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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

A study of the feasibility of ultraviolet spectrometry for cometary missions

Ultraviolet spectra fo the comet West obtained by sounding rocket experiments in 1976 are reproduced and interpreted in order to estimate the expected brightness of the emission features and determine the spatial extent of these features for the proposed Halley Flyby/Tempel 2 rendezvous and the possible Halley or Encke flybys close to perihelion. A coma model was constructed and evaluated for the physical condition of candidate targets such as heliocentric distance, gas production, and composition. In addition to brightness profiles, the neutral and ion densities of the principal species are also dervied. The brightness profiles can be used to determine the feasibility of utilizing the space telescope to provide supporting observations during the mission. Basic parameters identified are spectral range, wavelength resolution, spatial resolution, sensitivity and dynamic range, rejection of scattered light, and integration or accumulation time.

Feldman, P. D.↗
NASA NTRS1978

Multiple Discipline science assessment

Various science disciplines were examined to determine where and when it is appropriate to include their objectives in the planning of planetary missions. The disciplines considered are solar astronomy, stellar and galactic astronomy, solar physics, cosmology and gravitational physics, the geosciences and the applied sciences. For each discipline, science objectives are identified which could provide a multiple discipline opportunity utilizing either a single spacecraft or two spacecraft delivered by a single launch vehicle. Opportunities using a common engineering system are also considered. The most promising opportunities identified include observations of solar images and relativistic effects using the Mercury orbiter; collection of samples exposed to solar radiation using the Mars surface sample return; studies of interstellar neutral H and He, magnetic fields, cosmic rays, and solar physics during Pluto or Neptune flybys; using the Mars orbiter to obtain solar images from 0.2 AU synchronous or from 90 deg orbit; and the study of the structure and composition of the atmosphere using atmospheric probes and remotely piloted vehicles.

Wells, W. C.↗
NASA NTRS1979

Report of the Comet Science Working Group

Scientific objectives and mission strategies for a flyby of Halley's comet and rendezvous with Tempel-2 are discussed. Instrumentation to be carried by both the rendezvous spacecraft and the coma probe payload are listed. Known characteristics of both comets are summarized.

Source record↗
NASA NTRS1978

Io may have a bright dawn terminator

A test of the thermoluminescence properties of Io is proposed which can be performed with the unique optical observations programmed for Voyager 1's close flyby of that Jovian satellite. It is suggested that if Io's surface is thermoluminescent with a characteristic post-eclipse-brightening time of the order of 10 min, there should be a characteristic bright rim on the sunlit side of the satellite's dawn terminator. It is noted that the spatial resolution of six-color pictures to be taken by Voyager 1 should be sufficiently adequate to reveal the bright dawn terminator if it exists and is bright enough. A mathematical model illustrating the bright-dawn-terminator effect is presented.

Forman, M.↗
NASA NTRS1978

The albedo of Titan

It is shown that a two-layer model of Titan's atmosphere is required in order to match laboratory data on methane absorption to ground-based IR observations of Titan. An excellent fit to the observations is obtained with a model where the dust in Titan's atmosphere is confined to a region containing 50 m-am of methane and located above an atmosphere with 1.95 km-am of methane, with the amount of dust being the value required for an extinction optical depth of 5 at 0.5 micron. The surface reflectivity required by this model is about 40% to 50%, which is typical for both dirty snow and clouds. The possibility is discussed of distinguishing between two scenarios for the observed surface of Titan (a true surface or just cloud tops) on the basis of limb-darkening curves obtained at many wavelengths with an orbiting telescope or a flyby mission.

Podolak, M.↗
NASA NTRS1979

Jovian longitudinal control of Io-related radio emissions

A theoretical model is proposed to explain the control of Io-related radio emissions by Jupiter's rotational phase. The model is based on the hypothesis that the radio emissions are generated by Birkeland currents flowing between Io and the Jovian ionosphere. Specifically, it is suggested that the precipitation of radiation-belt electrons within a certain range of Jovian longitudes produces a restricted region of enhanced ionization and correspondingly enhanced conductivity in Jupiter's ionosphere and that the Io-Jupiter Birkeland current and the associated radio emissions are dramatically increased when Io's flux tube encounters this sector of enhanced ionization in Jupiter's ionosphere. The magnitude of the current is found to be about 100,000 A at most Jovian longitudes because of ionospheric resistance. It is estimated that within the favored longitudinal sector electron precipitation produces an enhancement of this current by one to three orders of magnitude. The model predictions are compared with observations made during the Pioneer 10 and 11 flybys, and satisfactory agreement is obtained.

