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Johnson, P.

Publications and source records attributed to Johnson, P..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Low Frequency Radar Sensor Observations of Tropical Forests in the Panama Canal Area

A synthetic aperture radar sensor operating in 5 bands between 80 and 120 MHz was flown over forested areas in the canal zone of the Republic of Panama in an experiment to measure biomass in heavy tropical forests. The sensor is a pulse coherent SAR flown on a small aircraft and oriented straight down. The doppler history is processed to collect data on the ground in rectangular cells of varying size over a range of incidence angles fore and aft of nadir (+45 to - 45 degrees). Sensor data consists of 5 frequency bands with 20 incidence angles per band. Sensor data for over 12+ sites were collected with forest stands having biomass densities ranging from 50 to 300 tons/ha dry above ground biomass. Results are shown exploring the biomass saturation thresholds using these frequencies, the system design is explained, and preliminary attempts at data visualization using this unique sensor design are described.

Imhoff, M. L.↗
NASA NTRS1988

Booster propulsion/vehicle impact study, 2

This is the final report in a study examining the impact of launch vehicles for various boost propulsion design options. These options included: differing boost phase engines using different combinations of fuels and coolants to include RP-1, methane, propane (subcooled and normal boiling point), and hydrogen; variable and high mixture ratio hydrogen engines; translating nozzles on boost phase engines; and cross feeding propellants from the booster to second stage. Vehicles examined included a fully reusable two stage cargo vehicle and a single stage to orbit vehicle. The use of subcooled propane as a fuel generated vehicles with the lowest total vehicle dry mass. Engines with hydrogen cooling generated only slight mass reductions from the reference, all-hydrogen vehicle. Cross feeding propellants generated the most significant mass reductions from the reference two stage vehicle. The use of high mixture ratio or variable mixture ratio hydrogen engines in the boost phase of flight resulted in vehicles with total dry mass 20 percent greater than the reference hydrogen vehicle. Translating nozzles for boost phase engines generated a heavier vehicle. Also examined were the design impacts on the vehicle and ground support subsystems when subcooled propane is used as a fuel. The most significant cost difference between facilities to handle normal boiling point versus subcooled propane is 5 million dollars. Vehicle cost differences were negligible. A significant technical challenge exists for properly conditioning the vehicle propellant on the ground and in flight when subcooled propane is used as fuel.

Johnson, P.↗
NASA NTRS1988

Manned Mars Mission program concepts

This paper describes the SRS Manned Mars Mission and Program Analysis study designed to support a manned expedition to Mars contemplated by NASA for the purposes of initiating human exploration and eventual habitation of this planet. The capabilities of the interactive software package being presently developed by the SRS for the mission/program analysis are described, and it is shown that the interactive package can be used to investigate the impact of various mission concepts on the sensitivity of mass required in LEO, schedules, relative costs, and risk. The results, to date, indicate the need for an earth-to-orbit transportation system much larger than the present STS, reliable long-life support systems, and either advanced propulsion or aerobraking technology.

Hamilton, E. C.↗
NASA NTRS1986

Models of polarized infrared emission from bipolar nebulae

Many stars with circumstellar dust shells show a high degree of linear polarization (Sato et al. 1985). We are developing a model which assumes that the polarization arises from scattering by circumstellar dust. Our model assumes a geometry in which the star is surrounded by an optically thin spherical dust shell and embedded within an optically thick disk. This geometry is consistent with that proposed for objects with bipolar molecular outflow. This is important because many bipolar flow objects have also been observed to be highly polarized. The high degree of linear polarization is produced because the disk differentially attenuates the light from the star. The light incident from the point source is attenuated by a factor of exp(-tau/cos theta) where theta is the angle between a ray from the point source to the scatterer and a ray normal to the disk; tau is the optical depth at the wavelength of interest. Hence, the light scattered from the regions directly above and below the disk give the largest contribution to the total flux. The scattering angle for light from these regions is near 90 deg., so the light is strongly polarized and, in the Rayleigh scattering regime, is polarized parallel to the disk. The Stokes parameters for the scattered light from each particle in the shell are calculated by using the scattering matrix elements generated by a Mie scattering program. After the Stokes parameters for each particle are computed they are summed to give the Stokes parameters for the entire shell. Two graphs are presented which show the intensity and polarization spectrum generated by our model using the optical constants for astronomical silicates as defined by Draine and Lee (1984).

