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The hydrogen coma of Comet P/Halley observed in Lyman-alpha using sounding rockets

Hydrogen Lyman-alpha (121.6 nm) images of Comet P/Halley were obtained using sounding rockets launched from White Sands Missile Range on 24.5 February and 13.5 March 1986. The second rocket was launched 13 hours before the fly-by of the Giotto spacecraft. An electrographic camera on both flights provided Lyman-alpha images covering a 20 field of view with 3 arcmin resolution. The data from both flights have been compared with a time-dependent model of hydrogen kinetics. To match the measured isophote contours, hydrogen sources with velocity components of 8 km/s and 20 km/s (from OH and H2O respectively) as well as a low velocity component (about 2 km/s) are required. This low velocity component is thought to result from thermalization of fast hydrogen atoms within the collision zone, providing an important diagnostic of temperature and density near the nucleus. Hydrogen production rates of 3.8 x 10 exp 30/s and 1.7 x 10 exp 30/s have been obtained for the two observations.

Mccoy, R. P.↗

Lunar horizon glow and the Clementine mission

The Clementine spacecraft is to be launched into Earth orbit in late January for subsequent insertion into lunar orbit in late February, 1994. There, its primary mission is to produce -- over a period of about two months -- a new photographic map of the entire surface of the Moon; this will be done, in a variety of wavelengths and spatial resolutions, in a manner greatly superior to that previously accomplished for the whole Moon. It will then go on to fly by and photograph the asteroid Geographos. A secondary goal that has been accepted for this mission is to take a series of photographs designed to capture images of, and determine the brightness and extent of, the Lunar Horizon Glow (LHG). One form of LHG is caused by the solar stimulation of emission from Na and K atoms in the lunar exosphere. The scale height of this exosphere is of the order of 100 km. There are also brighter LHG components, with much smaller scale heights, that appear to be caused by scattered off of an exospheric lunar dust cloud.

Zook, H. A.↗

Method and system for near-field spectroscopy using targeted deposition of nanoparticles

There is provided in one embodiment of the invention a method for analyzing a sample material using surface enhanced spectroscopy. The method comprises the steps of imaging the sample material with an atomic force microscope (AFM) to select an area of interest for analysis, depositing nanoparticles onto the area of interest with an AFM tip, illuminating the deposited nanoparticles with a spectrometer excitation beam, and disengaging the AFM tip and acquiring a localized surface enhanced spectrum. The method may further comprise the step of using the AFM tip to modulate the spectrometer excitation beam above the deposited nanoparticles to obtain improved sensitivity data and higher spatial resolution data from the sample material. The invention further comprises in one embodiment a system for analyzing a sample material using surface enhanced spectroscopy.

Anderson, Mark S.↗

Planetary and satellite x ray spectroscopy: A new window on solid-body composition by remote sensing

The rings and most of the satellites of the outer planets orbit within the radiation belts of their parent bodies. This is an environment with intense fluxes of energetic electrons. As a result, these objects are strong emitters of X-rays. The characteristic X-ray lines from these bodies depend on atomic composition, but they are not sensitive to how the material is arranged in compounds or mixtures. X-ray fluorescence spectral analysis has demonstrated its unique value in the laboratory as a qualitative and quantitative analysis tool. This technique has yet to be fully exploited in a planetary instrument for remote sensing. The characteristic X-ray emissions provide atomic relative abundances. These results are complementary to the molecular composition information obtained from IR, visible, and UV emission spectra. The atomic relative abundances are crucial to understanding the formation and evolution of these bodies. They are also crucial to the proper interpretation of the molecular composition results from the other sensors. The intensities of the characteristic X-ray emissions are sufficiently strong to be measured with an instrument of modest size. Recent developments in X-ray detector technologies and electronic miniaturization have made possible space-flight X-ray imaging and nonimaging spectrometers of high sensitivity and excellent energy resolution that are rugged enough to survive long-duration space missions. Depending on the application, such instruments are capable of resolving elemental abundances of elements from carbon through iron. At the same time, by measuring the bremsstrahlung intensity and energy spectrum, the characteristics of the source electron flux can be determined. We will discuss these concepts, including estimated source strengths, and will describe a small instrument capable of providing this unique channel of information for future planetary missions. We propose to build this instrument using innovative electronics packaging methods to minimize size and weight.

