Results for “uranus” — Page 2

NASA NTRS1976

The aerosol distribution in Uranus' atmosphere - Interpretation of the hydrogen spectrum

New observations of Uranus' combined H2 spectrum have been obtained. An analysis, emphasizing the leverage provided by the S3(0) quadrupole line, leads to strong constraints on Uranus' bulk aerosol distribution with depth. Using an inhomogeneous model atmosphere program which includes Rayleigh scattering for gases and a 1 plus cos theta phase function for cloud layers, it is shown that Uranus' H2 spectrum implies a cloud layer (or dense aerosol) at a depth from 500 to 700 km-amagat H2. It also implies negligible aerosol scattering in the 280 to 450 km-amagat H2 region and a thin haze at 100 to 200 km-amagat H2 depths. The low albedo in very strong CH4 bands indicates that there is negligible aerosol scattering in Uranus' radiative zone. Hydrogen appears to be more abundant than the sum of all other atmospheric gases. The C/H ratio deep in Uranus' atmosphere is strongly enhanced over the solar value. Accurate limb-darkening measurements in and near strong CH4 bands would be effective in specifying Uranus' aerosol distribution in greater detail.

Trafton, L.↗

NASA NTRS1983

Uranus - Variability of the microwave spectrum

Radio astronomical observations of Uranus show that the radio emission spectrum is evolving in time. Ammonia vapor must be depleted in the Uranian atmosphere as Gulkis et al. (1978) previously suggested. Since 1965, ammonia either has been decreasing in time or is a decreasing function of latitude, or both, provided that the radio emission is atmospheric in origin. If Uranus has an observable low-emissivity 'surface', these trends may be reversed. The microwave observations made in 1965, at the time when the spin axis of Uranus was nearly perpendicular to the sun-Uranus line, are consistent with an atmospheric opacity profile that would be produced by saturated ammonia vapor in a predominantly hydrogen atmosphere. At the present time, when the spin axis of Uranus is nearly aligned with the sun-Uranus line, the measurements require an opacity that would be produced by saturated water vapor. A large thermal gradient between the pole and equator is ruled out.

Gulkis, S.↗

NASA NTRS1984

Magnetosphere, Rings, and Moons of Uranus

The observation of an ultraviolet aurora on Uranus implies the existence of a magnetosphere. It is suggested that the magnetospheres of Uranus and Saturn may be very similar. Charged particle sputtering of water ice surfaces on the Uranian moons may maintain an oxygen ion plasma torus similar to the heavy ion plasma torus at Saturn. Atmospheric cosmic ray albedo neutron decay may sustain an inner radiation belt with omnidirectional proton fluxes. If the 100 keV ion fluxes near 7 RU are similar to Saturnian ion fluxes at such energies, the Uranian aurora may be maintained by ion precipitation from the radiation belts at nearly the strong diffusion rate. This mechanism predicts comparable aurorae over both magnetic poles of Uranus, in contrast with the Faraday disc dynamo mechanism, which powers an aurora only over the sunlit pole of uranus. If, however, the 100 kev ion fluxes at Uranus are comparable to those at Saturn, any exposed methane ice surfaces on the moons and rings of Uranus would be quickly transformed by ion impacts to a black, carbonaceous polymer.

Cheng, A. F.↗

NASA NTRS1985

Are Uranus' rings black because they are in the solar wind?

It is shown that the rings of Uranus are mostly immersed in the solar wind if Uranus lacks an intrinsic magnetic field. Except for geologically active moons like Io, the rings and moons of Jupiter and Saturn have either high albedos in a low radiation environment or low albedos in a high radiation environment. It is suggested that the moons of Uranus have a significantly lower albedo than those in the low radiation environment of Saturn, and that the rings of Uranus are black because the rings and moons of that planet are relatively depleted in their surface stores of ices. In this scenario, immersion of the Uranus rings in the solar wind provides a loss mechanism for ring ices. The black rings may thus imply the lack of an intrinsic Uranus magnetic field.