Burns, M. S.↗
NASA NTRS1986

Apollo 16 regolith breccias - Characterization and evidence for early formation in mega-regolith

The Apollo 16 regolith breccias were characterized in terms of petrography, grain-size distribution, porosity, major and trace element composition, noble gas contents, and ferromagnetic resonance properties. Significant variation was found with respect to density and porosity; the more dense breccias displayed substantial shock damage. The breccias resembled the soils in grain-size distribution and in petrological components, though many were found to be compositionally different from the Apollo 16 soils in that a mafic component was lacking. Nearly all breccias showed evidence of irradiation at the lunar surface, and analyses of disaggregated breccias indicated that this irradiation occurred before compaction. The concentration of surface irradiation parameters were far less than those of lunar soils or breccias of other Apollo missions. Observations with respect to the argon isotope ratio and Xe presence have led to the possibility that breccia-surface irradiation occurred as early as four billion years ago, and that most Apollo 16 regolith breccias were not formed from any known Apollo 16 soil.

Mckay, D. S.↗
NASA NTRS1985

Bone and muscle atrophy with suspension of the rat

In order to identify a suitable model for the study of muscle atrophy due to suspension in space, a modified version of the Morey tail suspension model was used to measure the atrophic responses of rat bone and muscle to 14-30 days of unloading of the hindlimbs. The progress of atrophy was measured by increases in methylene diphosphonate (MDP) uptake. It is found that bone uptake of methylene diphosphonate followed a phasic pattern similar to changes in the bone formation rate of immobilized dogs and cats. Increased MDP uptake after a period of 60 days indicated an accelerated bone metabolism. Maximum muscle atrophy in the suspended rats was distinctly different from immobilization atrophy. On the basis of the experimental results, it is concluded that the tail suspension model is an adequate simulation of bone atrophy due to suspension.

Leblanc, A.↗
NASA NTRS1984

In vivo nuclear magnetic resonance imaging

A number of physiological changes have been demonstrated in bone, muscle and blood after exposure of humans and animals to microgravity. Determining mechanisms and the development of effective countermeasures for long duration space missions is an important NASA goal. The advent of tomographic nuclear magnetic resonance imaging (NMR or MRI) gives NASA a way to greatly extend early studies of this phenomena in ways not previously possible; NMR is also noninvasive and safe. NMR provides both superb anatomical images for volume assessments of individual organs and quantification of chemical/physical changes induced in the examined tissues. The feasibility of NMR as a tool for human physiological research as it is affected by microgravity is demonstrated. The animal studies employed the rear limb suspended rat as a model of mucle atrophy that results from microgravity. And bedrest of normal male subjects was used to simulate the effects of microgravity on bone and muscle.

Leblanc, A.↗
NASA NTRS1984

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites

Three meteorites belonging to the rare group of SNC achondrites, which may have originated in the planet Mars, have been subjected to noble gas isotopic concentration measurements. The elemental and isotopic ratios obtained are unlike those for any other noble gas components except those obtained in analyses of the Martian atmosphere by Viking spacecraft. It is hypothesized that the Kr and Xe gases represent a portion of the Martian atmosphere which was shock-implanted in the case of Elephant Moraine A79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. If the SNC meteorites were ejected from Mars at the shergottite shock age of about 180 My ago, they must have been objects more than 6 m in diameter which experienced at least three space collisions to initiate cosmic ray exposure.

