Nitrogen Buffer Gas Experiments in Mercury Trapped Ion Frequency Standards
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atomic clock nitrogen gas Mercury↗
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Gravitational and radiative effects on the escape of helium from the moon
On the moon, and probably on Mercury and other similar regolith-covered bodies with tenuous atmosphere, the dominant gas is He-4. It arises as the radiogenic product of the decay of uranium and thorium within any planet, but its major source appears to be the alpha particle flux of the solar wind. The moon intercepts solar wind helium at an average rate of 1.1 times 10 to the 24th atom/sec, and loses it at the same rate. Some helium may escape directly as the result of the process of solar wind soil bombardment which may release previously trapped helium at superthermal speeds. Atmospheric models have been calculated with the total helium influx as source. Subsequent comparison of model and measured helium concentrations indicates that the fraction of helium escaping via the atmosphere may range from 20% to 100% of the solar wind influx. Of the escaping atmosphere, most of the helium (about 93%) becomes trapped in earth orbit, while about 5% gets trapped in satellite orbits about the moon. Owing to a 6 month lifetime for helium in solar radiation, the satellite atoms form a lunar corona that exceeds the lunar atmosphere in total abundance by a factor of 4 to 5.
Hodges, R. R., Jr.↗
Measurement of heavy cosmic-ray nuclei on LDEF (M0002-2)
The objective of this experiment is to measure the elemental and isotopic abundamces of heavy cosmic-ray nuclei with nuclear charge Z equal to or greater than 3. The chemical and energy spectra will be measured for particles that have energies in the range from 20 to 1000 MeV per atomic mass unit. Two points of great interest are geomagneticaly forbidden cosmic-ray particles and heavy ions of the trapped radiation.
Beaujean, R.↗
Progress in Spacecraft Environment Interactions: International Space Station (ISS) Development and Operations
The set of spacecraft interactions with the space flight environment that have produced the largest impacts on the design, verification, and operation of the International Space Station (ISS) Program during the May 2000 to May 2007 time frame are the focus of this paper. In-flight data, flight crew observations, and the results of ground-based test and analysis directly supporting programmatic and operational decision-making are reported as are the analysis and simulation efforts that have led to new knowledge and capabilities supporting current and future space explorations programs. The specific spacecraft-environment interactions that have had the greatest impact on ISS Program activities during the first several years of flight are: 1) spacecraft charging, 2) micrometeoroids and orbital debris effects, 3) ionizing radiation (both total dose to materials and single event effects [SEE] on avionics), 4) hypergolic rocket engine plume impingement effects, 5) venting/dumping of liquids, 6) spacecraft contamination effects, 7) neutral atmosphere and atomic oxygen effects, 8) satellite drag effects, and 9) solar ultraviolet effects. Orbital inclination (51.6deg) and altitude (nominally between 350 km and 460 km) determine the set of natural environment factors affecting the performance and reliability of materials and systems on ISS. ISS operates in the F2 region of Earth s ionosphere in well-defined fluxes of atomic oxygen, other ionospheric plasma species, solar UV, VUV, and x-ray radiation as well as galactic cosmic rays, trapped radiation, and solar cosmic rays. The micrometeoroid and orbital debris environment is an important determinant of spacecraft design and operations in any orbital inclination. The induced environment results from ISS interactions with the natural environment as well as environmental factors produced by ISS itself and visiting vehicles. Examples include ram-wake effects, hypergolic thruster plume impingement, materials out-gassing, venting and dumping of fluids, and specific photovoltaic (PV) power system interactions with the ionospheric plasma. Vehicle size (L) and velocity (v), combined with the magnitude and direction of the geomagnetic field (B) produce operationally significant magnetic induction voltages (VxB.L) in ISS conducting structure during high latitude flight (>+/- 45deg) during each orbit. In addition, ISS is a large vehicle and produces a deep wake structure from which both ionospheric plasma and neutrals species are largely excluded. ISS must fly in a very limited number of approved flight attitudes, so that exposure of a particular material or system to environmental factors depends upon: 1) location on ISS, 2) ISS flight configuration, 3) ISS flight attitude, and 4) variation of solar exposure (Beta angle), and hence thermal environment, with time. Finally, an induced ionizing radiation environment is produced by trapped radiation and solar/cosmic ray interactions with the relatively massive ISS structural shielding.
