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NASA NTRS1995

LDEF contributions to cosmic ray and radiation environments research

LDEF-1 carried three experiments which are producing significant advances in our knowledge of ultra heavy and anomalous cosmic rays, solar flare particles, and heavy nuclei in the trapped belts. Nine other experiments made measurements on the radiation environments or performed dosmetric monitoring. Data from those experiments, and from measurements of induced radioactivity in LDEF components have significantly improved our knowledge of LEO radiation environment. Measurements at various locations shielding depths of radiation absorbed dose, linear energy transfer spectra, proton, neutron and heavy ion fluences, and induced radioactivity have been made, and many of these results have been compared to models. This has allowed the assessment of accuracy, and the potential for improvement, of the models. Serendipitous results from the radiation measurements include the discovery of atmospheric Be-7 plated on the front surface of LDEF, which has motivated a series of new investigations. A sample of measurements and modeling results will be presented, as well as the status of archiving the measurements and models.

Parnell, Thomas A.↗

NASA NTRS1995

Status of LDEF activation measurements and archive

We review the status of induced radioactivity measurements for the LDEF spacecraft which includes studies of the nuclide, target, directional and depth dependences of the activation. Analysis of the data has focused on extraction of the specific activities for many materials to develop a global picture of the low Earth orbital environment to which the LDEF was subjected. Preliminary comparisons of data in a previous review showed that it was possible to make meaningful intercomparisons between results obtained at different facilities. Generally these comparisons were good and gave results to within 10-20 percent, although some analysis remains. These results clearly provide constraints for recent calculations being performed of the radiation environment of the LDEF. We are not anticipating a period of production of final activation results. An archive is being prepared jointly between NASA/Marshall and Eastern Kentucky University which will include gamma ray spectra and other intermediate results.

Harmon, B. Alan↗

NASA NTRS1995

Status of LDEF radiation modeling

The current status of model prediction and comparison with LDEF radiation dosimetry measurements is summarized with emphasis on major results obtained in evaluating the uncertainties of present radiation environment model. The consistency of results and conclusions obtained from model comparison with different sets of LDEF radiation data (dose, activation, fluence, LET spectra) is discussed. Examples where LDEF radiation data and modeling results can be utilized to provide improved radiation assessments for planned LEO missions (e.g., Space Station) are given.

Watts, John W.↗

NASA NTRS1995

The orbital characteristics of debris particle rings as derived from IDE observations of multiple orbit intersections with LDEF

During the first 346 days of the LDEF's almost 6 year stay in space, the metal oxide silicon detectors of the Interplanetary Dust Experiment (IDE) recorded over 15,000 impacts, most of which were separated in time by integer multiples of the LDEF orbital period (called multiple orbit event sequences, or MOES). Simple celestial mechanics provides ample reason to expect that a good deal of information about the orbits of the impacting debris particles can be extracted from these MOES, and so a procedure, based on the work of Greenberg, has been developed and applied to one of these events, the so-called 'May swarm'. This technique, the 'Method of Differential Precession,' allows for the determination of the geometrical elements of a particle orbit from the change in the position of the impact point with time. The application of this approach to the May swarm gave the following orbital elements for the orbit of the particles striking LDEF during this MOES: a = 6746.5 km; 0.0165 less than e less than 0.025; i = 66.55 deg; Omega(sub 0) = 179.0 deg plus or minus 0.2 deg; omega = 178.1 deg plus or minus 0.2 deg.

Cooke, William J.↗

NASA NTRS1995

Changes in chemical and optical properties of thin film metal mirrors on LDEF

Thin films of the metals Cu, Ni, Pt, Au, Sn, Mo, and W deposited on fused silica flats were exposed at ambient temperature on the leading and trailing faces of the LDEF. Reflectances of these films were measured from 250 to 2500 nm and compared with controls. The exposed films were subjected to the LDEF external environment including atomic oxygen, molecular contamination, and solar ultraviolet. Major changes in optical and infrared reflectance were seen for Cu, Mo, Ni, and W films on the leading face of LDEF and are attributed to partial conversion of metal to metal oxide. Smaller changes in optical properties are seen on all films and are probably caused by thin contaminant films deposited on top of the metal. The optical measurements are correlated with film thickness measurements, x-ray photoelectron spectroscopy, optical calculations, and, in the case of Cu, with x-ray diffraction measurements. In a few cases, comparisons with results from a similar UAH experiment on STS-8 have been drawn.

