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NASA NTRS1995

Arcs and Clumps in the Uranian Lambda Ring

Careful reprocessing of the Voyager images reveals that the Uranian lambda ring has marked longitudinal variations in brightness comparable in magnitude to those in Saturn's F ring and Neptune's Adams ring. The ring's variations show a dominant five-cycle (72-degree) periodicity, although additional structure down to scales of about 0.5 degree is also present. The ring's shape is defined by a small overall eccentricity plus a six-cycle (60-degree) sinusoidal variation of radial amplitude around 4 kilometers. Both of these properties can be explained by the resonant perturbations of a moon at a semimajor axis of 56,479 kilometers, but no known moon orbits at this location. Unfortunately, the mass required suggests that such a body should have been imaged by Voyager.

Showalter, Mark R.↗
NASA NTRS1992

Triton - Voyager's finale

The investigation of the Neptunian satellite Triton by the Voyager 2 is described with interpretations of the object's nature and composition. The orbit, seasonal cycle, and southern-hemisphere solstice are described, and the composition of the satellite is discussed. Triton's mass and radius are known, and the objects is made up of about 70 percent rock and organics and 30 percent ice by mass. Triton's interior is warm and geologically active considering its distance from the sun, and large amounts of frozen methane and nitrogen are theorized to contribute to the object's high reflectivity. Also noted in the Voyager color images are creeping ice, cryogenic lava, and dark streaks on the south polar cap from nitrogen gas leaks driven by a type of greenhouse effect. Triton represents a class of satellite that has not been observed previously: a moon-sized body in a retrograde inclined orbit from the class of objects that coalesced to form Neptune.

Brown, R. H.↗
NASA NTRS1990

A close-up view of Triton

Triton, the only large moon in the solar system with a retrograde motion, is investigated. The moon rotates about Neptune every 5.88 days and its annual cycle lasts 165 years. The orbit of Triton is 355,000 km from Neptune and it is inclined 23 deg relative to Neptune's equator. The precession of its orbital plane causes complications in its seasonal progression. Triton has a radius of 1353 km and a density of 2.07 gm/cu cm. Triton is believed to have a core of rock surrounded by water ice and a surface veneer of methane and nitrogen ice. The bright haze in its atmosphere could be small grains of particulates. Triton's surface features suggest that the moon should have remained molten until about 1 billion years ago. In order to explain the active geyser-like plumes observed near the subsolar latitude of about 50 deg south, various mechanisms are suggested including explosive escape of nitrogen gas, surface winds, and buoyancy of warmer gas. Voyager 2, which left Neptune and Triton in August 1989 and is now moving out of the solar system, is expected to provide the first glimpses of interstellar material.

Tsurutani, Bruce T.↗
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 NTRS1995

Chaotic Rotation of Nereid

The shape and spin of Neptune's outermost satellite Nereid are still unknown. Ground-based photometry indicates large brightness variations, but different observers report very different lightcurve amplitudes and periods. On the contrary, Voyager 2 images spanning 12 days show no evidence of variations greater than 0.1 mag. The latter suggest either that Nereid is nearly spherical, or that it is rotating slowly. We propose that tides have already despun Nereid's rotation to a period of a few weeks, during the time before the capture of Triton when Nereid was closer to Neptune. Since Nereid reached its present orbit, tides have further despun Nereid to a period on the order of a month. For Nereid's orbital eccentricity of 0.75, tidal evolution ceases when the spin period is still approximately 1/8 of the orbital period. Furthermore, the synchronous resonance becomes quite weak for such high eccentricities, along with other low-order spin orbit commensurabilities. In contrast, high-order resonances become very strong particularly the 6:1, 6.5:1, 7:1, 7.5:1, and 8:1 spin states. If Nereid departs by more than approximately 1% from a sphere, however, these resonances overlap, generating chaos. Our simulations show that Nereid is likely to be in chaotic rotation for any spin period longer than about 2 weeks.