Chenette, D. L.↗

Empirical Modeling of the Storm Time Innermost Magnetosphere Using Van Allen Probes and THEMIS Data: Eastward and Banana Currents

The structure of storm time currents in the inner magnetosphere, including its innermost region inside 4R(sub E), is studied for the first time using a modification of the empirical geomagnetic field model TS07D and new data from Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms missions. It is shown that the model, which uses basis-function expansions instead of ad hoc current modules to approximate the magnetic field, consistently improves its resolution and magnetic field reconstruction with the increase of the number of basis functions and resolves the spatial structure and evolution of the innermost eastward current. This includes a connection between the westward ring current flowing largely at R > or approx. 3R(sub E) and the eastward ring current concentrated at R < or approx. 3R(sub E) resulting in a vortex current pattern. A similar pattern coined 'banana current' was previously inferred from the pressure distributions based on the energetic neutral atom imaging and first-principles ring current simulations. The morphology of the equatorial currents is dependent on storm phase. During the main phase, it is complex, with several asymmetries forming banana currents. Near SYM-H minimum, the banana current is strongest, is localized in the evening-midnight sector, and is more structured compared to the main phase. It then weakens during the recovery phase resulting in the equatorial currents to become mostly azimuthally symmetric.

Stephens, G. K.↗

Comments on the observability of coronal variations

The observable variability of spectral lines in the soft X-ray and XUV region is discussed. Rapid variability of coronal emission, both in flaring and non-flaring structures, has been reported and is particularly prominent when high spatial resolution is available. Examination of the ionization and recombination time-scales for the formation and removal of ions with prominent solar emission lines shows that, even though ionization equilibrium generally prevails, the observable variability time-scales are often limited by these atomic processes, independent of the physical process which is causing the change in the solar atmosphere. Future observations of Mg X 609 A should be augmented by simultaneous observation at another wavelength, such as 63 A. In addition, with the ability to produce images in isolated spectral lines it becomes possible to select those for which rapid variability is observable, such as O VII, rather than lines which were selected on the basis of previous hardware constraints, such as O VIII.

Golub, L.↗

Planning In-Flight Calibration for XRISM

The XRISM X-ray observatory will fly two advanced instruments, the Resolve high-resolution spectrometer and the Xtend wide-field imager. These instruments, particularly Resolve, pose calibration challenges due to the unprecedented combination of spectral resolution, spectral coverage, and effective area, combined with a need to characterize the imaging fidelity of the full instrument system to realize the mission’s ambitious science goals. We present the status of the XRISM in-flight calibration plan, building on lessons from Hitomi and other X-ray missions. We present a discussion of targets and observing strategies to address the needed calibration measurements, with a focus on developing methodologies to plan a thorough and flexible calibration campaign and provide insight on calibration systematic error. We also discuss observations that exploit Resolve’s spectral resolution to calibrate atomic codes, and cross-calibration between the XRISM instruments and with other observatories.

Eric D Miller↗

X-ray and gamma ray astronomy detectors

X-ray and gamma ray astronomy was made possible by the advent of space flight. Discovery and early observations of celestial x-rays and gamma rays, dating back almost 40 years, were first done with high altitude rockets, followed by Earth-orbiting satellites> once it became possible to carry detectors above the Earth's atmosphere, a new view of the universe in the high-energy part of the electromagnetic spectrum evolved. Many of the detector concepts used for x-ray and gamma ray astronomy were derived from radiation measuring instruments used in atomic physics, nuclear physics, and other fields. However, these instruments, when used in x-ray and gamma ray astronomy, have to meet unique and demanding requirements related to their operation in space and the need to detect and measure extremely weak radiation fluxes from celestial x-ray and gamma ray sources. Their design for x-ray and gamma ray astronomy has, therefore, become a rather specialized and rapidly advancing field in which improved sensitivity, higher energy and spatial resolution, wider spectral coverage, and enhanced imaging capabilities are all sought. This text is intended as an introduction to x-ray and gamma ray astronomy instruments. It provides an overview of detector design and technology and is aimed at scientists, engineers, and technical personnel and managers associated with this field. The discussion is limited to basic principles and design concepts and provides examples of applications in past, present, and future space flight missions.