Peters, Palmer N.↗

NASA NTRS1995

Overview of the LDEF MSIG databasing activities

The Long Duration Exposure Facility (LDEF) and the accompanying experiments were composed of and contained a wide variety of materials, representing the largest collection of materials flown in low earth orbit (LEO) and retrieved for ground-based analysis to date. The results and implications of the mechanical, thermal, optical, and electrical data from these materials are the foundation on which future LEO spacecraft and missions will be built. The LDEF Materials Special Investigation Group (MSIG) has been charged with establishing and developing databases to document these materials and their performance to assure not only that the data are archived for future generations but also that the data are available to the spacecraft user community in an easily accessed, user-friendly form. This paper gives an overview of the current LDEF Materials Databases, their capabilities, and availability. An overview of the philosophy and format of a developing handbook on LEO effects on materials is also described.

Funk, Joan G.↗

NASA NTRS1995

Duplication and analysis of meteoroid damage on LDEF and advanced spacecraft materials

The analysis of exposed surfaces on LDEF since its retrieval in 1990 has revealed a wide range of meteoroid and debris (M&D) impact features in the sub-micron to millimeter size range, ranging from quasi-infinite target cratering in LDEF metallic structural members (e.g. inter-costals, tray clamps, etc.) to non-marginal perforations in metallic experimental surfaces (e.g. thin foil detectors, etc.). Approximately 34,000 impact features are estimated to exist on the exposed surfaces of LDEF. The vast majority of impact craters in metal substrates exhibit circular footprints, with approximately 50 percent retaining impactor residues in varying states of shock processing. The fundamental goals of this project were to duplicate and analyze meteoroid impact damage on spacecraft metallic materials with a view to quantifying the residue retention and oblique impact morphology characteristics. Using the hypervelocity impact test facility established at Auburn University a series of impact tests (normal and oblique incidence) were executed producing consistently high (11-12 km/s) peak impact velocities, the results of which were subsequently analyzed using Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS) facilities at Auburn University.

Hill, David C.↗

NASA NTRS1996

LDEF Satellite Radiation Analyses

This report covers work performed by Science Applications International Corporation (SAIC) under contract NAS8-39386 from the NASA Marshall Space Flight Center entitled LDEF Satellite Radiation Analyses. The basic objective of the study was to evaluate the accuracy of present models and computational methods for defining the ionizing radiation environment for spacecraft in Low Earth Orbit (LEO) by making comparisons with radiation measurements made on the Long Duration Exposure Facility (LDEF) satellite, which was recovered after almost six years in space. The emphasis of the work here is on predictions and comparisons with LDEF measurements of induced radioactivity and Linear Energy Transfer (LET) measurements. These model/data comparisons have been used to evaluate the accuracy of current models for predicting the flux and directionality of trapped protons for LEO missions.

Armstrong, T. W.↗

NASA NTRS1990

STS-32: LDEF Move from SAEF II to Hanger "C" CCAFS

One of the primary objectives of STS-32 was to retrieve the Long Duration Exposure Facility (LDEF) from space. The LDEF was designed to provide long-term data on the space environment and its effects on space systems and operations. This videotape shows the LDEF being moved from the Spacecraft Assembly and Encapsulation Facility to Hanger C in the Cape Canaveral Air Force Station after it had been retrieved from space. There are many views of the environment around the Kennedy Space Facility.

Source record↗

NASA NTRS1996

Induced radioactivity of LDEF materials and structural components

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.