Curtis, S. A.↗

NASA NTRS1994

Comparison of the field configurations of the magnetotails of Uranus and Neptune

The magnetic field configuration-states of the magnetotails of the planets Uranus and Neptune are compared. Earth's case is also briefly treated, as well as some related aspects of the other three magnetic planets. In Uranus' case, due to the large tilt (59 deg) of the planet's magnetic dipole with respect to its spin axis and the unusual obliquity of that axis, the angle of attack (alpha) of the solar wind with respect to dipole alignment goes through all possible angles, 0 deg to 180 deg, yielding a very broad spectrum of configuration-states of its tail. Cases are discussed where the planetary magnetic dipole is either aligned with the Sun-planet-line ('pole-on' state) or perpendicular to it and some intermediate states, for both Uranus and Neptune. Only Uranus experiences the pole-on state, which next occurs in November 1999 (+/- 2 months); last year (1993.2) it had the first 'perpendicular' state since Voyager encounter which resembles Earth's case. Neptune never has a pole-on configuration, but it gets as close as alpha = 14 deg from it; the next occurrence is early in 2003. At Voyager encounter Neptune's magnetotail apparently rapidly migrated through a broad spectrum of field structures with near extreme states resembling an Earth-like case on the one hand and a cylindrically symmetric one on the other. Magnetopause 'openness' should dramatically change in terms of the rapidly changing angle of attack throughout a planetary day for these two planets, and this has important implications for their magnetotails. Any future manetospheric mission plans for Uranus or Neptune should take in to consideration the allowed range of values for alpha for the epoch of interest; this is especially of concern for Uranus which has a pole-on state, and all possible alphas, around the middle of 2014, 20 years from now.

Lepping, R. P.↗

NASA NTRS2019

Voyager 2 Constraints on Plasmoid‐based Transport at Uranus

A magnetosphere controls a planet's evolution by suppressing or enhancing atmospheric loss to space. In situ measurements of Uranus' magnetosphere from the Voyager 2 flyby in 1986 provide the only direct evidence of magnetospheric transport processes responsible for this atmospheric escape at Uranus. Analysis of high‐resolution Voyager 2 magnetic field data in Uranus' magnetotail reveals the presence of a loop‐like plasmoid filled with planetary plasma traveling away from the planet. This first plasmoid observation in an Ice Giant magnetosphere elucidates that (1) both internal and external forces play a role in Uranus' magnetospheric dynamics, (2) magnetic reconnection contributes to the circulation of plasma and magnetic flux at Uranus, and (3) plasmoids may be a dominant transport mechanism for mass loss through Uranus' magnetotail.

DiBraccio, Gina A.↗

NASA NTRS2019

Voyager 2 Constraints on Plasmoid-Based Transport at Uranus

A magnetosphere controls a planet's evolution by suppressing or enhancing atmospheric loss to space. In situ measurements of Uranus' magnetosphere from the Voyager 2 flyby in 1986 provide the only direct evidence of magnetospheric transport processes responsible for this atmospheric escape at Uranus. Analysis of high‐resolution Voyager 2 magnetic field data in Uranus' magnetotail reveals the presence of a loop‐like plasmoid filled with planetary plasma traveling away from the planet. This first plasmoid observation in an Ice Giant magnetosphere elucidates that (1) both internal and external forces play a role in Uranus' magnetospheric dynamics, (2) magnetic reconnection contributes to the circulation of plasma and magnetic flux at Uranus, and (3) plasmoids may be a dominant transport mechanism for mass loss through Uranus' magnetotail.

Gina A Dibraccio↗

NASA NTRSDate not supplied

Auroral Electron Estimates From the Voyager 2 Plasma Observations During the Uranus Flyby and Comparisons to Earths Aurora

Introduction: We are fortunate enough to have plasma observations from the Voyager 2 flyby of Uranus in 1986. We revisit these data in the context of estimating what the auroral electrons in Uranus’ magnetosphere would look like and how they would compare to what we know about Earth’s aurora and how it is coupled to Earth’s magnetosphere. The Voyager 2 flyby of Uranus provided us with valua-ble information about the plasma environment in-side the magnetosphere of Uranus (Sittler, et al., 1987). The maximum energy of the Voyager 2 plasma instrument was 6 keV for electrons and it is clear from the data that the electron fluxes extended be-yond that range to higher energies. Future meas-urements should extend up to around 30 keV in or-der to fully capture the full electron distribution within the magnetosphere and those corresponding to the aurora. The electron temperatures were around 2 keV throughout the magnetosphere of Uranus and the densities ranged from 0.001 to 1.0 per cubic centimeter. These observed parameters will be compared to analogous structures within Earth’s magnetosphere and be used to estimate the energy and fluxes of electrons that would be observed within the auroral zones on Uranus.

