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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 253 records · Page 14
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NASA NTRS1983

The Voyager Mission - Encounters with Saturn

The trajectories of Voyager 1 and Voyager 2 through the Saturnian system were selected in such a way that the observations would be complementary. The Voyager 1 trajectory provided a close encounter with Titan, making possible atmospheric occultations, a search for an intrinsic Titanian magnetic field, and studies of the interaction of Titan with either the solar wind or the Saturnian magnetosphere. The trajectory was also designed to provide an optimum geometry for the transmission of S and X band radio waves from the spacecraft through the rings; in this way, the attenuation and scattering of 3.6- and 13-cm waves by the ring particles could be observed. Besides this, Voyager 1 provided radio and ultraviolet occultation studies of Saturn's atmosphere as well as close approaches to three of the moderate-sized icy satellites (Mimas, Dione, and Rhea) and to a number of minor satellites. The much later arrival of Voyager 2 and the Saturn flyby distance were chosen in such a way as to make possible a gravity-assisted continuation on to Uranus and Neptune.

Stone, E. C.↗
NASA NTRS1984

Helicopter rotor performance degradation in natural icing encounter

The analytical model described by Korkan et al. (1982) for predicting the performance degradation of propellers in a natural icing encounter is used to determine the feasibility of predicting helicopter performance degradation in hover during natural icing. The flight condition selected for analysis involves an altitude of 3000 ft and a free-air temperature of 1 F. The values of degradation yielded by the model for rime ice accretion are representative of those experienced in actual flight.

Korkan, K. D.↗
NASA NTRS1984

Analysis of aircraft control strategies for microburst encounter

Analyses have indicated that improved control strategies could reduce the threat posed by the presence of microburst-type wind shear during aircraft takeoffs and landings. The attenuation of flight path response to microburst inputs by feedback control to elevators and throttle was studied for the cases of a jet transport and a general aviation aircraft, using longitudinal equations of motion, root locus analysis, Bode plots of altitude response to wind inputs, and nonlinear numerical simulation. Energy management relative to the airmass, a pitch-up response to the decreasing airspeed, increased phugoid mode damping, and decreased phugoid natural frequency, are found to improve microburst penetration aircraft behavior. Aircraft stall, and throttle saturation, are limiting factors in an aircraft's ability to maintain a given flight path during a microburst encounter.

Stengel, R. F.↗
NASA NTRS1984

Close encounters and collisions of comets with the earth

A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.

Sekanina, Z.↗
NASA NTRS1986

Arecibo Observatory support of the US international cometary Explorer mission encounter at comet Giacobini-Zinner

The Arecibo Observatory in Puerto Rico participated in the support of the U.S. International Cometary Explorer (ICE) mission when the ICE spacecraft passed through the tail of comet Giacobini-Zinner on September 11, 1985. The Arecibo Observatory is a research facility of the National Astronomy and Ionosphere Center (NAIC) operated by Cornell University under contract to the National Science Foundation (NSF). Coverage of the encounter involved the use of the observatory's 305-m (1000-ft) radio reflector antenna and RF and data system equipment fabricated or modified specifically for support of the ICE mission. The successful implementation, testing, and operation of this temporary receive, record, and data relay capability resulted from a cooperative effort by personnel at the Arecibo Observatory, the Goddard Space Flight Center, and the Jet Propulsion Laboratory.

Gordon, D. D.↗
NASA NTRS1986

Intercontinental antenna arraying by symbol stream combining at ICE Giacobini-Zinner encounter

Deep space tracking stations on different continents were arrayed during the encounter of the International Cometary Explorer (ICE) spacecraft with the comet Giacobini-Zinner during September 9 through 12, 1985. This is the first time that telemetry signals received on different continents have been combined to enhance signal to noise ratio. The arraying was done in non-real time using the method of symbol stream combining. The improvement in signal to noise ratio was typically 2 dB over the stronger of the two stations in each array.

Hurd, W. J.↗
NASA NTRS1986

ISEE 3 observations during a plasma sheet encounter at 140 earth radii - Evidence for enhancement of reconnection at the distant neutral line

A plasma sheet encounter of the ISEE-3 spacecraft in the distant tail at 140 earth radii on March 20, 1983 is studied using magnetic field, energetic particle, and plasma electron data sets. The H-component magnetograms from auroral magnetometer stations, intensity-time profiles, high resolution magnetic field measurements, and electron and proton angular distributions are analyzed. The dynamics of the plasma sheet displayed by the strong tailward and earthward directed ion beams, large northward and southward magnetic fields excursions, and short tailward and earthward plasma flows are described.