Dobrovolskis, Anthony R.↗
NASA NTRS1989

Neptune's Triton: A moon rich in dry ice and carbon

The encounter of the spacecraft Voyager 2 with Neptune and its large satellite Triton in August 1989 will provide a crucial test of ideas regarding the origin and chemical composition of the outer solar system. In this pre-encounter publication, the possibility is quantified that Titron is a captured moon which, like Pluto and Charon, originally condensed as a major planetesimal within the gas ring that was shed by the contracting protosolar cloud at Neptune's orbit. Ideas of supersonic convective turbulence are used to compute the gas pressure, temperature and rat of catalytic synthesis of CH4, CO2, and C(s) within the protosolar cloud, assuming that all C is initially present as CO. The calculations lead to a unique composition for Triton, Pluto, Charon: each body consists of, by mass, 18 1/2 percent solid CO2 ice, 4 percent graphite, 1/2 percent CH4 ice, 29 percent methanated water ice and 48 percent of anhydrous rock. This mix has a density consistent with that of the Pluto-Charon system and yields a predicted mean density for Triton of 2.20 + or - 0.5 g/cu cm, for satellite radius equal to 1,750 km.

Prentice, A. J. R.↗
NASA NTRS1990

Deep atmospheric probe missions to Uranus and Neptune

The impressive results of the Voyager 2 flybys of Uranus (January 1986) and Neptune (August 1989) revealed many surprises and produced a new set of scientific questions regarding the formation and evolution of the planets, their atmospheres, the rings that surround them, and their satellite systems. A new set of intensive exploration missions to these outer planets is currently being examined by NASA and the scientific community. These missions, like Galileo to Jupiter and Cassini to Saturn, are to provide longer-term high-resolution orbital observations together with in situ atmospheric measurements. This paper will examine the feasibility of using deep atmospheric probes in connection with orbital missions to Uranus and Neptune to obtain the desired scientific measurements down to atmospheric pressure levels of at least 200 bars. The key design parameters of the atmospheric mission and system are defined, examined in some detail, and feasible options are suggested for further study.

Swenson, Byron L.↗
NASA NTRS2002

Laboratory Evaluation and Application of Microwave Absorption Properties under Simulated Conditions for Planetary Atmospheres

Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments, entry probe radio signal absorption measurements, and earth-based or spacecraft-based radio astronomical (emission) observations can be used to infer abundances of microwave absorbing constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. The use of theoretically-derived microwave absorption properties for such atmospheric constituents, or the use of laboratory measurements of such properties taken under environmental conditions that are significantly different than those of the planetary atmosphere being studied, often leads to significant misinterpretation of available opacity data. Laboratory measurements have shown that the centimeter-wavelength opacity from gaseous phosphine (PH3) under simulated conditions for the outer planets far exceeds that predicted from theory over a wide range of temperatures and pressures. This fundamentally changed the resulting interpretation of Voyager radio occultation data at Saturn and Neptune. It also directly impacts planning and scientific goals for study of Saturn's atmosphere with the Cassini Radio Science Experiment and the Rossini RADAR instrument. The recognition of the need to make such laboratory measurements of simulated planetary atmospheres over a range of temperatures and pressures which correspond to the altitudes probed by both radio occultation experiments and radio astronomical observations, and over a range of frequencies which correspond to those used in both spacecraft entry probe and orbiter (or flyby) radio occultation experiments and radio astronomical observations, has led to the development of a facility at Georgia Tech which is capable of making such measurements. It has been the goal of this investigation to conduct such measurements and to apply the results to a wide range of planetary observations, both spacecraft- and earth-based, in order to determine the identity and abundance profiles of constituents in those planetary atmospheres,

Steffes, Paul G.↗
NASA NTRS1986

Interplanetary optical navigation - Voyager Uranus encounter

Optical observations were necessary during the Voyager II Uranus approach and encounter in order to meet mission navigational requirements. In this paper, the overall optical navigation system and operation at Uranus are outlined. The formulation of the optical observables are presented, as well as their subsequent integration into the overall orbit determination products. Optical data analysis and some resulting orbit determination results from Uranus are shown. Certain scientifically important results were natural outcomes of the optical data analysis, and these are also reviewed. Finally, certain optical navigation related problems are anticipated at the forthcoming Neptune encounter in 1989; these are briefly discussed as are plans to deal with them.

