Initiation of spherical detonation in hydrocarbon/air mixtures

Spherical detonations have been initiated by solid explosive (Tetryl) charges in well-mixed stoicheiometric air mixtures with each of the hydrocarbons, ethane, propane, n-butane, isobutane and ethylene at atmospheric pressure. Prior to initiation, the gases were contained in plastic bags; total gas volume and available path length were up to 1.6 m³ and 2 m, respectively. The detonations were shown to be self-sustained by continuous measurement of detonation velocity using X-band microwave interferometry. Measured detonation velocities were in all cases close to calculated C-J values.In a few experiments close to the limits of detonability, velocity and blast pressure/time records indicated that the propagating wave system is sometimes irregular. The irregularity that occurs just after initiation is characterised by a reaction front velocity very much lower than the constant detonation velocity, but subsequently attaining the latter by an acceleration process. These observations indicate the existence of a dissociated phase in which shock and reaction fronts may no longer be coupled.Because similar experimental conditions were used throughout, it was possible to establish the relative susceptibilities of the various fuel gases to detonation. Comparison is made with the Zeldovich criterion and a detonation kernel theory of Lee.     

Voyager photopolarimeter observations of Saturn and Titan

The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 Å and in the near-IR at 7500 Å. Measurements of Saturn's limb brightening and polarization at several phase angles up to 70° indicate that a significant optical depth of UV absorbers are present in the top 100 mbar of Saturn's atmosphere in the Equatorial Zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation.The whole-body polarization of Titan is strongly positive in both the UV and near IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith [1], characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties.     

Lunar Reconnaissance Orbiter Overview: The Instrument Suite and Mission

NASA’s Lunar Precursor Robotic Program (LPRP), formulated in response to the President’s Vision for Space Exploration, will execute a series of robotic missions that will pave the way for eventual permanent human presence on the Moon. The Lunar Reconnaissance Orbiter (LRO) is first in this series of LPRP missions, and plans to launch in October of 2008 for at least one year of operation. LRO will employ six individual instruments to produce accurate maps and high-resolution images of future landing sites, to assess potential lunar resources, and to characterize the radiation environment. LRO will also test the feasibility of one advanced technology demonstration package. The LRO payload includes: Lunar Orbiter Laser Altimeter (LOLA) which will determine the global topography of the lunar surface at high resolution, measure landing site slopes, surface roughness, and search for possible polar surface ice in shadowed regions, Lunar Reconnaissance Orbiter Camera (LROC) which will acquire targeted narrow angle images of the lunar surface capable of resolving meter-scale features to support landing site selection, as well as wide-angle images to characterize polar illumination conditions and to identify potential resources, Lunar Exploration Neutron Detector (LEND) which will map the flux of neutrons from the lunar surface to search for evidence of water ice, and will provide space radiation environment measurements that may be useful for future human exploration, Diviner Lunar Radiometer Experiment (DLRE) which will chart the temperature of the entire lunar surface at approximately 300 meter horizontal resolution to identify cold-traps and potential ice deposits, Lyman-Alpha Mapping Project (LAMP) which will map the entire lunar surface in the far ultraviolet. LAMP will search for surface ice and frost in the polar regions and provide images of permanently shadowed regions illuminated only by starlight. Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which will investigate the effect of galactic cosmic rays on tissue-equivalent plastics as a constraint on models of biological response to background space radiation. The technology demonstration is an advanced radar (mini-RF) that will demonstrate X- and S-band radar imaging and interferometry using light weight synthetic aperture radar. This paper will give an introduction to each of these instruments and an overview of their objectives.     

Tailored analyses of 24 Galactic WN stars

The fundamental properties of 24 Galactic WN stars are determined from analyses of their optical, UV and IR spectra using sophisticated model atmosphere codes (Hillier, 1987, 1990). Terminal velocities, stellar luminosities, temperatures, mass loss rates and abundances of hydrogen, helium, carbon, nitrogen and oxygen are determined. Stellar parameters are derived using diagnostic lines and interstellar reddenings found from fitting theoretical continua to observed energy distributions.Our results confirm that the parameters of WN stars span a large range in temperature (T_*=30–90,000 K), luminosity (log L_*/L=4.8–5.9), mass loss (M=0.9–12×10^–5 M yr^–1) and terminal velocity (v =630–3300 km s^–1). Hydrogen abundances are determined, and found to be low in WNEw and WNEs stars (<15% by mass) and considerable in most WNL stars (1–50%). Metal abundances are also determined with the nitrogen content found to lie in the range N/He=1–5×10^–3 (by number) for all subtypes, and C/N 0.02 in broad agreement with the predictions of Maeder (1991). Enhanced O/N and O/C is found for HD 104994 (WN3p) suggesting a peculiar evolutionary history. Our results suggest that single WNL+abs stars may represent an evolutionary stage immediately after the Of phase. Since some WNE stars exist with non-negligible hydrogen contents (e.g. WR136) evolution may proceed directly from WNL+abs to WNE in some cases, circumventing the luminous blue variable (LBV) or red supergiant (RSG) stage.     