NASA Discipline Radiation Health↗

NASA NTRS1996

Neutron fluences and dose equivalents measured with passive detectors on LDEF

Neutron fluences were measured on LDEF in the low energy (< 1 MeV) and high energy (> 1 MeV) ranges. The low energy detectors used the 6Li(n,alpha)T reaction with Gd foil absorbers to separate thermal (< 0.2 eV) and resonance (0.2 eV-1 MeV) neutron response. High energy detectors contained sets of fission foils (181Ta, 209Bi, 232Th, 238U) with different neutron energy thresholds. The measured neutron fluences together with predicted spectral shapes were used to estimate neutron dose equivalents. The detectors were located in the A0015 and P0006 experiments at the west and Earth sides of LDEF under shielding varying from 1 to 19 g/cm2. Dose equivalent rates varied from 0.8 to 3.3 microSv/d for the low energy neutrons and from 160 to 390 microSv/d for the high energy neutrons. This compares with TLD measured absorbed dose rates in the range of 1000-3000 microGy/d near these locations and demonstrates that high energy neutrons contribute a significant fraction of the total dose equivalent in LEO. Comparisons between measurements and calculations were made for high energy neutrons based on fission fragment tracks generated by fission foils at different shielding depths. A simple 1-D slab geometry was used in the calculations. Agreement between measurements and calculations depended on both shielding depth and threshold energy of the fission foils. Differences increased as both shielding and threshold energy increased. The modeled proton/neutron spectra appeared deficient at high energies. A 3-D model of the experiments is needed to help resolve the differences.

LDEF Project↗

NASA NTRS1996

Absorbed dose measurements on LDEF and comparisons with predictions

The radiation environment on LDEF was monitored by cumulative absorbed dose measurements made with TLDs at different locations and shielding depths. The TLDs were included in four experiments: A0015(a) Biostack, P0004 Seeds in Space and P0006 Linear Energy Transfer Spectrum Measurements at the trailing edge (west side) of the satellite; M0004 Fiber Optics Data Link at the leading edge (east side); and A0015(b) Biostack at the Earth side. The shielding depths varied between 0.48 and 15.4 g/cm2, Al equivalent. Both the directional dependence of trapped protons incident on the satellite and the shielding thickness were reflected in absorbed dose values. The trapped proton anisotropy was measured by TLDs at the east and west sides of LDEF. At the east side doses ranged from 2.10 to 2.58 Gy under shielding of 2.90 to 1.37 g/cm2 (M0004) while on the west side doses ranged from 2.66 to 6.48 Gy under shielding of 15.4 to 0.48 g/cm2 (P0006). The west side doses were more than a factor of two higher, where the vertical shielding thicknesses to space were equal. Other west side doses of 3.04 to 4.49 Gy under shielding of 11.7 to 3.85 g/cm2 (A0015(a)) and 2.91 to 6.64 Gy under shielding of 11.1 to 0.48 g/cm2 (P0004) generally agreed with the P0006 results. The Earth side doses of 2.41 to 3.93 Gy under shielding of 10.0 to 1.66 g cm2 (A0015(b)) were intermediate between the east side and west side doses. Calculations utilizing a model of trapped proton spectra were performed by Watts et al. (1993) and comparisons of dose measurement and calculations may be found in a companion paper (Armstrong et al., 1996).

Flight Experiment↗

NASA NTRS1996

Secondary particle contribution to LET spectra on LDEF

Four experiments utilizing passive detectors (P0006, P0004, A0015, M0004) were flown on LDEF to study the radiation environment. These experiments have been summarized in a companion paper (Benton et al., 1996). One of the experimental goals was to measure LET spectra at different locations and shielding depths with plastic nuclear track detectors (PNTD). It was found that the LET spectra extended well above the LET cutoff imposed by the geomagnetic field on GCR particle penetration into LEO. The high LET particles detected were mostly short-range (range < 2000 m), indicating that they were secondaries produced locally within the PNTD. The presence of these high LET particle fluences is important for the determination of dose equivalent because of the high Quality Factors (Q) involved. A relatively small fraction of particle fluence can contribute a large fraction of dose equivalent. Short-range, inelastic secondary particles produced by trapped protons in the South Atlantic Anomaly (SAA) were found to be a major contributor to the LET spectra above 100 keV/micrometer. The LET spectra were found to extend beyond the approximately 137 keV/micrometer relativistic GCR Fe peak to over 1000 keV/micrometer. The high LET tail of the LET spectra was measured in CR-39 and polycarbonate PNTDs using different techniques. GCR made a relatively modest contribution to the LET spectra as compared to the contributions from short-range secondary particles and stopping protons. LET spectra intercomparisons were made between LDEF measurements and exposures to 154 MeV accelerated proton beams. The similarities support the role of nuclear interactions by trapped protons as the major source of secondary particles in the PNTDs. Also techniques were employed to reduce the range cutoff for detection of the short-range secondaries to approximately 1 micrometer, so that essentially all secondary particles were included in the LET spectra. This has allowed a more realistic assessment of secondary contribution to dose equivalent. Comparisons of measured and calculated LET spectra have been made that demonstrate the need for more accurate modeling of secondary particles in radiation transport codes. Comparisons include preliminary calculations in which attempts have been made to include secondary particles.