Synnott, S. P.↗
NASA NTRS1993

Neptune's magnetic field - Calculation of field geometric invariants derived from the I8E1 GSFC/BRI model

L shell values along the Voyager 2 encounter trajectory and those associated with the N1 through N6 moons and N1R through N6R rings of Neptune are computed numerically on the basis of a simplified description of the Neptunian magnetic field derived from the Goddard Space Flight Center/Bartol Research Institute I8E1 model, which includes internal terms up to and including the octupole (but no external terms). Like Uranus, the large tilt between the dipole term and the rotation axis causes the moons and rings to sweep a very large range of L shells. Their orbital motion introduces additional periodicities, causing the maxima and minima in L space to vary systematically with time.

Acuna, M. H.↗
NASA NTRS1991

Studies in planetary rings

Progress made in research devoted to examining Voyager imaging data for the purpose of understanding the kinematics and dynamics of the Neptune ring arcs is reported. It was found that a radial distortion of amplitude 30 km is traveling through the arcs with a perturbation by the nearby satellite 1989N4. Two new and smaller arcs within the outer Neptunian ring were also discovered. The longitudinal spacing of all arcs is roughly that expected for corotational arc shepherding by 1989N4 provided that not all corotation sites in the arc region are filled. The semimajor axis of the arcs' orbit inferred from their observed mean motion and the latest Neptune gravitational parameters is, within uncertainties, identical to the location of the 42:43 corotation-inclination resonance of 1989N4: an observation which supports the notion of corotational arc-shepherding. This hypothesis also explains that the 15 km radial width of the arcs are due to a spread in forced orbital eccentricities. Clumps within the arcs were observed at a greater frequency than previously reported. These features may be the manifestation of big bodies within the arcs providing a source of ring dust, or they may be a kinematical effect associated with the crossing of eccentric orbits at quadrature. The inclusion of ground based observations of the arcs taken up to five years ago will lengthen the baseline over which arc dynamics may be studied and should lead to a refinement in the position of the Neptune spin and invariable plane poles.

Porco, Carolyn C.↗
NASA NTRS2022

Mission Planning for Trident: Discovery proposal to Neptune’s moon, Triton

Trident was one of the four Discovery-class Step-1 mission proposals selected by NASA in 2020 for further development and study; however, in 2021, the Step-2 proposal was not down-selected to transition into the next phase of mission development, i.e., a mission for flight.Neptune’s largest moon, Triton, was the primary focus of study for Trident. Triton’s physical and orbital characteristics make it a unique planetary target for scientific exploration, providing opportunities for investigations in a wide variety of scientific fields, including geomorphological, atmospheric, geophysical, magnetospheric, and ionospheric studies. The science objectives of the Trident mission encompassed an in-depth interior-to-exterior set of objectives, focused on multiple outstanding questions resulting from the 1989 encounter of Voyager 2, and subsequent analysis.Ball Aerospace Corp. was tasked with building the Trident spacecraft, with JPL responsible for providing Engineering Support (Mission Design & Navigation, Mission Planning, Flight Operations, Ground Data Systems, Systems Engineering) and leading Project Management. The observatory would carry a wide-ranging suite of scientific instruments onboard, including an Infrared Spectrometer (IRS) and Narrow Angle Camera (NAC) to be provided by Ball Aerospace Corp., a Wide Angle Camera (WAC) from JPL, a Magnetometer from UCLA, a contributed Plasma Science Suite from IRF (Sweden), and a contributed Radio Science instrument from ASI (Italy). All of these instruments would be used to collect unique datasets during the Triton encounter. Trident would have taken advantage of an ~13-yr, nearly-ballistic trajectory to Triton, utilizing a timely Jupiter Gravity Assist, to execute a 10-day long encounter in the Neptunian system. Launch was planned for October 2025, with Triton arrival scheduled for December 2038. The timeline for this mission would have been sub-divided into seven major phases: Launch, Commissioning, Inner Planet Cruise, Outer Planet Cruise, Approach, Encounter, and Science Data Return. Multiple planetary flybys were planned to be performed during the cruise, including three Earth flybys and one Venus flyby in the Inner Planet Cruise phase, and one Jupiter flyby in the Outer Planet Cruise phase. Along with conventional (Range and Doppler) tracking data, Delta-DOR and Optical Navigation data were also to be acquired to assist with spacecraft navigation during the Approach and Encounter phases. A 3 meter X-Band High Gain Antenna would allow playback of all science data at 1 kbps within 1 year after the Triton Encounter. The Mission Planning element on Trident encompassed and informed multiple aspects of this proposal, ranging from science observation planning during the Triton Encounter phase, to generation of activity timelines for all mission phases; performing ground coverage analysis for science observations to be acquired by all instruments and tracing them to science requirements; evaluation of spacecraft resources including data volume stored onboard, power/energy consumption, telecom (commanding/telemetry) requirements, and overall, working at the interface of science and engineering teams on the mission. All of these functions that were performed by the Mission Planning team on this proposal are discussed in this paper.