R G Michell↗

NASA NTRSDate not supplied

Aerodynamic Implications of Aerocapture Systems for Uranus Orbiters

Exploration of the Ice Giants, Uranus in particular, has been indicated as the highest priority new flagship-class mission by the 2022 Planetary Science Decadal Survey. However, due to it’s location in the solar system, extensive travel time and resources are required for a mission to reach Uranus. Aerocapture offers shorter interplanetary transit times and reduced propellant requirements for such a mission when compared to a traditional propulsive-capture orbiter. Considerations for the aerodynamics of an aerocapture system enabling flagship-class science mission to Uranus are presented. The applicability of a low lift-to-drag entry vehicle aeroshell with flight heritage to a new planet and atmosphere is evaluated. A preliminary aerodatabase is formulated from low-fidelity methods and heritage flight mission data to provide initial estimates of trajectory space to be considered for inserting a spacecraft into orbit at Uranus using aerocapture. Computational fluid dynamics calculations in the free molecular, transitional, and continuum regimes using Direct Simulation Monte Carlo and Navier-Stokes solutions are used to assess the validity of this preliminary aerodatabase. The considered entry vehicle is found to varying levels of agreement with work done by previous flight missions when evaluated for flight in Uranus’ atmosphere. Implications for the aerodynamics of this vehicle as applied in an aerocapture system in flight at Uranus as found by the current work are discussed.

Eli R. Shellabarger↗

NASA NTRSDate not supplied

Aerocapture Solutions for Uranus Flagship-class Orbiter and Probe

Recent planetary mission assessments, such as the 2022 Planetary Science Decadal Survey released by the National Academies of Science, have focused on the need for a flagship-class mission to the Ice Giant planets, particularly Uranus. The Uranus Orbit and Probe mission proposal was used by the National Academies of Science as the baseline while making its recommendation as the top flagship-class mission for NASA in the 2020-2030\'s. This mission which was based on a launch date of 2031 or 2032, used a fully-propulsive orbit insertion maneuver at Uranus (requiring 60-70% of total mass for propellant), and reached the planet after 13 years of interplanetary cruise before the 2049 equinox, a goal of the science community. A pre-2033 launch date is required to conduct a necessary Jupiter fly-by as the giant planet will be not be in the proper alignment for a later launch date. However, as the budget constraints of the NASA budget have pushed back the start date of a Uranus mission, a launch date of a flagship mission before 2033 seems unreasonable. Aerocapture, an orbit insertion maneuver that uses the atmosphere to decelerate, can reduce the propellant load needed for a captured orbit. Additionally, the aerocapture maneuver can decelerate safely while approaching the planet at higher arrival velocities, thus allowing a mission to use a highly energetic trajectory and reduce the cruise time by several years. An aerocapture-enabled solution has launch opportunities in the mid 2030's, including as late as 2038 to reach Uranus by 2049. This paper will discuss the feasibility of an aerocapture option for a Flagship-class mission to Uranus.