Toward optimizing lighting as a countermeasure to sleep and circadian disruption in space flight

Light is being used as a pre-launch countermeasure to circadian and sleep disruption in astronauts. The effect of light on the circadian system is readily monitored by measurement of plasma melatonin. Our group has established an action spectrum for human melatonin regulation and determined the region of 446-477 nm to be the most potent for suppressing plasma melatonin. The aim of this study was to compare the efficacy of 460 and 555 nm for suppressing melatonin using a within-subjects design. Subjects (N=12) were exposed to equal photon densities (7.18 x 10(12) photons/cm2/s) at 460 and 555 nm. Melatonin suppression was significantly stronger at 460 nm (p<0.02). An extension to the action spectrum showed that 420 nm light at 16 and 32 microW/cm2 significantly suppressed melatonin (p<0.04 and p<0.002). These studies will help optimize lighting countermeasures to circadian and sleep disruption during spaceflight.     

Autonomous Landing Guidance Systems

All weather tactical aircraft recovery and high sortie generation rates from forward, possibly battle damaged landing areas will reqire autonomous landing guidance systems which are independent of ground-based cooperative aids. A recently completed study has examined the operational requirements and assessed current and near term technology for an answer to this need. The Landing Systems Requirements/Synthesis Study has defined the Pilot/vehicle Interface and imaging sensor suite required, based on the concept of augmenting pilot forward vision with sensor imagery and guidance symbology on a HUD during low visibility landings. Image processing technology was also assessed for potential enhancement of the information presented to the pilot. This paper summarizes the final report of the Study, which is in publication.     

A transmission electron microscopy study of silica and kerogen biosignatures in approximately 1.9 Ga gunflint microfossils

Microfossils preserved in chert from the;1.9 Ga Gunflint Formation (Schreiber Beach, Ontario, Canada) were studied with transmission electron microscopy (TEM) and analytical TEM (ATEM). Our goals were to uncover the style of silicification relative to the distribution of organic matter, and to evaluate the distribution and evolution of organic matter, at submicroscopic spatial scales. Petrographically the microfossils typically display filamentous or coccoidal morphologies, and consist of quartz crystals surrounded by kerogen along grain boundaries. ATEM analysis revealed that quartz associated with kerogen consists of 200-500nm-sized, round crystallites, whereas the chert matrix is comprised of randomly oriented, polygonal microquartz (5-10 microm). Silica spheroids found within some fossils consist of quartz subgrains in an amorphous to poorly crystalline matrix, suggesting that precipitation of opaline silica on organic matter occurred with subsequent but incomplete transformation to quartz. Some coccoidal microfossils surround large euhedral quartz crystals (up to 5 microm in diameter) that appeared to have influenced the distribution of kerogen during crystal growth. These euhedral quartz crystals commonly contain elongated (50-100 nm) iron-rich crystallites. Energy-loss, near-edge structure analysis of kerogen associated with a coccoidal microfossil showed that it is composed of amorphous carbon with no evidence of graphitization. TEM results revealed significant differences in the style of silicification between microbe-shaped microfossils and their surrounding chert matrix, as well as the presence of amorphous kerogen.     

The Swift Ultra-Violet/Optical Telescope

The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (∼1 min) UV and optical photons from the afterglow of gamma-ray bursts in the 170–600 nm band as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey–Chrétien design with micro-channel plate intensified charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.     

Characterizing the near-earth object hazard and its mitigation

Within observational constraints and analytic orbit determinations, potential NEO hazards and mitigations are characterized in terms of orbit displacements to establish (arbitrary) “safe” closest approach distances and corresponding energies that must be externally applied to achieve appropriate orbit displacements from the Earth. Required orbital velocity changes depend on projected closest Earth approach distances and time to (near) impact. Energy to achieve orbital displacement depends on NEO mass, required orbital velocity change, and the energy–momentum coupling coefficient. Errors in these parameters introduce uncertainties into hazard index and mitigation procedures. Hazard avoidance levels and mitigation indices for nine near-Earth asteroids, including 1997 XF₁₁ and 1999 AN₁₀, with non-zero Earth-impact probabilities are computed as examples of the proposed methodology, generating insight into the dilemma of predicting near impacts. This zeroth order approximation should not be construed as solving an orbital mechanics problem, nor establishing a particular set of criteria for mitigation action, but rather as a “survival index”.