Koontz, Steve↗
The anomalous nuclear component in the three-dimensional heliosphere
The anomalous nuclear component is neither of solar nor galactic cosmic ray origin. Its presence in the heliosphere is an independent probe for both interplanetary electrodynamical investigations--especially solar modulation--and probably the most direct means for determining the elemental and isotopic composition of those neutral atoms in the local interstellar medium that have high first ionization potentials (e.g., He, N, O, Ne, Ar, etc.). This report is a brief account of the evolution of our knowledge of this component for readers not specializing in the subject. Included are the initial discoveries of the component, its composition, spectra, heliospheric radial and latitudinal intensity gradients, modulation over the approximately 22 year solar magnetic cycle, trapping in the magnetosphere and its use for estimating the location of a heliospheric termination shock. Recent measurements from the ULYSSES spacecraft have provided conclusive evidence that incoming neutral atoms, after photoionization by solar uv, are picked up by the solar wind, thus lending support for the concept that after their acceleration--probably at a termination shock--they return to the inner heliosphere as pseudo-cosmic rays. ULYSSES spacecraft investigations extending to approximately 56 deg south latitude reveal, for both the anomalous nuclear component and the galactic cosmic rays, that there is a surprisingly small latitudinal intensity gradient. Thus, for the current phase of the solar cycle, modulation is much more spherically symmetric in the inner solar system than had generally been believed. A further surprise is the continual presence of approximately 26 day recurrent modulation at hight latitudes, without corresponding magnetic field compressions. These results are changing our ideas and requiring modification of our models for solar modulation in three dimensions.
Simpson, J. A.↗
High Stability Frequency Standards Based on (sup 1999)Hg+Ions in a Linear Ion Trap
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Microwave-Optical Mercury ions helium buffer gas a↗
High resolution threshold photoelectron spectroscopy by electron attachment
A system is provided for determining the stable energy levels of a species ion, of an atomic, molecular, or radical type, by application of ionizing energy of a predetermined level, such as through photoionization. The system adds a trapping gas to the gaseous species to provide a technique for detection of the energy levels. The electrons emitted from ionized species are captured by the trapping gas, only if the electrons have substantially zero kinetic energy. If the electrons have nearly zero energy, they are absorbed by the trapping gas to produce negative ions of the trapping gas that can be detected by a mass spectrometer. The applied energies (i.e. light frequencies) at which large quantities of trapping gas ions are detected, are the stable energy levels of the positive ion of the species. SF6 and CFCl3 have the narrowest acceptance bands, so that when they are used as the trapping gas, they bind electrons only when the electrons have very close to zero kinetic energy.
Chutjian, A.↗
Method and apparatus for producing microshells
A method is described for forming hollow particles, or shells, of extremely small size. The shell material is heated to a molten temperature in the presence of a gas that is at least moderately soluble in the shell material, to form a solution of the molten shell material and the soluble gas. The solution is atomized to form a multiplicity of separate droplets that are cooled while in free fall. Cooling of a droplet from the outside traps the desolved gas and forces it to form a gas bubble at the center of the droplet which now forms a gas filled shell. The shell is reheated and then cooled in free fall, in an environment having a lower pressure than the gas pressure in the shell. This causes expansion of the shell and the formation of a shell having a small wall thickness compared to its diameter.
