite and the ring particles. It is remarkable that the optical depth varies on a distance scale which is much finer than that over which angular momentum can be transferred between a satellite and the ring material. The important features of this phenomenon are: (1) a perturbed band of width Delta a/a of about (satellite mass/planetary mass) to the 1/2 power adjacent to the edge within which the angular momentum transfer occurs, (2) streamlines perturbed such that the angular momentum luminosity decreases smoothly across the band to zero at the edge even though the optical depth remains constant, and (3) dynamical equilibrium requires a relation between the random velocity, the rate of deformation and the optical depth.

Borderies, N.↗
NASA NTRS1982

The rings of Uranus

Stellar occultation data on the Uranian rings are summarized. Spatial resolution is 0.1 km, for positions of ring segments, 4 km for structural details. The system consists of 9 narrow rings with average geometric albedo of 0.02. The data are well fitted by a kinematic model that describes the rings as ellipses, inclined to the equatorial plane of Uranus and precess due to harmonic terms in the Uranian potential fields. The main characteristics of the Uranian ring system that need a confirmed theoretical explanation are: narrow rings; sharp edges; uniform orbital precession; origin of the eccentricities and inclinations; the structure of the epsilon ring; the structure of the eta ring; and the origin of the ring system.

Elliot, J. L.↗
NASA NTRS1972

The planets Uranus, Neptune, and Pluto (1971)

Design criteria relating to spacecraft intended to investigate the planets of Uranus, Neptune, and Pluto are presented. Assessments were made of the potential effects of environmental properties on vehicle performance. Pertinent data on the mass, radius, shape, mean density, rotational pole location, and mean orbital elements for the three planets are given in graphs and tables.

Palluconi, F. D.↗
NASA NTRS1985

Dynamical evolution of the Oort cloud

New studies of the dynamical evolution of cometary orbits in the Oort cloud are made using a revised version of Weissman's (1982) Monte Carlo simulation model, which more accurately mimics the perturbation of comets by the giant planets. It is shown that perturbations by Saturn and Jupiter provide a substantial barrier to the diffusion of cometary perihelia into the inner solar system. Perturbations by Uranus and Neptune are rarely great enough to remove comets from the Oort cloud, but do serve to scatter the comets in the cloud in initial energy. The new model gives a population of 1.8 to 2.1 x 10 to the 12th comets for the present-day Oort cloud, and a mass of 7 to 8 earth masses. Perturbation of the Oort cloud by giant molecular clouds in the galaxy is discussed, as is evidence for a massive 'inner Oort cloud' internal to the observed one. The possibility of an unseen solar companion orbiting in the Oort cloud and causing periodic comet showers is shown to be dynamically plausible but unlikely, based on the observed cratering rate on the earth and moon.

Weissman, P. R.↗
NASA NTRS2000

Photochemical Modeling of CH3 Abundances in the Outer Solar System

Recent measurements of methyl radicals (CH3) in the upper atmospheres of Saturn and Neptune by the Infrared Space Observatory (ISO) provide new constraints to photochemical models of hydrocarbon chemistry in the outer solar system. The derived column abundances of CH3 on Saturn above 10 mbar and Neptune above the 0.2 mbar pressure level are (2.5 - 6.0) x 10(exp 13) / sq cm and (0.7 - 2.8) x 10(exp 13) / sq cm, respectively. We use the updated Caltech/Jet Propulsion Laboratory photochemical model, which incorporates hydrocarbon photochemistry, vertical molecular and bulk atmospheric eddy diffusion, and realistic radiative transfer modeling, to study the CH3 abundances in the upper atmosphere of the giant planets and Titan. We identify the key reactions that control the concentrations of CH3 in the model, such as the three-body recombination reaction, CH3 + CH3 + M yields C2H6 + M. We evaluate and extrapolate the three-body rate constant of this reaction to the low-temperature limit (1.8 x 10(exp -16) T(sup -3.75) e(sup -300/T), T < 300 K) and compare methyl radical abundances in five atmospheres: Jupiter, Saturn, Uranus, Neptune, and Titan. The sensitivity of our models to the rate coefficients for the reactions H + CH3 + M yields CH4 + M, H + C2H3 yields C2H2 + H2, (sup 1)CH2 + H2 yields CH3 + H, and H + C2H5 yields 2CH3, the branching ratios of CH4 photolysis, vertical mixing in the five atmospheres, and Lyman alpha photon enhancement at the orbit of Neptune have all been tested. The results of our model CH3 abundances for both Saturn (5.1 x 10(exp 13) / sq cm) and Neptune (2.2 x 10(exp 13) / sq cm) show good agreement with ISO Short Wavelength Spectrometer measurements. Using the same chemical reaction set, our calculations also successfully generate vertical profiles of stable hydrocarbons consistent with Voyager and ground-based measurements in these outer solar system atmospheres. Predictions of CH3 column concentrations (for p <= 0.2 mbar) in the atmospheres of Jupiter (3.3 x 10(exp 13) /sq cm), Uranus (2.5 x 10(exp 12) / sq cm), and Titan (1.9 x 10(exp 15) / sq cm) may be checked by future observations.