Soumyo Dutta↗

NASA NTRSDate not supplied

Aerocapture Solutions for Uranus Flagship-class Orbiter and Probe

Recent planetary mission assessments, such as the 2022 Planetary Science Decadal Survey released by the National Academies of Science, have focused on the need for a flagship-class mission to the Ice Giant planets, particularly Uranus. The Uranus Orbit and Probe mission proposal was used by the National Academies of Science as the baseline while making its recommendation as the top flagship-class mission for NASA in the 2020-2030\'s. This mission which was based on a launch date of 2031 or 2032, used a fully-propulsive orbit insertion maneuver at Uranus (requiring 60-70% of total mass for propellant), and reached the planet after 13 years of interplanetary cruise before the 2049 equinox, a goal of the science community. A pre-2033 launch date is required to conduct a necessary Jupiter fly-by as the giant planet will be not be in the proper alignment for a later launch date. However, as the budget constraints of the NASA budget have pushed back the start date of a Uranus mission, a launch date of a flagship mission before 2033 seems unreasonable. Aerocapture, an orbit insertion maneuver that uses the atmosphere to decelerate, can reduce the propellant load needed for a captured orbit. Additionally, the aerocapture maneuver can decelerate safely while approaching the planet at higher arrival velocities, thus allowing a mission to use a highly energetic trajectory and reduce the cruise time by several years. An aerocapture-enabled solution has launch opportunities in the mid 2030's, including as late as 2038 to reach Uranus by 2049. This paper will discuss the feasibility of an aerocapture option for a Flagship-class mission to Uranus.

Soumyo Dutta↗

NASA NTRS1976

Possible radio emission from Uranus at 0.5 MHz

Radio emission from the direction of Uranus has been detected in data from the Goddard radio astronomy experiment on the IMP-6 spacecraft. Previously, emission from the direction of Jupiter and Saturn had been observed by IMP-6 at a number of frequencies near 1 MHz and were identified through an analysis of the phase of the observed modulated signal detected from the spinning dipole antenna. This technique was applied to the direction of Uranus with possible positive results. Over the approximately 500 days of data, three to six bursts with unique spectral characteristics have been found. The events persisted less than 3 minutes and are strongest in intensity near 0.5 MHz. Identification with Uranus is confused by the likely presence of low-level terrestrial and solar emission. Because of the unfavorable angular separation of earth and Uranus, there is a possibility that the bursts are atypical terrestrial magnetospheric phenomena, although the uniqueness of the set of events indicates the probable detection of radiation from Uranus.

Brown, L. W.↗

NASA NTRS1980

Uranus mission options

Options for delivery of Uranus payloads are covered extensively. Launch energy charts are supplied for ballistic trajectories to Uranus for direct, Jupiter gravity-assist, and Saturn gravity-assist missions launching from 1982 to 1995. Both early reconnaissance (flyby) and orbiter missions are considered. Performance charts for low-thrust, Delta Vega, planetary gravity-assist, and combinations of these modes are plotted in terms of net orbited mass and trip time to Uranus. Discussion and illustration of the effects of the unique Uranus system oritentation on Uranus encounter geometry with trajectory arrival date is covered. Example mission designs are characterized for several delivery modes.

Wallace, R. A.↗

NASA NTRS1981

Observations of Uranus, Neptune, and Titan by the International Ultraviolet Explorer

The ultraviolet spectra of Uranus, Neptune, and Titan do not reveal absorption features at 7-A resolution from 2100 to 3200 A. Upper limits of from 1 to 3 A are set for the equivalent widths of narrow absorptions, which corresponds to a CO/H2 mixing ratio less than 2 x 10 to the -4th in the case of Uranus. The slopes of the continuum reflectivities of Uranus and Neptune are consistent with the semi-infinite Rayleigh-Raman scattering model of Cochran, while the absolute levels are matched only if solar photometry is modified within acceptable limits. An alternative, but less satisfactory explanation of the new data is that the UV reflectivities of both Uranus and Neptune are depressed uniformly by a continuum absorber. The suggestion by Savage et al (1980) that the albedo of Uranus decreases by approximately 20% from 2200 to 1800 A is not confirmed. For Titan, the albedo decreases monotonically toward shorter wavelengths down to 2300 A. New limits are set for the pressure level in the atmosphere of Titan up to which the real, but presently unidentified, UV absorber there must extend, and for the fraction of Titan that must be covered by this absorber

Caldwell, J.↗

NASA NTRS1983

To Uranus and beyond

Known characteristics of the planet Uranus are reviewed, together with a status report on the Voyager spacecraft. Uranus was discovered in 1781, and has been found to have a north pole that is pointed directly at the sun. Data is still needed on the Uranus atmosphere, its possible magnetic field, and its rotation frequency. Five moons have been sighted, accompanied by nine rings. The Voyager 2 spacecraft will encounter Uranus in December 1986 at a 14.7 km/sec velocity, imposing limitations on the observation sequence. The spacecraft carries 11 scientific packages, including a camera, and will pass within 50,000 km of the moon Miranda and 107,000 km of Uranus. Radio signals will be broadcast through the rings, which will be imaged by a UV spectrometer. Voyager 2 will then continue on to a Neptune encounter in 1989 before moving toward interstellar space.