Scholer, M.↗
NASA NTRS1985

Angle-of-attack estimation for analysis of CAT encounters

Recent studies of clear-air turbulence (CAT) encounters involving wide-body airliners have been based upon flight-path wind estimates made by analyzing digital flight-data-recorder (DFDR) records and radar records. Such estimates require a time history of the aircraft angle of attack, a record that is not usually included in the DFDR measurement set. This paper describes a method for reconstructing angle of attack that utilizes available flight record and aircraft-specific information associated with an aerodynamic model of the lift coefficient. Results from two wide-body incidents in which vane measurements of angle of attack were recorded show good agreement between measured and calculated time histories. This research has been performed in cooperation with the National Transportation Safety Board to provide a better understanding of the CAT phenomenon.

Bach, R. E., Jr.↗
NASA NTRS1986

Voyager engineering improvements for Uranus encounter

Engineering changes made on the Voyager spacecraft during their explorations of the outer solar system are discussed. A recovery from a scan actuator fault, achieved by ground testing, in flight capability enhancement and testing, conservation of actuator usage and a set of contingency plans to substitute roll motion of the spacecraft for motion of the scan platform in the AZ direction, is discussed. General engineering improvements made are addressed, including a scan platform anticreep patch modification, an improvement of dynamic performance during maneuvers, and adaptive control of the attitude control deadband. Changes made in the power management are described. Finally, engineering improvements made in order to enhance scientific findings at the Uranus encounter are reviewed in detail. The two most important were the increased gyro drift turn rate capability to accommodate image motion compensation for the close fly-by of Miranda and the reduction in spacecraft rates to accommodate increased imaging exposure times without incurring excessive image smear.

Marderness, H. P.↗
NASA NTRS1986

VLA feedhorn for Voyager encounter of Neptune

A high gain, low noise corrugated feedhorn was designed and developed by JPL for use in Very Large Array (VLA) antennas, near Soccoro, New Mexico. The new feedhorn will enable the VLA to support the Voyager encounter of Neptune in August of 1989. This will significantly enhance the receiving capability of the United States for that historic event.

Manshadi, F.↗
NASA NTRS1986

Voyager Uranus encounter 0.2lbf T/VA short pulse test report

The attitude control thrusters on the Voyager spacecraft were tested for operation at electrical pulse widths of less than the current 10-millisecond minimum to reduce impulse bit and, therefore, reduce image smear of pictures taken during the Uranus encounter. Thrusters with the identical configuration of the units on the spacecraft were fired in an altitude chamber to characterize impulse bit and impulse bit variations as a function of electrical pulse widths and to determine if the short pulses decreased thruster life. Pulse widths of 4.0 milliseconds provide approximately 45 percent of the impulse provided by a 10-ms pulse, and thruster-to-thruster and pulse-to-pulse variation is approximately plus or minus 10 percent. Pulse widths shorter than 4 ms showed wide variation, and no pulse was obtained at 3 ms. Three thrusters were each subjected to 75,000 short pulses of 4 ms or less without performance degradation. A fourth thruster exhibited partial flow blockage after 13,000 short pulses, but this was attributed to prevous test history and not short pulse exposure. The Voyager attitude control thrusters should be considered flight qualified for short pulse operation at pulse widths of 4.0 ms or more.

Source record↗
NASA NTRS1986

CO near the Pleiades: Encounter of a star cluster with a small molecular cloud

Although there is a large amount of interstellar matter near the Pleiades star cluster, the observed dust and gas is not a remnant of the placental molecular cloud from which the star cluster was formed. Carbon monoxide (CO) associated with the visible reflection nebulae was discovered by Cohen (1975). Its radial velocity differs from that of the cluster by many times the cluster escape velocity, which implies that the cloud-cluster association is the result of a chance encounter. This circumstance and the proximity of the Pleiades to the sun creates an unique opportunity for study of interstellar processes at high spatial resolution. To study the molecular component of the gas, a 1.7 square degree field was mapped with the AT&T Bell Laboratories 7-meter antenna (1.7' beam) on a 1' grid in the J=1.0 C(12)O line, obtaining over 6,000 spectra with 50 kHz resolution. The cloud core was mapped in the J=1-0 line of C(13)O. Further observations include an unsuccessful search for CS (J=2-1) at AT&T BL, and some C(12)O J=2-1 spectra obtained at the Millimeter Wave Observatory of the University of Texas.

