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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
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NASA NTRS1985

Long-term changes in Saturn's troposphere

Attention is given to the results of a long term monitoring study of Saturn's H2 quadrupole and CH4 band absorptions outside the equatorial zone, over an interval of half a Saturn year that covers most of the perihelion half of Saturn's elliptical orbit (which is approximately bounded by the equinoxes). Marked long term changes are noted in the CH4 absorption, accompanied by weakly opposite changes in the H2 absorption. Seasonal changes are inferred on the basis of temporal variations in absorption. Spatial measurements have also been made in the 6450 A NH3 band since the 1980 equinox.

Trafton, L.↗
NASA NTRS1986

Current issues in the X-ray properties of active galactic nuclei

Some issues raised by soft X-ray spectra and hours variability observations of active galactic nuclei (AGN), made possible by improved energy calibration of the Einstein IPC and the launch of Exosat into a unique 72-hour highly elliptical orbit, are presented. Explanations for steep soft excesses in quasars include the emission from the hot tail of an accretion disk spectrum, and from optically thin bremsstrahlung at 1-2 x 10 to the 6th K from a large volume. Mechanisms for the approximately 1.0 slope in the 0.2-4 keV soft IPC X-ray band quasars include direct synchrotron emission, unsaturated Comptonization of an arbitrary seed spectrum, and synchro-Compton scattering from the infrared. The Exosat observation of NGC 4051 revealed six 1-hour cycles with spectral changes during the variations. AGN time variations may now be able to limit emission models through studying, for example, the lag times between soft and hard variations.

Elvis, M.↗
NASA NTRS1986

Changes in Saturn's south-temperate haze distribution during the summer of 1973-1980

Attention is given to the results of a long term monitoring study of Saturn's H2 quadrupole and CH4 band absorptions outside the equatorial zone, over an interval of half a Saturn year that covers most of the perihelion half of Saturn's elliptical orbit (which is approximately bounded by the equinoxes). Marked long term changes are noted in the CH4 absorption, accompanied by weakly opposite changes in the H2 absorption. Seasonal changes are inferred on the basis of temporal variations in absorption. Spatial measurements have also been made in the 6450 A NH3 band since the 1980 equinox.

Trafton, L.↗
NASA NTRS1986

Diatoms in comets

The fossil record of the microscopic algae classified as diatoms suggests they were injected to earth at the Cretaceous boundary. Not only could diatoms remain viable in the cometary environment, but also many species might replicate in illuminated surface layers or early interior layers of cometary ice. Presumably they reached the solar system on an interstellar comet as an already-evolved assemblage of organisms. Diatoms might cause color changes to comet nuclei while their outgassing decays and revives around highly elliptical orbits. Just as for interstellar absorption, high-resolution IR observations are capable of distinguishing whether the 10-micron feature arises from siliceous diatom material or mineral silicates. The 10-30-micron band and the UV 220-nm region can also provide evidence of biological material.

Hoover, R.↗
NASA NTRS1986

Harmonically structured ULF pulsations observed by the AMPTE CCE magnetic field experiment

Spectrograms of ULF waves in the 0 to 80 mHz frequency range have been prepared from magnetic field data obtained by the elliptically orbiting AMPTE/CCE satellite (with an apogee of approximately 8.8 earth radii). The most prominent feature of these spectrograms (which cover a full 15.6-h orbit) is the presence of harmonically structured, azimuthally polarized pulsations in the outer magnetosphere during daytime hours. The frequencies of these pulsations decrease with increasing radial distance from the earth, indicating that they represent independent resonances of local magnetic flux tubes. The latitudinal structure of these harmonic pulsations, observed as AMPTE/CCE traveled to + or - 16 deg magnetic latitude, is consistent with accepted field line resonance models.

Engebretson, M. J.↗
NASA NTRS1986

Comment

Frank et al.'s (1986) suggestion that the earth is impacted by small (100-Mg) comets at the rate of 20/min is reconsidered in the light of recent comet models. The possibility of such a swarm of comets is found to be problematical because of the large number of comets required and because thermal stability arguments, while not excluding such a possibility, imply that it is implausible. In a reply, Frank et al. reaffirm their findings that such small comets, following direct elliptical orbits about the sun, are distributed in a disk centered on the sun and lying near the ecliptic plane at heliocentric radial distances 1-10,000 AU. The presence of these comets in the inner solar system is attributed to the gravitational action of the outer planets, passing stars, or interstellar clouds on comets is more distant orbits.