Lee, Mark C.↗
The Interstellar Gas Experiment: Analysis in progress
The Interstellar Gas Experiment (IGE) exposed thin metallic foils aboard the LDEF spacecraft in low Earth orbit in order to collect neutral interstellar particles which penetrate the solar system due to their motion relative to the sun. By mechanical penetration these atoms were imbedded in the collecting foils along with precipitating magnetospheric ions and, possibly, with ambient atmospheric atoms. During the entire LDEF mission, seven of these foils collected particles arriving from seven different directions as seen from the spacecraft. After the foils were returned to Earth, a mass spectrometric analysis of the noble gas component of the trapped particles was begun. The isotopes of He-3, He-4, Ne-20, and Ne-22 were detected. We have given a first account of the experiment. In order to infer the isotopic ratios in the interstellar medium from the concentrations found in the foils, several lines of investigation had to be initiated. The flux of ambient atmospheric noble gas atoms moving toward the foils due to the orbital motion of LDEF was estimated by detailed calculations. Any of these particles which evaded the baffles in the IGE collector could be entrapped in the foils as a background flux. However, the calculations have shown that this flux is negligible, which was the intent of the experiment hardware design. This conclusion is supported by the measurements. However, both the concentration of trapped helium and its impact energy indicate that the flux of magnetospheric ions which was captured was larger than had been expected. In fact, it appears that the magnetospheric particles constitute the largest fraction of the particles in the foils. Since little is known about this particle flux, their presence in the IGE foils appears fortunate. The analysis of these particles provides information about their isotropic composition and average flux.
Buehler, F.↗
The effects of atomic oxygen on the thermal emittance of high temperature radiator surfaces
Radiator surfaces on high temperature space power systems such as SP-100 space nuclear power system must maintain a high emittance level in order to reject waste heat effectively. One of the primary materials under consideration for the radiators is carbon-carbon composite. Since carbon is susceptible to attack by atomic oxygen in the low earth orbital environment, it is important to determine the durability of carbon composites in this environment as well as the effect atomic oxygen has on the thermal emittance of the surface if it is to be considered for use as a radiator. Results indicate that the thermal emittance of carbon-carbon composite (as low as 0.42) can be enhanced by exposure to a directed beam of atomic oxygen to levels above 0.85 at 800 K. This emittance enhancement is due to a change in the surface morphology as a result of oxidation. High aspect ratio cones are formed on the surface which allow more efficient trapping of incident radiation. Erosion of the surface due to oxidation is similar to that for carbon, so that at altitudes less than approximately 600 km, thickness loss of the radiator could be significant (as much as 0.1 cm/year). A protective coating or oxidation barrier forming additive may be needed to prevent atomic oxygen attack after the initial high emittance surface is formed. Textured surfaces can be formed in ground based facilities or possibly in space if emittance is not sensitive to the orientation of the atomic oxygen arrival that forms the texture.
Rutledge, Sharon K.↗
High-resolution threshold photoelectron spectroscopy by electron attachment
A new technique for measuring high-resolution threshold photoelectron spectra of atoms, molecules, and radicals is described. It involves photoionization of a gaseous species, attachment of the threshold, or nearly zero electron to some trapping molecule (here SF6 or CFCl3), and mass detection of the attachment product (SF6/-/ or Cl/-/ respectively). This technique of threshold photoelectron spectroscopy by electron attachment was used to measure the spectra of argon and xenon at 11 meV (FWHM) resolution, and was also applied to CFCl3.
Ajello, J. M.↗
Radiation damage in scientific charge-coupled devices
Radiation damage is reported on CCDs (charge-coupled devices) that have been primarily exposed to 1.25-MeV gamma rays (Co-60 source). Two important classes of radiation damage are discussed, namely, bulk and ionization effects. Bulk damage or displacement damage is a process in which silicon atoms are displaced from their normal lattice positions by high-energy photons or particles. Single atomic displacements or cluster defect damage is produced, depending on the energy and type of radiation experienced by the detector. Bulk damage creates trapping sites within the CCD's signal channel which in turn degrades charge-transfer efficiency.
Janesick, James↗
Molecular hydrogen formation by excited atom radiative association
The results from a semiclassical calculation of the thermal rate coefficient for the radiative association process H(n = 2) + H(n = 1) - H2 + hv are presented (n is the principal quantum number of the separated hydrogen atoms). The relative importance of this reaction in various environments is briefly discussed. Models of the early universe around the epoch of recombination and protostellar winds have been calculated which include the excited atom process. Not surprisingly, it is shown that the excited atom process will not be important in the general interstellar medium, except possibly in environments where the amount of Ly-alpha photon trapping is large. Examples may be the material surrounding quasars, active galactic nuclei, and bright H II regions. The most likely application of this process might be within rapidly evolving systems where a large transient n = 2 population of neutral hydrogen could result in a burst of molecular hydrogen formation.