Prockter, Louise↗
NASA NTRS1983

Mariner Mark II and the exploration of the solar system

NASA's Solar System Exploration Committee has proposed the Mariner Mark II concept for the next generation of deep-space planetary observation missions, which will yield Voyager- and Galileo-quality focused data sets at a fraction of the costs associated with those spacecraft. Cost-limiting strategies under consideration include the use of reconfigurable designs, a multimission ground support system, and the incorporation of such selected new technologies as an all-X band telemetry system, fiber-optic rate sensors for use as an inertial reference system, image compression, and electronic and optical disk data distribution. It is believed possible to meet cost goals of 150-300 million/mission. The missions under consideration are comet rendezvous and sample return, main belt asteroid flybys, and orbits, flybys and probes of Saturn, Titan, Neptune and Uranus.

Neugebauer, M.↗
NASA NTRS1990

Convection in Neptune's magnetosphere

It is assumed that nonthermal escape from Triton's atmosphere produces a co-orbiting torus of unionized gas (presumably nitrogen and hydrogen) that subsequently becomes ionized by electron impact to populate a partial Triton plasma torus analogous to the Io plasma torus in Jupiter's magnetosphere. Centrifugal and magnetic-mirror forces confine the ions to a plasma sheet located between the magnetic and centrifugal equators. The ionization rate, and hence the torus ion concentration, is strongly peaked at the two points (approximately 180 deg apart in longitude) at which Triton's orbit intersects the plasma equator. During the course of Neptune's rotation these intersection points trace out two arcs roughly 75 deg in longitudinal extent, which we take to be the configuration of the resulting (partial) plasma torus. The implied partial ring currents produce a quadrupolar (four-cell) convection system that provides rapid outward transport of plasma from the arcs. Ring-current shielding, however, prevents this convection system from penetrating very far inside the plasma-arc distance. It is suggested that this convection/shielding process accounts for the radial confinement of trapped particles (150 keV or greater) within L = 14.3 as observed by the Voyager LECP instrument.

Hill, T. W.↗
NASA NTRS1989

The Voyager 2 encounter with the Neptunian system

An overview of the Voyager 2 encounter with Neptune is presented, including a brief discussion of the trajectory, the planned observations, and highlights of the results described in the 11 companion papers. Neptune's blue atmosphere has storm systems reminiscent of those in Jupiter's atmosphere. An optically thin methane ice cloud exists near the 1.5-bar pressure level, and an optically thick cloud exists below 3 bars. Neptune's magnetic field is highly tilted and offset from the planet's center; it rotates with a period of 16.11 hours. Two narrow and two broad rings circle the planet; the outermost of these rings has three optically thicker arc segments. Six new moons were discovered in circular prograde orbits, all well inside Triton's retrograde orbit. Triton has a highly reflective and geologically young surface, a thin nitrogen atmosphere, and at least two active geyser-like plumes.