Decher, Rudolf↗

Abundances, planetary nebulae, and stellar evolution

Among Henry Norris Russell's many achievements were his contributions to solar and stellar spectroscopy, in particular, to an analysis of the chemical composition of the solar atmosphere. The question of composition differences between stars was hotly debated; some distinguished astronomers argued that all stars had the solar composition. Some early challenges to this doctrine are described. Determinations of chemical compositions of gaseous nebulae were much more difficult. If we observe the lines of a given chemical element in one ionization stage in a stellar spectrum, we can deduce readily the abundance of that element. No such luxury is available for a planetary or diffuse gaseous nebula. We must measure lines of as many ionization stages as we can. Furthermore, a nebula is an extended object. Often detailed spectroscopy is at hand only for narrow pencil columns taken through the image. Different observers use a variety of apertures. Fortunately it is possible to calculate theoretical spectra for any arbitrary cross section taken through a symmetrical model, so UV, optical, and IR observations all can be compared properly with a prediction. The value of high-resolution spectra obtained with instruments such as the Hamilton Echelle Spectrograph at Lick Observatory is emphasized. Improved fluxes for weak but important transitions are found. Close blends of lines of different ions can be resolved, and checks can be made on predictions of atomic parameters such as Einstein A-values and collision strengths. High spectral resolution data have been obtained and reduced for 22 planetary nebulae of varying size, structure, stellar population membership, dustiness, level of excitation, evolutionary status, and chemical compositions. The promise seems justified that with such extensive, high quality data, additional insights on nebular genesis and late states of stellar evolution can be found. The present survey is confined to nebulae of high surface brightness, but among these are found some engaging objects such as NGC 7027, IC 4997, and NGC 6572.

Aller, Lawrence H.↗

Experimental and Theoretical Needs for the JWST Early Release Science Program on Radiative Feedback from Massive Stars (PDRs4ALL): III. A Quantum Chemistry Perspective

Massive stars disrupt their natal molecular cloud material by dissociating molecules, ionizing atoms and molecules, and heating the gas and dust. These processes drive the evolution of interstellar matter in our Galaxy and throughout the Universe from the era of vigorous star formation at z=1-3 to the present day. Much of this interaction occurs in Photo- Dissociation Regions (PDRs) where far-ultraviolet photons of these stars create a largely neutral, but warm region of gas and dust. PDR emission dominates the IR spectra of starforming galaxies and provides a unique tool to study the physical and chemical processes that are relevant for inter- and circumstellar media including diffuse clouds, molecular cloud and protoplanetary disk surfaces, and starburst galaxies. The ERS program ID1288 is dedicated to provide template data as well as data processing and analysis tools for PDRs. To this end, it will observe the Orion Bar, the proto-typical PDR situated in the nearby Orion Nebula, using NIRSpec IFU, MIRI IFU, and NIRCAM and MIRI imaging. These observations will, for the first time, spatially resolve and perform a tomography of the PDR, revealing the individual IR spectral signatures from the key zones and sub-regions within the ionized gas, the PDR, and the molecular cloud. Quantum chemistry can be used to determine molecular data such as ionization potentials, chemical reaction rates, electronic spectra, dissociation energies, channels, and branching ratios, etc. These are often used in synergy with experiment to take advantage of the strengths of each approach. An area of active research is modeling the infrared (IR) emission spectra produced by PAH molecules, including the effects of anharmonicity. This will be important in the interpretation of JWST data which will yield higher-resolution spectral data compared to Spitzer or ISO. Quantum chemistry methods can compute accurate anharmonic IR absorption spectra of PAH molecules up to about 25 C atoms, and the results have been benchmarked against high-resolution experimental spectra. These feed into a quantum Monte Carlo (QMC) approach to generate temperature-dependent anharmonic IR emission spectra and then used to generate fully anharmonic cascade emission IR spectra of PAH molecules -- the very type of spectra that JWST will observe. Work continues to extend these methods to larger PAH molecules. We will discuss the current state of this work, as well as