Kerridge, S. J.↗

NASA NTRS1984

Temperature Structures and Infrared-Derived Properties of the Atmosphere of Uranus and Neptune

Temperature structures of Uranus and Neptune are currently derivable from a combination of stellar occultation and thermal radiance observations. The globally averaged temperatures of Uranus and Neptune appear quite similar in their convective regions, reaching temperatures near 75 K at 1 bar and 150 K near 10 bar. Temperature minima near 200 mbar are also similar, near 53 to 55 K. The temperature in the stratosphere of Neptune rises much more steeply with altitude than for Uranus in the range of 0.1 to 100 mbar. Above this level, near 1 ubar, both atmospheres reach temperatures of 120 to 160 K. The bolometric radiant energy output is equivalent to the output of a blackbody at 58.3 + or - 2.0 K for Uranus and 60.3 + or - 2.0 for Neptune. These values imply approximate equilibrium with absorbed sunlight for Uranus but an internal energy source 1.9 to 2.6 times the absorbed sunlight for Neptune. The implication of the thermal spectra is that while the bulk composition may be consistent with a solar mixture of H2 and He, the mixing ratio of CH4 is probably about 2% in the deep atmospheres of both planets.

Glenn S Orton↗

NASA NTRS1984

Magnetospheric Structures: Uranus and Neptune

Magnetospheric structures that might be encountered at Uranus and Neptune are described. Statistics indicate a sufficiently high probability to warrant consideration of their likely properties in advance of the Voyager encounters. Because the spin axis of Uranus lies nearly in the ecliptic and presently points approximately sunward, Voyager is likely to encounter the unique pole on configuration that has special theoretical significance. Corotation in the magnetospheres of Uranus and Neptune would probably exclude solar wind drive convection as an important driver of global magnetospheric dynamics, as it does at Jupiter and Saturn. The magnetospheres of Uranus and Neptune probably lack sufficient internal sources of plasma to produce significant levels of rotationally driven convection. The reported observation of auroral emission from Uranus has therefore motivated the development of an alternative model in which solar wind motion is coupled directly to the rotation of the ionosphere to establish a dynamo circuit which generates Birkeland currents and polar cap aurora. This model predicts the strength and configuration of the aurora as functions of the magnitude and polarity, respectively, of the planetary magnetic moment.

T W Hill↗

NASA NTRS1986

The HD/H2 ratio in the atmosphere of Uranus

High resolution spectra of HD and H2 were brought together to derive the D/H ratio for Uranus. The deuterium concentration in the dominant molecular hydrogen phase is least susceptible to the effects of isotope fractionation in the planetary atmosphere, and the determination of relative abundances of HD and H2 is unambiguous due to nearness and relative weakness of the chosen spectral lines. The HD 5-0 R(0) and R(1) dipole lines and the H2 4-0 S(0), S(1), and S(2) quadrupole lines were obtained with a PEPSIOS instrument at the Palomar 5-meter telescope. The H2 spectra, which resolve the asymmetric line profiles resulting from pressure shifts in the deep stratified Uranus atmosphere, unambiguously define the line-of-sight hydrogen abundance for comparison with the HD spectra. The 5-0 band of HD was chosen to minimize interference from blended CH4 lines. However, weak interfering lines were found in the 5-0 bands from Uranus as well, and some uncertainties remain regarding the intrinsic line strengths in molecular hydrogen, complicating the analysis of the HD/H2 data. Nevertheless, it is established that the D/H ratio in the atmosphere of Uranus is smaller than the Jovian value, and is significantly smaller than recent theoretical predictions for Uranus based on estimates of isotope fractionation in the pre-planetary solar nebulae.

Trauger, J. T.↗