Mckay, C. P.↗
NASA NTRS1986

Efficient inverse solution of Kepler's equation

A bicubic polynomial approximation to Kepler's equation for elliptic orbits is shown to provide accurate starting values for efficient numerical solution of this equation for eccentric anomaly. In the approximate equation, a cubic in mean anomaly is set equal to a cubic in eccentric anomaly. The coefficients in the two cubics are obtained as functions of eccentricity by specifying values of function and slope at the midpoint and the endpoint of the complete interval (0 to pi). The initial estimate of eccentric anomaly to use in an iteration formula is obtained by evaluating the cubic in mean anomaly and finding the single real root of the cubic in eccentric anomaly. Numerical results are presented which indicate that the estimate accuracy of this method is roughly an order of magnitude better than that of other recently-reported formulas.

Boltz, Frederick W.↗
NASA NTRS1987

Geomagnetic field-line resonant harmonics measured by the Viking and AMPTE/CCE magnetic field experiments

The first simultaneous observations of multiple harmonic, azimuthally polarized, ULF pulsations at two points along a geomagnetic flux tube in space are reported. In March 1986, the elliptically orbiting equatorial AMPTE/CCE satellite was oriented with the apogee near 0830 h MLT, and the orbital plane of the polar-orbiting Viking satellite was at 1000 MLT. The satellites were situated within approximately the same flux tube but with an effective separation of approximately 10 R(e) near L = 8 on the inbound pass of the AMPTE/CCE orbit. Structured harmonic pulsations were observed by the magnetic field experiments on both spacecraft, and they appeared to turn off and on simultaneously at both locations. Both the observations and the relative amplitudes along the magnetic field lines support recent ideas of multiple field-line resonances of Alfven waves.

Zanetti, L. J.↗
NASA NTRS1988

Satellite Interference Analysis and Simulation Using Personal Computers

This report presents the complete analysis and formulas necessary to quantify the interference experienced by a generic satellite communications receiving station due to an interfering satellite. Both satellites, the desired as well as the interfering satellite, are considered to be in elliptical orbits. Formulas are developed for the satellite look angles and the satellite transmit angles generally related to the land mask of the receiving station site for both satellites. Formulas for considering Doppler effect due to the satellite motion as well as the Earth's rotation are developed. The effect of the interfering-satellite signal modulation and the Doppler effect on the power received are considered. The statistical formulation of the interference effect is presented in the form of a histogram of the interference to the desired signal power ratio. Finally, a computer program suitable for microcomputers such as IBM AT is provided with the flowchart, a sample run, results of the run, and the program code.

Kantak, Anil↗
NASA NTRS1987

The rings of Uranus

The rings of Uranus are not all circular; nor do they all lie in the plane of Uranus' equator. The epsilon ring of Uranus, which is the largest and outermost one, ranges in width from 20 to 96 km in proportion to the distance of the ring material from Uranus. Typical rings can be modeled by assuming that its edges are defined by a combination of smooth elliptical orbits and random 'wiggles'. A transition from a few large moons in outer regions to many small moons just outside ring-filled inner regions is a feature noted to be held in common by the ring and moon systems of Saturn, Uranus, and Neptune.

Cuzzi, Jeffrey N.↗
NASA NTRS1989

Magnetic fields near Mars - First results

The magnetic fields of Mars have been measured from Phobos 2 with high temporal resolution in the tail and down to an 850-km altitude. During four successive highly elliptical orbits, the position of the bow shock as well as that of a transition layer, the 'planetopause', were identified. Subsequent circular orbits at 6000-km altitude provided the first high-resolution data in the planetary tail and indicate that the interplanetary magnetic field mainly controls the magnetic tail. Magnetic turbulence was also detected when the spacecraft crossed the orbit of Phobos, indicating the possible existence of a torus near the orbit of this moon.

Riedler, W.↗
NASA NTRS1990

Martian bow shock - Phobos observations

Data obtained with the Magma magnetometer on the subsolar passes of the Phobos spacecraft during its 3 elliptic orbits reveals a turbulent bow shock with a strong foot consistent with the reflection of solar wind protons. The bow shock lies at a subsolar distance of 1.47 + or - .03 R(M). The circular orbit phase of the mission reveals a bow shock with a highly varying location. The median terminator crossing lies at 2.72 Mars radii. The location of the bow shock in the terminator plane is sensitive to neither the EUV flux nor to planetary longitude.