Bally, J.↗
NASA NTRS1986

Pointing calibration of the MKIVA DSN antennas Voyager 2 Uranus encounter operations support

The MKIVA DSN introduced significant changes to the pointing systems of the 34-meter and 64-meter diameter antennas. To support the Voyager 2 Uranus Encounter, the systems had to be accurately calibrated. Reliable techniques for use of the calibrations during intense mission support activity had to be provided. This article describes the techniques used to make the antenna pointing calibrations and to demonstrate their operational use. The results of the calibrations are summarized.

Stevens, R.↗
NASA NTRS1986

The Voyager encounter with Uranus and Neptune

Voyager 2 approaches Uranus at a relative low phase angle and high southerly latitude. Only when the spacecraft is very close to Uranus does the geometry change appreciably. Most of the important observations occur within six hours of closest approach. Voyager flies through an Earth and solar occulation zone and leaves Uranus at a relatively high phase angle of about 145 degrees. There isn't much of an opportunity to look at the equatorial region of the planet. At Neptune, on the other hand, the approach is more nearly equatorial (about 35 deg S lat). Voyager 2 will come much closer to Nepture than to any of the other gas giants as it skims within about 2000 km of Neptune's cloudtops. It will pass through earth and solar occultation zones at both Neptune and its satellite, Triton. Again, Voyager 2 will leave Neptune at about 35 deg S latitude. Voyager operational instrument, interplanetary trajectories and planetary encounters are briefly discussed.

Miner, Ellis D.↗
NASA NTRS1986

Ultraviolet spectrophotometry of comet Giacobini-Zinner during the ICE encounter

The IUE spectrophotometry of Comet P/Giacobini-Zinner was acquired in support of the International Cometary Explorer (ICE) mission. The abundances (or upper limits) of UV-active species were calculated. During the ICE encounter the H2O production rate was 3 times 10 to the 28th power/sec, + or - 50%, consistent with values derived from the ICE experiments. Comparison of the abundance of CO2(+) ions with the total electron density measured by the plasma electron experiment on ICE indicates a deficiency of ions relative to electrons indicating a population of ions not detected by remote sensing. The absence of detectable Mg(+) rules out this species as a possible ion of M/Q = 24 detected by the Ion Composition Instrument, part of the ICE complement of instruments.

Ahearn, Michael F.↗
NASA NTRS1986

The variability of Halley's Comet during the Vega, Planet-A, and Giotto encounters

Narrowband photometry of Halley obtained at Cerro Tololo Inter-American Observatory (CTIO), Mauna Kea Observatory (MKO), and Perth Observatory was combined to determine the relative level of activity during the interval spanning the spacecraft encounters. Measurements of the flux from the comet in emission bands of OH, NH, CN, C3, and C2, as well as at 2 continuum points, were obtained at CTIO on each night between 5 March and 17 March 1986. Observations were made on many of these same dates at MKO and Perth using comparable interference filters. The date clearly show variation of a factor of 2.5 in the production of all observed species with a characteristic time scale of a few days.

Schleicher, D. G.↗
NASA NTRS1986

Mass distribution of particulates measured by Giotto's Dust Impact Detection System (DIDSY) in the close encounter period

The cumulative mass distribution of the comet Halley dust efflux in the close encounter period -300 to +300 sec was analyzed. Analysis of the cumulative mass distribution index (alpha) shows considerable short time scale variation. There is clear evidence from the flux rates of passage through at least one major dust enhancement (dust jet) just after closest approach and this is associated with a steepening of the alpha. Comparison with measurements from other sources, and with preencounter predictions, is also made.

Zarnecki, J. C.↗
NASA NTRS1986

Measurements of thermal ion energy spectra from the Giotto encounter with Comet Halley

Ram ion measurements made by the Giotto neutral mass spectrometer show the spatial evolution of the composition and energy spectrum of coma ions. Near the nucleus the plasma is cold and has many constituents, but at distances of the order of 200,000 km the main ions are the terminal products of water dissociation, H(+) and O(+). The ion velocity distribution at the time of the Giotto encounter is highly anisotropic. Ion flow is generally away from the comet with energies increasing with distance to the order of 100 eV in the comet frame of reference. Similarities in the proton and oxygen ion energy spectra in the comet frame suggest an acceleration mechanism other than v x B pickup.

Hodges, R. R.↗