Dessler, A. J.↗
NASA NTRS1979

Jupiter's radiation belts and atmosphere

Maps and stripscans of the radio emission from Jupiter were made during the Pioneer 10 flyby in December 1973 at wavelengths of 6 cm, 21 cm, and 50 cm using the Westerbork telescope in the Netherlands. With this instrument the disk of the planet was resolved at 6 and 21 cm. The pictures are averaged over 15 deg of Jovian longitude. At 21 cm the stripscans clearly show the existence of a 'hot region' in the radiation belts at a System III longitude (1965.0) of 255 + or - 10 deg. Its flux is about 9% of the total nonthermal flux, and it has a volume emissivity enhanced by a factor of about 1.6 with respect to the general radiation belts. The temperature of the thermal disk at 21 cm appears to be 290 + or - 20 K. This is likely due to a high ammonia mixing ratio in the atmosphere, a factor of 4-5 larger than the expected solar value of 0.00015.

De Pater, I.↗
NASA NTRS1979

Liquid content of the lower clouds of Venus as determined from Mariner 10 radio occultation

S- and X-band radio occulation data obtained during the Mariner 10 flyby of Venus are analyzed to determine the effects of signal attenuation due to the clouds of Venus. The signal-amplitude data are corrected for errors introduced by spacecraft motion and antenna steering during the occultation, absorption-coefficient profiles are computed, and the liquid content of the Venusian clouds is estimated. Excess attenuation effects are found and ascribed to absorption by small liquid particles, consisting of a water-sulfuric acid mixture containing 75% + or - 25% sulfuric acid, in the clouds. Three absorbing cloud layers at respective heights of 68, 60, and 48 km are suggested by the X-band absorptivity profile, but only the 68- and 48-km layers are clearly indicated by the S-band profile. The relatively slow S-band absorption at 60 km is tentatively attributed to an unknown constituent that is much more absorbing in the X-band than in the S-band. The results are compared with nephelometer measurements performed at 0.9 micron by the Venera 9 and 10 probes.

Kliore, A. J.↗
NASA NTRS1979

The NASA space program

Current and planned NASA space programs are outlined. The major current activity is the development and installation of the Space Transportation System consisting of the Space Shuttle, Spacelab and upper stages for payload launching from the Shuttle. NASA's space sciences program includes spaceborne astronomy by means of the future Space Telescope and Spacelab instruments as well as the current HEAO and IUE satellites. In the area of solar-terrestrial physics, the Solar Maximum Mission and the Solar Polar Mission are scheduled for 1980 and 1983, respectively. Planetary programs generally include ground-based and flyby observations to specify planetary environments, exploration missions to acquire knowledge of planetary processes, history and evolution, and study missions to investigate specific problems. Missions underway include the Voyager mission to Jupiter and Saturn and the Pioneer Venus mission. Research involving the inner planets, the outer planets and asteroids and comets is being planned.

Naugle, J. E.↗
NASA NTRS1979

Measurements of turbulence in the Venus atmosphere deduced from Pioneer Venus multiprobe radio scintillations

The 2.3-GHz log-amplitude fluctuations observed in the radio links of the Pioneer Venus entry probes during Venus encounter have been used to study turbulence in the Venus atmosphere. The deduced estimates of the upper bound of the structure constant of the refractive index fluctuations (less than approximately 4 x 10 to the -8th/cu root cm) are inconsistent with similar entry probe measurements by Veneras 4 to 8 but are consistent with the radio occultation measurements by flyby (Mariners 5 and 10) and orbiting (Venera 9) spacecraft. The Pioneer Venus measurements therefore provide a resolution of the long-standing order of magnitude discrepancy between these earlier measurements of the structure constant.

Woo, R.↗
NASA NTRS1979

Performance verification and system parameter identification of spacecraft tape recorder control servo

Design adequacy of the lead-lag compensator of the frequency loop, accuracy checking of the analytical expression for the electrical motor transfer function, and performance evaluation of the speed control servo of the digital tape recorder used on-board the 1976 Viking Mars Orbiters and Voyager 1977 Jupiter-Saturn flyby spacecraft are analyzed. The transfer functions of the most important parts of a simplified frequency loop used for test simulation are described and ten simulation cases are reported. The first four of these cases illustrate the method of selecting the most suitable transfer function for the hysteresis synchronous motor, while the rest verify and determine the servo performance parameters and alternative servo compensation schemes. It is concluded that the linear methods provide a starting point for the final verification/refinement of servo design by nonlinear time response simulation and that the variation of the parameters of the static/dynamic Coulomb friction is as expected in a long-life space mission environment.