Timothy Lee↗

A Three-Year Program of Micro- and Nano-System Technology Development for X-Ray Astronomy

For many years the work at MIT aimed at the development of new concepts and technologies for space experiments in high-energy astrophysics, but not explicitly supported by flight programs, has been supported. This work has yielded new devices and techniques for X-ray astronomy, primarily low-noise, deep-depletion charge-coupled devices (CCDS) for spectrally-resolved X-ray imaging, and high-performance transmission gratings for high-resolution X-ray spectroscopy. Among the most significant recent achievements have been the development by G. Ricker and associates of the X-ray CCD camera flying on ASCA, and currently in development for AXAF and Astro-E, and the development by C. Canizares and associates of thick, 200 nm-period transmission gratings employing the phenomenon of phase shifting for high-resolution X-ray spectroscopy up to energies of 8- 1 0 keV that is essential for the operation of the AXAF High Energy Transmission Grating Spectrometer (HETGS). Through the current SR&T grant, the latter technology is now being extended successfully to the fabrication of 100 nm-period transmission gratings, which have twice the dispersion of the AXAF gratings. We note that, among other outcomes, the modest investments of past SR&T Grants at MIT resulted in the development of the key technologies for fully one-half of the scientific instrumentation on AXAF. In addition, NASA flight programs that have benefited from previous SR&T support at MIT include the SAS 3 X-ray Observatory, which carried the first rotation modulation collimator, the Focal Plane Crystal Spectrometer (FPCS) on the Einstein Observatory, the CCD cameras on ASCA and planned for Astro-E, the High Energy Transient Experiment (HETE), the Solar EUV Monitor on the Solar and Heliospheric Observatory (SOHO), the Medium Energy Neutral Atom imager (MENA) on the Image for Magnetopause-to-aurora Global Exploration (IMAGE) mission, and the recently-approved Two Wide-Angle Imaging Neutral-atom Spectrometers (TWINS) Mission of Opportunity.

Canizares, Claude R.↗

Determining the Molecular Growth Mechanisms of Protein Crystal Faces by Atomic Force Microscopy

A high resolution atomic force microscopy (AFM) study had shown that the molecular packing on the tetragonal lysozyme (110) face corresponded to only one of two possible packing arrangements, suggesting that growth layers on this face were of bimolecular height. Theoretical analyses of the packing also indicated that growth of this face should proceed by the addition of growth units of at least tetramer size corresponding to the 43 helices in the crystal. In this study an AFM linescan technique was devised to measure the dimensions of individual growth units on protein crystal faces as they were being incorporated into the lattice. Images of individual growth events on the (110) face of tetragonal lysozyme crystals were observed, shown by jump discontinuities in the growth step in the linescan images as shown in the figure. The growth unit dimension in the scanned direction was obtained from these images. A large number of scans in two directions on the (110) face were performed and the distribution of lysozyme growth unit sizes were obtained. A variety of unit sizes corresponding to 43 helices, were shown to participate in the growth process, with the 43 tetramer being the minimum observed size. This technique represents a new application for AFM allowing time resolved studies of molecular process to be carried out.