long duration↗

NASA NTRS1996

Prediction of LDEF exposure to the ionizing radiation environment

Predictions of the LDEF mission's trapped proton and electron and galactic cosmic ray proton exposures have been made using the currently accepted models with improved resolution near mission end and better modeling of solar cycle effects. An extension of previous calculations, to provide a more definitive description of the LDEF exposure to ionizing radiation, is represented by trapped proton and electron flux as a function of mission time, presented considering altitude and solar activity variation during the mission and the change in galactic cosmic ray proton flux over the mission. Modifications of the AP8MAX and AP8MIN fluence led to a reduction of fluence by 20%. A modified interpolation model developed by Daly and Evans resulted in 30% higher dose and activation levels, which better agreed with measured values than results predicted using the Vette model.

Flight Experiment↗

NASA NTRS1974

Passive stabilization of the Long Duration Exposure Facility (LDEF)

The results of a study on the application of the magnetically anchored rate damper to gravity gradient stabilization of the Long Duration Exposure Facility (LDEF) are presented. The analyses and simulations required to investigate the use of an existing viscous magnetic rate damper for rate stabilizing the LDEF spacecraft were performed. The following tasks were included: linear performance estimates, capture and damper requirements, and performance prediction. Each of these tasks was performed for two gravity gradient stabilization configurations; an axisymmetric configuration for two-axis (pitch and roll) stability; and a non-axisymmetric configuration for three-axis stability. The results are presented by stabilization configuration.

Das, A.↗

NASA NTRS1979

LDEF transverse flat plate heat pipe experiment /S1005/

The paper describes the Transverse Flat Plate Heat Pipe Experiment. A transverse flat plate heat pipe is a thermal control device that serves the dual function of temperature control and mounting base for electronic equipment. In its ultimate application, the pipe would be a lightweight structure member that could be configured in a platform or enclosure and provide temperature control for large space structures, flight experiments, equipment, etc. The objective of the LDEF flight experiment is to evaluate the zero-g performance of a number of transverse flat plate heat pipe modules. Performance will include: (1) the pipes transport capability, (2) temperature drop, and (3) ability to maintain temperature over varying duty cycles and environments. Performance degradation, if any, will be monitored over the length of the LDEF mission. This information is necessary if heat pipes are to be considered for system designs where they offer benefits not available with other thermal control techniques, such as minimum weight penalty, long-life heat pipe/structural members.

Robinson, G. A., Jr.↗

NASA NTRS1982

The LDEF benefits

The Long Duration Exposure Facility (LDEF) is described, and experiments planned for the first LDEF mission are discussed. Four of the eight involve scientific studies of interstellar gas, micrometeoroids, cosmic rays, and crystal growth in zero gravity, and four involve technology studies of the space environmental effects on solar cells, composite materials, thermal coatings, fiber optics, and electronic instruments. For each experiment, the objectives and methods are discussed.

Kinard, W. H.↗

NASA NTRS1985

The LDEF interplanetary dust experiment

Explorer 46 data and the Long Duration Exposure Facility (LDEF) interplanetary dust experiment are examined. Analysis of the Explorer 46 data reveals the existence of particles of 0.1 micron and a mass of 1 x 10 to the -16th gm, the injection of the submicron particles directly by a comet (injection mass of about 5000 tons), and a submicron particle lifetime of about three years. The applications of LDEF data to particles in hyperbolic orbits, particle swarms, morning-to-evening asymmetry, the effects of the earth's orbit eccentricity, and the presence of interstellar dust are discussed. The effects of space debris on data collection are considered.

Singer, S. F.↗