Mukhopadhyay, A. K.↗
NASA NTRS1979

Aerocapture - A system design for planetary exploration

The paper presents the results of a study that developed conceptual designs of an atmospheric entry system with moderate to high L/D ratios to establish the concepts of aerocapture into low circular Mars orbits from hyperbolic flyby trajectories. The payloads considered in this study were those currently conceived for the Mars Sample Return (MSR) mission including single mission (SM) and multi-mission (MM) modes. The design concepts developed are also applicable (with small modification) for establishment of closed orbits about Venus and Saturn. The MSR mission payload requires an orbiter, lander, ascent vehicle, and rover, with a total mass of approximately 4000 kg. The emphasis during the study was placed on development of concepts for aerodynamic configuration, the aeroshell design, and a guidance, navigation, and control subsystem.

Cruz, M. I.↗
NASA NTRS1979

Satellite orbit determination

The determination of the Mars-centered ephemerides of the Viking Orbiters and positions of the Landers from two way Doppler and range data is described. An overview of mission satellite orbit determination functions and methods is given. Topics covered include: postmaneuver orbit convergence, local orbit knowledge accuracies, the effects of interplanetary media, use of constrained solutions, and solving through trim burns. Procedure and results relevant to the determination of the planet gravity field are included. Results relative to sensing Mars' gravity field during the extended mission are presented along with a discussion of the Phobos Flyby Experiment conducted during February 1977.

Hildebrand, C. E.↗
NASA NTRS1980

Utilization of multi-body trajectories in the Sun-Earth-Moon system

An overview of three uncommon trajectory concepts for space missions in the Sun-Earth-Moon System is presented. One concept uses a special class of libration-point orbits called 'halo orbits.' It is shown that members of this orbit family are advantageous for monitoring the solar wind input to the Earth's magnetosphere, and could also be used to establish a continuous communications link between the Earth and the far side of the Moon. The second concept employs pretzel-like trajectories to explore the Earth's geomagnetic tail. These trajectories are formed by using the Moon to carry out a prescribed sequence of gravity-assist maneuvers. Finally, there is the 'boomerang' trajectory technique for multiple-encounter missions to comets and asteroids. In this plan, Earth-swingby maneuvers are used to retarget the original spacecraft trajectory. The boomerang method could be used to produce a triple-encounter sequence which includes flybys of comets Halley and Tempel-2 as well as the asteroid Geographos.

Farquhar, R. W.↗
NASA NTRS1980

Voyager 1: Encounter with Saturn

The history of the Voyager Project is reviewed as well as known facts about Saturn and its satellites. Important results of encounters with Jupiter are summarized. Scientific objectives of the flyby of Saturn involve the planet's atmosphere, rings, and magnetic field interactions with the solar wind and satellites. The search for additional satellites, and various aspects of Titan, Rhea, Dione, Mimas, Iapetus, Hyperion, and Enceladas are also of interest. The instruments developed to obtain these goals are described.

Panagakos, N.↗
NASA NTRS1979

Solar electric propulsion system /SEPS/ program plans and system definition

The status of the NASA Solar Electric Propulsion System (SEPS) program is reviewed. The plans for SEPS definition and development include the initiation of the SEPS definition phase, the procurement for full-scale development in 1980, delivery of the SEPS for integration and tests with the comet rendezvous spacecraft in 1984, and a launch in 1985 that involves a flyby of Halley's comet in 1985 and a subsequent rendezvous with the comet Tempel 2 in 1988. In preparation for this, the acquisition process for SEPS has been initiated based on mission requirements rather than system requirements.

Austin, R. E.↗
NASA NTRS1979

Plasma wave observations near Jupiter - Initial results from Voyager 2

The Voyager 2 flyby of Jupiter, which occurred in July 1979, provided the second opportunity to study plasma waves in the vicinity of Jupiter (first measurements made by Voyager 1 in March 1979). Because of the somewhat different trajectory and plasma conditions at Jupiter, the Voyager 2 mission provided new perspectives for analyzing many of the phenomena detected by Voyager 1, and also revealed the presence of several new types of plasma waves. A survey is presented of the initial results from the Voyager 2 plasma wave instrument, with special emphasis on the new observations and comparisons with the Voyager 1 results. The data base for the present discussion starts with the first detection of radio emissions from Jupiter about six months before closest approach and ends about two weeks after closest approach

Gurnett, D. A.↗
NASA NTRS1979

Jupiter's cloud distribution between the Voyager 1 and 2 encounters - Results from 5-micrometer imaging

As part of a continuing effort of ground-based support for Voyager target selection, infrared images in the 5-micrometer wavelength region were acquired in preparation for the Voyager 2 flyby of Jupiter. Observations were made during May 1979 from the Palomar 5-meter telescope and the new 3-meter NASA Infrared Telescope Facility at Mauna Kea and are compared to previous observations. Variations seen in the 5-micrometer flux distribution suggest global patterns of clouding over of some Jovian belts and clearing of others. These data were used to predict the Jovian cloud distribution at the time of the Voyager 2 encounter in order to target the imaging and infrared experiments to areas free of high obscuring clouds.

Terrile, R. J.↗