Lee, Anthony Y. T.↗
NASA NTRSDate not supplied

Applications of Nuclear Thermal Propulsion Systems for Deep Space Science Missions

Nuclear thermal propulsion (NTP) systems occupy a unique area in the space propulsion technology landscape, due to their ability to combine moderate-to-high thrust systems normally seen in chemical propulsion systems with specific impulses that are higher than those of more traditional chemical propulsion systems. Thus, NTP systems have the potential to greatly expand our access to deep space and can enable or enhance capabilities for a variety of missions that achieve science goals as outlined in NASA’s decadal surveys. This paper leverages previous work performed on applying NTP systems for multiple science missions. It also expands the mission portfolio to additional NTP-powered science mission profiles, evaluating the system performance benefits delivered by this propulsion option. This paper will also outline efforts to improve the fidelity of the existing NTP design concepts and vehicles that are utilized, leveraging previous work on nuclear propulsion systems for human-Mars missions to enhance the fidelity of results obtained from previous science mission concept studies – specifically Triton lander, solar polar orbiter, and the interstellar medium probe missions. The work outlined in this paper will also examine additional missions that have been highlighted in the recent decadal surveys, which may include robotic missions to the ice giant Uranus, robotic missions to Venus, and missions to various Kuiper belt objects (KBOs).

K A Polzin↗
NASA NTRS1980

Generic aerocapture atmospheric entry study, volume 1

An atmospheric entry study to fine a generic aerocapture vehicle capable of missions to Mars, Saturn, and Uranus is reported. A single external geometry was developed through atmospheric entry simulations. Aerocapture is a system design concept which uses an aerodynamically controlled atmospheric entry to provide the necessary velocity depletion to capture payloads into planetary orbit. Design concepts are presented which provide the control accuracy required while giving thermal protection for the mission payload. The system design concepts consist of the following elements: (1) an extendable biconic aerodynamic configuration with lift to drag ratio between 1.0 and 2.0; (2) roll control system concepts to control aerodynamic lift and disturbance torques; (3) aeroshell design concepts capable of meeting dynamic pressure loads during aerocapture; and (4) entry thermal protection system design concepts to meet thermodynamic loads during aerocapture.

Source record↗
NASA NTRS1983

Planetary rings - 2-2/3 centuries of nearly total ignorance, 4 years of information explosion

Saturn lies at nearly twice Jupiter's distance from the sun and nearly all parts of its system are characterized by much smaller scales than those which are important in the case of Jupiter. This appears in the structures of the planet's atmosphere, in the sizes of classical satellites other than Titan vis-a-vis those of the Galilean satellites, in the plethora of small Saturnian satellites, especially Lagrangian co-orbiters, in the structure of Saturn's F-Ring as contrasted with that of Jupiter's Ring and finally in the highly structured detail in Saturn's Rings, much finer than seriously considered in past theoretical discussions. Uranus' Rings were unknown until five years ago. The discovery and observation of these rings have revived contributions to theory originally intended for application to Saturn's Rings. Models have also been generated for eccentric rings for application to Uranus' Rings which also apply to those of Saturn. These two classes of model are reviewed in the present paper along with the first tentative steps made down the road to unravelling the complexity of Saturn's Rings.

Cook, A. F.↗
NASA NTRS1984

The Structure of the Uranian Rings and the Search for Rings Around Neptune

The nine narrow rings of Uranus, presently the only confirmed features of this system, have been observed with the diffraction-limited resolution (3.5 km) of ground-based occultations since their discovery in 1977. These data have been used to establish an orbit model, from which the five Keplerian orbit parameters for each ring, the pole of the mean ring plane, and the gravitational harmonic coefficients J sub 2 and J sub 4 have been determined. The rings are typically a few kilometers wide with eccentricities of about 0.001 and inclinations of a few hundredths of a degree, although a few have no measurable eccentricity or inclination. Interesting Voyager investigations would include probing the structure of the rings at higher spatial resolution, searching for new rings in the system (including inter-ring material), locating the postulated shepherd satellites, and searching for satellites inside the orbit of Miranda that might have dynamical influence on the rings. The high precision (approx 2 km) of the ring orbits might prove useful for spacecraft navigation. For Neptune, occultation searches have revealed no rings to a limit of a few hundreths optical depth, within a few hundred kilometers from the top of the planet's atmosphere (for equatorial rings).