Stone, E. C.↗
NASA NTRSDate not supplied

Aerosciences Implications for Uranus Aerocapture

Exploration of Uranus has been indicated as the highest priority new flagship planetary science missions for NASA in the 2022 Decadal Survey. Due to Uranus’ location as the second furthest planet from the Sun, interplanetary flight times of a traditional propulsive-capture orbiter can expect to take 13-15 years to reach Uranus while requiring up to two-thirds of the launch mass budget to be allocated to propellant alone. Aerocapture presents an alternative means to capture orbit around Uranus by using aerodynamic forces generated from flight through the planetary atmosphere to decelerate to orbital insertion while reducing time in interplanetary transit time and increasing on-orbit payload mass. To date, only one spacecraft (Voyager 2) has visited Uranus, and there have been no missions which have entered its atmosphere. While knowledge of planetary entry at rocky terrestrial planets with Air (N2/O2) and CO2 dominated atmospheres has greatly advanced over the past decades with a variety of missions to learn from, the H2/He composition and much larger atmosphere of ice giants such as Uranus poses new aerosciences implications which are not fully understood. These implications must be better understood not only for aerocapture, but for planetary entry at Uranus as a whole (such as a Uranus probe, as proposed by the 2022 Decadal Survey), as well as other ice giants like Neptune. To conduct a proper analysis of aerocapture methods, aerodynamic and aerothermal analysis of flight trajectories in Uranus atmosphere are required. Previous work has investigated aerocapture at Neptune (with similar atmosphere to Uranus) using a Mid-L/D configuration. Additional previous work considers the convective heating experienced in H2/He atmospheres. The current work looks to use heritage Low-L/D configurations, adding technical depth and maturation to analysis previously performed by Girija. Atmospheric flight at ice giants presents unique challenges, including but not limited to: fidelity of atmospheric models, large entry velocities due to location in the solar system, extended flight in rarefied flow conditions due to atmospheric scale, and unusual aerodynamic flow properties specific to Uranus and Ice Giant atmospheres.

Eli Shellabarger↗
NASA NTRS1992

Distant magnetotails of the outer magnetic planets

The distant planetary magnetotails of Jupiter, Saturn, Uranus, and Neptune are assumed to be partially open, hot, long plasma cavities generally in pressure equilibrium with the solar wind. Most of the magnetosheath magnetic field lines drape around the magnetotails. Conservation of momentum density, magnetic field, plasma density, and energy density fluxes are invoked at the tail boundaries to determine the shape of the magnetotails and the variations of plasma and magnetic field characteristics with distance down the magnetotail. Voyager observations are used to initialize calculations in the near-planet portions of each magnetotail. Estimates of magnetotail cross sections, magnetic field strengths, and plasma densities are described as a function of downstream distance. The model accurately predicts properties of the Jovian magnetotail at least as far as Saturn's orbit.

Macek, W. M.↗
NASA NTRS2020

50 Years of Spaceflight with Fourier Transform Spectrometers (FTS) Built at NASA GSFC

Over the past 50 years, NASA Goddard Space Flight Center (GSFC) has been developing, building, testing and flying a series of Fourier Transform Spectrometers (FTS). This began with the IRIS instruments on the Earth-orbiting Nimbus satellites and progressed to more sophisticated designs optimized for interplanetary spacecraft sent to Mars and later to the outer solar system. Adaptions have been made over time, including progressively higher spectral resolution, sensitivity, numbers of detectors and complexity. Instrument operating temperatures have decreased to enable remote sensing of the cold giant planet systems. In this paper we describe the historical evolution of this instrument line, comparing and contrasting different aspects such as optical design and materials, detector types and data handling. We conclude by looking towards the future. At present the CIRS-Lite prototype is being tested at NASA GSFC for potential use on a future mission to the ice giants, Uranus and Neptune. Surpassing the previous performance of the Voyager IRIS instruments remains challenging, and new technologies that could enable these measurements are discussed.

Nixon, Conor A.↗