Bogard, D. D.↗
NASA NTRS1984

Cosmic ray exposure of SNC meteorites and constraints on their derivation from Mars

There is chemical, mineralogical, and isotopic evidence that several rare meteorites originated from the planet Mars. The eight specimens (4 sphergottites, 3 nakhlites, and 1 chassignite) possess a variety of characteristics that suggest a mutual relationship. It can be predicted with reasonable confidence that these SNC meteorites were exposed to cosmic rays at relatively shallow depths for three distinct periods of time during the past approximately 10 million years. Within the context of a Martian origin, this implies that either relatively large objects were ejected from Mars at a common time and underwent three disruptive events in space or that three ejection events occurred within the past approximately 10 million years on restricted segments of Martian terrain with 1300 million year-old rocks. If mechanisms can be identified which satisfy the geochemical properties of SNC meteorites, as well as their cosmic ray exposure, and dynamical problems associated with ejection out of a large gravity field, then the case for the SNC meteorites being samples of Mars can become stronger.

Bogard, D. D.↗
NASA NTRS1983

Trapped noble gases indicate lunar origin for Antarctic meteorite

The isotopic abundances of the noble gases (He, Ne, Ar, Kr, Xe) are reported for Antarctic ALHA 81005. It contains solar wind-implanted gases whose absolute and relative concentrations are quite similar to lunar regolith samples but not to other meteorites. ALHA 81005 also contains a large excess Ar-40 component which is identical to the component in lunar fines implanted from the lunar atmosphere. Large concentrations of cosmogenic Ne-21, Kr-82, and Xe-126 in ALHA 81005 indicate a total cosmic ray exposure age of at least 200 million years. The noble gas data alone are strong evidence for a lunar origin of this meteorite.

Bogard, D. D.↗
NASA NTRS1983

Martian gases in an Antarctic meteorite?

Significant abundances of trapped argon, krypton, and xenon have been measured in shock-altered phases of the achondritic meteorite Elephant Moraine 79001 from Antarctica. The relative elemental abundances, the high ratios of argon-40 to argon-36 (equal to or greater than 2000), and the high ratios of xenon-129 to xenon-132 (equal to or greater than 2.0) of the trapped gas more closely resemble Viking data for the Martian atmosphere than data for noble gas components typically found in meteorites. These findings support earlier suggestions, made on the basis of geochemical evidence, that shergottites and related rare meteorites may have originated from the planet Mars.

Bogard, D. D.↗
NASA NTRS1983

High resolution bone mineral densitometry with a gamma camera

A technique by which the regional distribution of bone mineral can be determined in bone samples from small animals is described. The technique employs an Anger camera interfaced to a medical computer. High resolution imaging is possible by producing magnified images of the bone samples. Regional densitometry of femurs from oophorectomised and bone mineral loss.

Leblanc, A.↗
NASA NTRS1982

The Apennine Front core 15007/8 - Irradiational and depositional history

The Apennine Front core 15007/8 was subjected to ferromagnetic resonance (FMR), magnetic and noble gas measurements. Ferromagnetic resonance intensity I(s)/FeO surface exposure (maturity) index values and FeO concentrations are reported for 0.5 cm depth intervals, and noble gas and FMR data are given for 13 soils and two clasts selected from various depths in the 57 cm-long core. These data indicate four core units whose contacts are at about 18, 49 and 55 cm. The 0-18 cm unit has cosmogenic Xe-131/Xe-126 values that are higher than underlying soil and indicate irradiation at depths greater than 50 cm, showing it to probably be the ejecta from the crater on whose rim the core was collected. The 18-49 cm unit has relatively constant I(s)/FeO values and cosmogenic gas abundances, and may represent a mixed soil zone. I(s)/FeO drops by 30 units at the 49-cm contact. Comparison of the 15007/8 core to other ones suggests that soils on slopes tend to be mixed and to not contain units with long in situ radiation profiles.

Bogard, D. D.↗