Latter, William B.↗
Spacecraft Materials in the Space Flight Environment: International Space Station - May 2002 to May 2007
The performance of ISS spacecraft materials and systems on prolonged exposure to the low-Earth orbit (LEO) space flight is reported in this paper. In-flight data, flight crew observations, and the results of ground-based test and analysis directly supporting programmatic and operational decision-making are presented. The space flight environments definitions (both natural and induced) used for ISS design, material selection, and verification testing are shown, in most cases, to be more severe than the actual flight environment accounting for the outstanding performance of ISS as a long mission duration spacecraft. No significant ISS material or system failures have been attributed to spacecraft-environments interactions. Nonetheless, ISS materials and systems performance data is contributing to our understanding of spacecraft material interactions in the spaceflight environment so as to reduce cost and risk for future spaceflight projects and programs. Orbital inclination (51.6o) and altitude (nominally near 360 km) determine the set of natural environment factors affecting the functional life of materials and systems on ISS. ISS operates in an electrically conducting environment (the F2 region of Earth s ionosphere) with well-defined fluxes of atomic oxygen, other charged and neutral ionospheric plasma species, solar UV, VUV, and x-ray radiation as well as galactic cosmic rays, trapped radiation, and solar cosmic rays (1-4). The LEO micrometeoroid and orbital debris environment is an especially important determinant of spacecraft design and operations (5, 6). The magnitude of several environmental factors varies dramatically with latitude and longitude as ISS orbits the Earth (1-4). The high latitude orbital environment also exposes ISS to higher fluences of trapped energetic electrons, auroral electrons, solar cosmic rays, and galactic cosmic rays (1-4) than would be the case in lower inclination orbits, largely as a result of the overall shape and magnitude of the geomagnetic field (1-4). As a result, ISS exposure to many environmental factors can vary dramatically along a particular orbital ground track, and from one ground track to the next, during any 24-hour period.
Golden, John↗
Environmental Effects on ISS Materials Aging (1998 to 2008)
The performance of ISS spacecraft materials and systems on prolonged exposure to the low- Earth orbit (LEO) space flight are reported in this paper. In-flight data, flight crew observations, and the results of ground-based test and analysis directly supporting programmatic and operational decision-making are described. The space flight environments definitions (both natural and induced) used for ISS design, material selection, and verification testing are shown, in most cases, to be more severe than the actual flight environment accounting, in part, for the outstanding performance of ISS as a long mission duration spacecraft. No significant ISS material or system failures have been attributed to spacecraft-environments interactions. Nonetheless, ISS materials and systems performance data is contributing to our understanding of spacecraft material interactions with the spaceflight environment so as to reduce cost and risk for future spaceflight projects and programs. Orbital inclination (51.6 deg) and altitude (nominally near 360 km) determine the set of natural environment factors affecting the functional life of materials and systems on ISS. ISS operates in an electrically conducting environment (the F2 region of Earth s ionosphere) with well-defined fluxes of atomic oxygen, other charged and neutral ionospheric plasma species, solar UV, VUV, and x-ray radiation as well as galactic cosmic rays, trapped radiation, and solar cosmic rays. The LEO micrometeoroid and orbital debris environment is an especially important determinant of spacecraft design and operations. The magnitude of several environmental factors varies dramatically with latitude and longitude as ISS orbits the Earth. The high latitude orbital environment also exposes ISS to higher fluences of trapped energetic electrons, auroral electrons, solar cosmic rays, and galactic cosmic rays than would be the case in lower inclination orbits, largely as a result of the overall shape and magnitude of the geomagnetic field. As a result, ISS exposure to many environmental factors can vary dramatically along a particular orbital ground track, and from one ground track to the next, during any 24-hour period. The induced environment results from ISS interactions with the natural environment as well as environmental factors produced by ISS itself and visiting vehicles fleet. Examples include ram-wake effects, hypergolic thruster plume impingement, materials out-gassing, venting and dumping of fluids, and specific photovoltaic (PV) power system interactions with the ionospheric plasma (7-11). Vehicle size (L) and velocity (V), combined with the magnitude and direction of the geomagnetic field (B) produce operationally significant magnetic induction voltages (VxB.L) in ISS conducting structure during flight through high latitudes (> +45deg) during each orbit. Finally, an induced ionizing radiation environment is produced by cosmic ray interaction with the relatively thick ISS structure and shielding materials. The intent of this review article is, therefore, to provide a summary of selected aspects and elements of the ISS vehicle with regard to LEO space environment effects, associated with the much larger and more complicated vehicle that ISS has become since 1998, but also with an eye towards performance life extension to the year 2016 and beyond.