Nadarajah, Arunan↗

Large Solar-Rejection Filter

NASA utilized Image Intensified Video Cameras for ATV data acquisition from a jet flying at 12.8 km. Afterwards the video was digitized and then analyzed with a modified commercial software package, Image Systems Trackeye. Astrometric results were limited by saturation, plate scale, and imposed linear plate solution based on field reference stars. Time-dependent fragment angular trajectories, velocities, accelerations, and luminosities were derived in each video segment. It was evident that individual fragments behave differently. Photometric accuracy was insufficient to confidently assess correlations between luminosity and fragment spatial behavior (velocity, deceleration). Use of high resolution digital video cameras in future should remedy this shortcoming. A scenically accurate description of matter interpreted as a substance made up of corpuscular constituents was established during the course of the 19th century. In this description, atoms--the building blocks of the matter--form molecules. The properties of the molecules were described by chemistry or thermodynamics depending on what characteristics of the matter were investigated. In both theories, the molecules can dissociate to atoms when the kinetic energies of the atoms exceed the strength of the chemical bonds. The number of atoms is always preserved in a closed system. This is not true, however, when the matter takes up much higher energies at relativistic scales. New particles can be produced at the expense of the kinetic energy. The number of particles is no longer preserved. There are other conserved quantities, however, these quantities, the charge, baryon number, lepton number, are associated with particles that are considered elementary today. The properties and behavior of these elementary particles is the subject of Particle Physics or High Energy Physics. Practice Page: A crewmember touching a positively charged surface was thought to be galvanically isolated from the vehicle ground analogous to a bird on a high voltage power wire. Recent analysis confirms that positive floating potentials, ionospheric currents to the EVA suit, can be hazardous. The analysis is wrong in that the ionospheric plasma itself can close the circuit. Parametric analysis of very low voltage exposures (2 to 15 volts) could cause pain and/or involuntary muscle tetani or spinal cord shock. NASA worked with the Naval Health Research Center Detachment Directed Energy Bioeffects Laboratory to examine the affects electrical hazards could have on extravehicular activity using two models. The results of the two computational models were combined to predict areas of the body in which neurons of different diameters would be excited. They predicted that physiologically active current could be conducted across the crew member causing catastrophic hazards. Future work to analyze additional current paths was proposed. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10 degrees). Assuming an average WD mass of 0.8 solar mass leads to a distance of the order of approximately 650pc, consistent with the extremely low galactic reddening in its direction, and a mass accretion rate of 10 (exp -9) solar mass a year. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant: silicon is 3 times solar, and sulfur is 20 times solar. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10 degrees). Assuming an average WD mass of 0.8 solar mass leads to a distance of the order of approximately 650pc, consistent with the extremely low galactic reddening in its direction, and a mass accretion rate of 10 (exp -9) solar mass a year. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant: silicon is times solar, and sulfur is 20 times solar. From its previously observed optical emission lines, P831-57 (WD 0334 6400 or Ret 1 in A Catalog and Atlas of Cataclysmic Variables: Living Edition) has been suspected to contain an accretion disk associated with a companion star in orbit around a subdwarf star with a temperature T is greater than 21,000K. P831-57 has therefore been classified as a nova-like. However, our present observations show it to be a DA + dMe binary. The analysis of its Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum (continuum and lines) reveal an average mass white dwarf (Log(g) approximately equals 7.8 plus or minus 0.1)with a temperature T approximately equals 37,000 plus or minus 500K, an extremely low projected rotational velocity, and a distance of about 115 plus or minus 5pc. The photosphere contains C, N, Si, and S (at about 1% of solar abundances). The dMe star is seen as a flux excess in near-infrared photometry and appears to show occasional flaring of about one magnitude as seen in the Harvard plates. There is no evidence of periodic variability in the spectroscopic or photometric data. We find no evidence of a an accretion disk, instead we find evidence of wind accretion as the stellar carbon abundance (N(C)/N(H) = 2.5 x 10 (exp -6) is about ten times larger than predicted by radiative levitation for such a gravity and temperature. The power needs and solutions for the space exploration and lunar mobility program are discussed. Long term missions in space and on the lunar surface require high energy batteries. Rechargeable batteries for mobility systems and portable utility pallet are needed for successful exploration missions. Nanomaterial usage increases the energy density of the cells apart from increasing the power density. The symptoms and threats from acute mountain sickness (AMS) are discussed. The underlying assumptions concerning spacecraft atmosphere mean there is a potential risk to astronauts. The baseline worst case scenario is about 25% based on direct ascent to 8.0 psia with 32% O2. The practical mitigation approach is a staged depressurization scheme. Due to the uncertainty about potential AMS risk the flight surgeon should prepare, focused research should proceed, and current analytical efforts should continue. This slide presentation reviews the threat astronauts face from acute mountain sickness (AMS). The incidence of AMS is highly variable and the risk to astronauts is unclear. However, the potential risk is higher than current EAAs suggest and precautions should be implemented. Due to the uncertain nature of the risk flight surgeons should prepare, and focused research should proceed. Quasiparticle tunneling can be understood at low temperatures using a non-equilibrium kinetic theory. Low-temperature tunneling can be suppressed using existing techniques: island-lead gap engineering, lead cooling with SIN junctions, and quasiparticle traps. The slide presentation reviews the science goals and mission planning for the proposed Jupiter System Observer (JSO). The spacecraft would accommodate 9 instruments making the instrument payload more capable than any previously flown to Jupiter. The would-be mission includes study programs for Jupiter's satellites, interior, magnetosphere and atmosphere. The power needs and solutions for the space exploration and lunar mobility program are discussed. Long term missions in space and on the lunar surface require high energy batteries. Rechargeable batteries for mobility systems and portable utility pallet are needed for successful exploration missions. Nanomaterial usage increases the energy density of the cells apart from increasing the power density. This slide presentation reviews the power requirements for the space exploration and the lunar surface mobility programs. It includes information about the specifications for high energy batteries and the power requirements for lunar rovers,