Schwingenschuh, K.↗
NASA NTRS1990

Magellan mission to Venus

The Magellan spacecraft arrived at Venus on August 10, 1990 to begin global high-resolution radar image and altimeter mapping and gravity field determination of the planet. Mapping of this cloud covered planet is done from an elliptical orbit with a 3.15 hour period and a 275 km periapsis. After the initial checkout of the spacecraft, the primary missions lasts for 243 days (one Venus day); the time it takes for Venus to make one revolution beneath the spacecraft. The data collected by Magellan is expected to provide an understanding of the geological and geophysical nature of the planet Venus, and the general processes that control planetary evolution.

Boyce, Joseph M.↗
NASA NTRS1991

Magellan imaging radar mission to Venus

The Magellan imaging-radar mapping mission has collected and processed data from the spacecraft in an elliptical orbit around Venus. A brief description is given of the mission and the spacecraft, followed by a more detailed description of the radar system design, which used earth-orbiting SAR experience and several innovations in its design to operate from an orbit around another planet. The radar sensor, ground processing, and data products are described. This multimode radar is the only science instrument on the mission and has the objective of mapping at least 70 percent of the planet surface. It has three modes: SAR, altimetry, and passive radiometry. The radar system has produced maps of almost all of the Venusian surface with a resolution better than 600-m equivalent optical line pair, and the best resolution obtained is equivalent to less than 300 m. Some of the early radar images are shown.

Johnson, William T. K.↗
NASA NTRS1991

CRAF/Cassini (C/C)

The Comet Rendezvous Asteroid Flyby (CRAF) is a mission to rendezvous with the comet Tempel 2 and to station-keep at the comet for a period of 2.6 years, including the comet perihelion. There is a flyby of the asteroid Mandeville prior to the arrival at Tempel 2. The Cassini is a mission to place a spacecraft in a highly elliptical orbit around the planet Saturn and deliver a probe to the surface of its satellite Titan. There is a flyby of the asteroid 1989 UR1 prior to arrival at Saturn. Coverage goals for the two missions are explained. Information is given in tabular form for frequency assignments, telemetry, command, navigation, and tracking support responsibility.

Draper, R.↗
NASA NTRS1991

Magellan

The Magellan Mission consists of a single spacecraft to be placed in an elliptical orbit around Venus. The main objective of the mission is to perform radar imaging, altimetry, and reflectivity of 90 percent of the planet for one Venusian year (243 days). A flight profile is given, and information is presented in tabular form on the following topics: Deep Space Network support, frequency assignments, telemetry, command, and tracking support responsibilities.

Scott, J.↗
NASA NTRS1991

Pioneer 12 (PN-12)

The DSN (Deep Space Network) mission support requirements for Pioneer 12 are summarized. The Pioneer 12 spacecraft is in a 24-hour elliptical orbit around Venus. Atmospheric and altimetry data are obtained mainly around periapsis, and planetary imaging is normally performed around apoapsis. The Pioneer 12 mission objectives are outlined and the DSN support requirements are defined through the presentation of tables and narratives describing the spacecraft flight profile; DSN support coverage; frequency assignments; support parameters for telemetry, command and support systems; and tracking support responsibility.

Lozier, D.↗
NASA NTRS1992

Electromagnetic ion cyclotron waves observed near the oxygen cyclotron frequency by ISEE 1 and 2

The first results of observations of ion cyclotron waves by the elliptically orbiting ISEE 1 and 2 pair of spacecraft are reported. The most intense waves (8 nT) were observed in the outer plasmasphere where convection drift velocities were largest and the Alfven velocity was a minimum. Wave polarization is predominantly left-handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by cold plasma propagation theory are identified in the wave data. Computations of the experimental wave spectra during the passage through the plasmapause show that the spectral slots relate to the local plasma parameters, possibly suggesting an ion cyclotron wave growth source near the spacecraft. A regular wave packet structure seen over the first 30 min of the event is attributed to the modulation of this energy source by the Pc 5 waves seen at the same time.

Fraser, B. J.↗