Alred, John↗
Frequency standards from industry over the next 25 years
Present and possible future performance for many of the existing and new commercial frequency standards is presented here. Recent progress in the gas cell atomic standards with regards to size and cost is significant and considerable improvement is expected. Cesium beam standards will benefit in stability and accuracy from optical pumping. Cooled hydrogen masers will offer extremely good stability. Advances in trapped ion and cesium fountain technologies make them good high performance candidates for the future. The quartz oscillator field is more mature and consequently performance improvements for the future are going to be less spectacular. Oscillators stabilized to GPS will have many applications. Recent performance of cooled microwave dielectric resonator oscillators is very good and they offer the promise of serving as flywheel oscillators for advanced performance atomic standards.
Cutler, Leonard S.↗
Tholins: Can They Provide a Substrate, Carbon and Nitrogen for Plant Production?
Tholin is a word coined to describe the entire class of complex organic solids produced in laboratory experiments where pre-biotic gaseous chemicals are subject to bombardment by high energy. The atomic composition of Titan tholin produced from 10 percent CH4 and 90 percent N2 in a simulation of Titan atmosphere irradiated by charged particles trapped in the magnetosphere of Saturn gave 67 percent C and 33 percent N. Hydrolysis of Titan tholin with 6N HCl produced a racemic mixture of biological and non-biological amino acids that was confirmed by GC/MS. Other tholins, that revealed the presence of amino acids, were UV tholin produced under possible primitive Earth conditions by irradiation of a mixture of gases (CH4, C2H6, NH3, H2S and liquid H2O) with long-wavelength ultraviolet light, representing the most abundant useful energy source for prebiological organic synthesis; Spark tholin in a crude simulation of Jupiter atmosphere using electrical discharge through a mixture of CH4, NH3, and H2O vapor. Pyrolytic GC/MS of Titan tholin produced more than one hundred organic compounds including saturated and unsaturated aliphatic hydrocarbons, substituted polycyclic aromatics, nitriles, amines, pyrroles, pyrazines, pyridines, pyrimidines, and the purine, adenine. Similar rich pyrolytic products were obtained with UV as well as Spark tholins. A range of two to four ring PAHs (Polycyclic Aromatic Hydrocarbons) in Spark as well as Titan tholins, some with one to four alkylation sites, were identified by two-step laser desorption/multiphoton ionization mass spectrometry and also confirmed by the synchronous fluorescence technique. Previous studies have demonstrated the potential for use of tholins as a source of carbon and energy by microbes. This paper describes studies that evaluate the potential for using different types of tholins as (a) a substrate for growing plants and (b) a source of carbon and nitrogen for plants. The data are interpreted in terms of the potential for using such tholins to grow plants for food in extraterrestrial habitats and also to speculate on the possibilities of abiotic evolution of plants.
Wignarajah, Kanapathipillai↗
A Radiation Shielding Code for Spacecraft and Its Validation
The HZETRN code, which uses a deterministic approach pioneered at NASA Langley Research Center, has been developed over the past decade to evaluate the local radiation fields within sensitive materials (electronic devices and human tissue) on spacecraft in the space environment. The code describes the interactions of shield materials with the incident galactic cosmic rays, trapped protons, or energetic protons from solar particle events in free space and low Earth orbit. The content of incident radiations is modified by atomic and nuclear reactions with the spacecraft and radiation shield materials. High-energy heavy ions are fragmented into less massive reaction products, and reaction products are produced by direct knockout of shield constituents or from de-excitation products. An overview of the computational procedures and database which describe these interactions is given. Validation of the code with recent Monte Carlo benchmarks, and laboratory and flight measurement is also included.
Shinn, J. L.↗