Roberts, William↗

Extreme ultraviolet spectrum of a solar active region from SERTS

We present wavelengths and absolute intensities for 243 emission lines from a single active region observed by the Solar Extreme Ultraviolet (EUV) Rocket Telescope and Spectrograph (SERTS) on 1989 May 5. For this catalog, the imaged spectra have been spatially averaged over a field of view 7 sec x 276 sec cutting through the center of AR5464 at S18 W45. Wavelength coverage is 170-450 A with a spectral resolution approaching 10,000. Most of the line positions are determined to 5 mA or better, representing the highest accuracy yet obtained for solar wavelengths throughout this spectral interval. The relative photometric calibration of the instrument is good to +/- 20% over its first-order range, and has been placed onto an absolute scale that should be correct to within a factor less than 2. Where known, identifications, atomic transitions and formation temperatures are also given. The identified lines arise from temperatures that cover the range log T greater than or equal to 4.7 and less than or equal to 6.8, providing information about the Sun's corona and upper transition region. Upper limits to the intensity of any emission line not included here can be estimated from the measured instrumental sensitivity. This averaged EUV spectrum should prove useful as a source of accurate wavelengths and intensities for emission characteristic of the high-temperature plasma associated with a solar active region and small subflare.

Thomas, Roger J.↗

Thermal and Nonthermal Contributions to the Solar Flare X-Ray Flux

The relative thermal and nonthermal contributions to the total energy budget of a solar flare are being determined through analysis of RHESSI X-ray imaging and spectral observations in the energy range from approx. 5 to approx. 50 keV. The classic ways of differentiating between the thermal and nonthermal components - exponential vs. sources - can now be combined for individual flares. In addition, RHESSI's sensitivity down to approx. 4 keV and energy resolution of approx. 1 keV FWHM allow the intensities and equivalent widths of the complex of highly ionized iron lines at approx. 6.7 keV and the complex of highly ionized iron and nickel lines at approx. 8 keV to be measured as a function of time. Using the spectral line and continuum intensities from the Chianti (version 4.2) atomic code, the thermal component of the total flare emission can be more reliably separated from the nonthermal component in the measured X-ray spectrum. The abundance of iron can also be determined from RHESSI line-to-continuum measurements as a function of time during larger flares. Results will be shown of the intensity and equivalent widths of these line complexes for several flares and the temperatures, emission measures, and iron abundances derived from them. Comparisons will be made with 6.7-keV Fe-line fluxes measured with the RESIK bent crystal spectrometer on the Coronas-F spacecraft operating in third order during the peak times of three flares (2002 May 31 at 00:12 UT, 2002 December 2 at 19:26 UT, and 2003 April 26 at 03:OO UT). During the rise and decay of these flares, RESIK was operating in first order allowing the continuum flux to be measured between 2.9 and 3.7 keV for comparison with RHESSI fluxes at its low-energy end.