Elliot, J. L.↗
NASA NTRS1991

Energetic particles at Uranus

The energetic particle measurements by the low-energy charged-particle and cosmic-ray instruments on the Voyager 2 spacecraft in the magnetosphere of Uranus are reviewed. Upstream events were observed outside the Uranian bow shock, probably produced by ion escape from the magnetosphere. Evidence of earthlike substorm activity was discovered within the Uranian magnetosphere. A proton injection event was observed within the orbit of Umbriel and proton events were observed in the magnetotail plasma-sheet boundary layer that are diagnostic of earthlike substorms. The magnetospheric composition is totally dominated by protons, with only a trace abundance of H(2+) and no evidence for He or heavy ions; the Uranian atmophere is argued to be the principal plasma source. Phase-space densities of medium energy protons show inward radial diffusion and are quantitatively similar to those observed at the earth, Jupiter, and Saturn. These findings and plasma wave data suggest the existence of structures analogous to the earth's plasmasphere and plasmapause.

Cheng, Andrew F.↗
NASA NTRS1977

Time scale for the formation of the earth and planets and its role in their geochemical evolution

The initial mass of the solar nebula is discussed. Models of a massive nebula (two solar masses and more) encounter serious difficulties: an effective mechanism of transfer of the momentum from the central part of the nebula outward, capable of leading to formation of the sun and removal of half the mass of the nebula from the solar system has not been found. As a consequence of the instability of these models, their evolution can end with the formation, not a planetary system, but of a binary star. The possibility is demonstrated of obtaining acceptable growth rates for Uranus and Neptune by prolonging the thickening of preplanetary dust in the region of large masses. The important role of large bodies in the process of formation of the planets is noted. The impacts of such bodies, moving in heliocentric orbits, could have imparted considerable additional energy to the forming Moon, which, together with the energy given off by the joining of a small number of large protomoons, could have led to a high initial temperature of the moon.

Safronov, V. S.↗
NASA NTRS1986

A model of the spatial and temporal variation of the Uranus thermal structure

Seasonal variability of the temperature structure of Uranus is modeled for all latitudes in the .0004 to 2 bar pressure range in anticipation of the Voyager encounter in January 1986. Atmospheric heating in the model results on the one hand from an internal heat source and, on the other hand, from absorption of solar energy by methane and by non-conservative aerosols located between the 0.5 and 2 bar levels. Various cases for the behavior of the internal heat flux are investigated, such as constant with latitude or constrained to yield a time-averaged thermal emission independent of latitude. Meridional transport of heat in the stably stratified atmosphere is not taken into account. The results indicate that the Voyager encounter time, very small north-south temperature asymmetry should be expected. Moreover, the northern hemisphere, although not illuminated, should emit as much energy (within one percent) as the southern hemisphere at this date. At a given latitude, extreme temperatures are reached at the equinoxes. At the poles, seasonal amplitudes of about 10 K in the upper stratosphere and 6 K at the 0.6 bar level are predicted, and the variation with time of the emission to space is found to be at most 20 percent. The atmosphere of Uranus appears to be characterized by very long radiative response times (mainly due to its cold temperature) which inhibit the large seasonal variations that one could otherwise expect in view of the high obliquity of the planet and its long orbital period.

Bezard, B.↗
NASA NTRS1973

Single thrust period missions to Uranus for unmanned nuclear-electric propulsion systems

The effects of trip time, propulsion time, and specific powerplant mass are studied for optimized unmanned probe spacecraft on missions to Uranus with nuclear-electric propulsion systems. Electric propulsion is confined to a single thrust period at the beginning of each mission. Mission profiles include both high-thrust and electric-propulsion Earth-departure modes for planet flyby and orbital capture. Effects of propulsion time and propulsion system parameters are evaluated, and typical design features of the nuclear-electric spacecraft are outlined. Payload capability comparisons are made with systems employing ballistic transfer and solar-electric propulsion.

Zola, C. L.↗
NASA NTRS1979

The rings of Uranus - Nature and origin

Effects of a resonance between a set of ring particles and individual satellites are considered. It is suggested that the resonances are 1:1 for the rings of Uranus and that each ring must contain an as-yet unseen satellite. A model is proposed in which the rings of Uranus were caused by small satellites that entered the Roche zone due to tidal drag. According to this model, the satellites gradually lost material from their surfaces in this zone, the particles left a satellite with relatively low speeds and must have entered orbits similar to that of the satellite, and the gravitational force of the satellite compelled these particles to remain in a narrow sharply defined ring.

Dermott, S. F.↗