Dennis, Brian R.↗

New AFM Techniques for Investigating Molecular Growth Mechanisms of Protein Crystals

Atomic Force Microscopy (AFM) has emerged as a powerful technique for investigating protein crystal growth. Earlier AFM studies were among the first to demonstrate that these crystals grew by dislocation and 2D nucleation growth mechanisms [1]. These investigations were restricted to the micron range where only surface features, such as dislocation hillocks and 2D islands are visible. Most AFM instruments can scan at higher resolutions and have the potential to resolve individual protein molecules at nanometer ranges. Such scans are essential for determining the molecular packing arrangements on crystal faces and for probing the growth process at the molecular level. However, at this resolution the AFM tip influences the image produced, with the resulting image being a convolution of the tip shape and the surface morphology [2]. In most studies this problem is resolved by deconvoluting the image to obtain the true surface morphology. Although deconvolution routines work reasonably well for simple one- dimensional shapes, for complex surfaces this approach does not produce accurate results. In this study we devised a new approach which takes advantage of the precise molecular order of crystal surfaces, combined with the knowledge of individual molecular shapes from the crystallographic data of the protein and the AFM tip shape. This information is used to construct expected theoretical AFM images by convoluting the tip shape with the constructed crystal surface shape for a given surface packing arrangement. By comparing the images from actual AFM scans with the constructed ones for different possible surface packing arrangements, the correct packing arrangement can be conclusively determined. This approach was used in this study to determine the correct one from two possible packing arrangements on (I 10) faces of tetragonal lysozyme crystals. Another novel AFM technique was also devised to measure the dimension of individual growth units of the crystal faces. Measuring these units was not attempted before and most studies have assumed that the growth unit consisted of individual protein molecules. The linescan mode of AFM instruments allows the crystal surface to be scanned along a single line. By scanning across a growth step an image showing the motion of the step is obtained. Normally such an image shows a straight line for continuous and constant step velocity. In this study by increasing the scan rate and by decreasing the step velocity (by decreasing the supersaturation), we were able to capture images of individual growth events, shown by jump discontinuities in the step line. By suitable integration of the image the growth unit dimension in the scanned direction can be obtained. Since multiple units can be involved in the growth process it is necessary to collect a statistically relevant sample before drawing conclusions about the growth mechanism. This technique was successfully employed to obtain the dimensions of growth units for the (110) face, showing that they consisted of various aggregates corresponding to the 43 helices in the crystal structure.

Li, Huayu↗

Corrosion of Highly Specular Vapor Deposited Aluminum (VDA) on Earthshade Door Sandwich Structure

High-resolution infrared (IR) imaging requires spacecraft instrument design that is tightly coupled with overall thermal control design. The JPL Tropospheric Emission Spectrometer (TES) instrument measures the 3-dimensional distribution of ozone and its precursors in the lower atmosphere on a global scale. The TES earthshade must protect the 180-K radiator and the 230-K radiator from the Earth IR and albedo. Requirements for specularity, emissivity, and solar absorptance of inner surfaces could only be met with vapor deposited aluminum (VDA). Circumstances leading to corrosion of the VDA are described. Innovative materials and processing to meet the optical and thermal cycle requirements were developed. Examples of scanning electronmicroscope (SEM), atomic force microscope (AFM), and other surface analysis techniques used in failure analysis, problem solving, and process development are given. Materials and process selection criteria and development test results are presented in a decision matrix. Examples of conditions promoting and preventing galvanic corrosion between VDA and graphite fiber-reinforced laminates are provided.

Plaskon, Daniel↗

Determining the Molecular Growth Mechanisms of Protein Crystal faces by Atomic Force Microscopy

A high resolution atomic force microscopy (AFM) study had shown that the molecular packing on the tetragonal lysozyme (110) face corresponded to only one of two possible packing arrangements, suggesting that growth layers on this face were of bimolecular height (Li et al., 1998). Theoretical analyses of the packing had also indicated that growth of this face should proceed by the addition of growth units of at least tetramer size corresponding to the 43 helices in the crystal. In this study an AFM linescan technique was devised to measure the dimensions of individual growth units on protein crystal faces. The growth process of tetragonal lysozyme crystals was slowed down by employing very low supersaturations. As a result images of individual growth events on the (110) face were observed, shown by jump discontinuities in the growth step in the linescan images. The growth unit dimension in the scanned direction was obtained by suitably averaging these images. A large number of scans in two directions on the (110) face were performed and the distribution of lysozyme aggregate sizes were obtained. A variety of growth units, all of which were 43 helical lysozyme aggregates, were shown to participate in the growth process with a 43 tetramer being the minimum observed size. This technique represents a new application for AFM allowing time resolved studies of molecular process to be carried out.

Li, Huayu↗