NASA Long Duration Exposure Facility Research Articles

Multi-faceted simulation of atomic oxygen erosion of deorbit sail for cleaning space debris

Deorbit sails are used in low earth orbit to reduce the growing amount of orbital debris by rapidly removing satellites at the end of their operational missions. However, in low earth orbit, atomic oxygen (AO) is a potential threat to long-duration exposure facilities. In this work, a multi-faceted simulation for the AO erosion of the Taurus Deorbit Sail was developed to calculate the erosion yield for the membrane material and the undercutting profile at the crease site. The disintegration and erosion detail for the deorbit sail membrane during AO impacts can be studied on an atomic scale. The ReaxFF reaction force field program is used to provide a large enough calculation speed in the size of the system to describe all the chemical reactions of the reaction. The on-orbit flight data from the Materials International Space Station Experiment 2 were used to validate the simulated erosion rate in this study. The defects on the deorbit sails were experimentally measured using a scanning electron microscope and then modeled by the Monte Carlo method. The computational predictions of AO undercutting at the defect sites were validated by experimental data from the NASA long-duration exposure facility. The disintegration of the membrane could be divided into two stages: the physical reaction stage and the chemical reaction stage, after 150 AO collisions the membrane becomes highly volatilized. The maximum undercutting depth for the deorbit drag sail within the predicted 37-month deorbit time was 5.5 μm and 7.5 μm for a defect width of 500 nm and 1 μm, respectively. The undercutting width for a deorbit sail with a width of 500 nm or 1 μm was predicted to be 1.6 μm and 2 μm, respectively, and the breaker was formed at the bottom of the undercutting profile. AO erosion has been studied by researchers in terms of evaluating the effectiveness of protective coatings. This current study, however, seeks to understand the fundamental interactions between AO and creases in the deorbit sail, which provides deeper knowledge than what has been developed so far. The multi-faceted simulation results can provide design references for engineering applications for the deorbit sail.

New Experimental Bounds on the Contributions to the Cosmological Density Parameter Ω from Strongly Interacting Massive Particles

Strongly interacting neutral massive particles (SIMPs) have been proposed as candidates for dark matter, as the lightest supersymmetric particle, as a possible explanation for ultra-high-energy cosmic rays, and as a dark matter solution to galactic structure problems. If bound to nuclei, SIMPs could manifest themselves as anomalously heavy isotopes of known elements. We analyze the results from a recent experimental search for SIMPs in a collection of gold and iron samples with various exposures to cosmic rays and to a SIMP component of dark matter. The samples included gold flown on the NASA Long-Duration Exposure Facility, as well as geological samples and an iron meteorite. We show that the bounds on SIMPs from that experiment can be used to set nontrivial constraints on the SIMP contribution to the cosmological density parameter Ω.

Intensity and Directionality of Flare‐accelerated α‐Particles at the Sun

We have studied γ-ray line emission from fusion of accelerated α-particles with ambient helium in 19 solar flares observed by the Solar Maximum Mission Gamma-Ray Spectrometer from 1980 to 1989. An isotropic or fan-beam distribution of accelerated particles provides good fits to the line profiles for most of the flares. In contrast, a downward beam of accelerated particles can be ruled out at high confidence levels, 99.99% and 99.8%, for the two most intense flares and provides significantly poorer fits than isotropic or fan-beam distributions in two other flares. The fluences in the α-α lines, with a few exceptions, show a correlation with fluences of the narrow nuclear de-excitation lines from elements with high first ionization potential. If we assume that all accelerated ions have the same spectral shape, and if we adopt accelerated particle and ambient abundances derived from other γ-ray line studies, we find that an accelerated α/proton ratio of 0.5 is consistent with the spectra from the 19 flares, whereas a ratio of 0.1 is not. We also suggest the possibility that 7Be produced in α-4He fusion during the intense flares occurring in late 1989 may have reached Earth in a concentration high enough to account, in part, for 7Be observed in NASA's Long-Duration Exposure Facility satellite after its recovery in early 1990.

Observations of surface structures and reaction products for materials exposed in space in low-earth orbit

A preliminary study of materials exposed in space in low-earth orbit for nearly six years (in the NASA Long-Duration Exposure Facility (LDEF)) has revealed a range of micrometeorite and microparticle impact craters and blisters of unknown origin on aluminum alloy 6061-T6 samples on the leading edge of the satellite. In addition these samples were covered with an enormous range of reaction products which included crystals and polycrystalline aggregates which originated in part from other reactions and atomic oxygen degradation taking place in systems proximate to the alloy samples.A replication-type lift-off techniques was developed to remove reaction products (crystals and polycrystals, etc.) and debris from the specimen surfaces in order to isolate them from the background substrate without creating microchemical or microstructural artifacts. This technique culminates in extracted surface features resting on a carbon support film which is virtually “invisible” to observation by electron microscopy and non-dispersive x-ray analysis.

Effects of space environment on structural materials: A preliminary study and development of materials characterization protocols

A preliminary study of materials exposed in space in a low-earth orbit for nearly 6 years (in the NASA Long-Duration Exposure Facility) has revealed a wide range of micrometeorite or microparticle impact craters ranging in size from 1–100 μm diameter, debris particles from adjacent and distant materials systems, reaction products and other growth features on the specimen surfaces, and related phenomena. The exposed-surface features included finegrained and nearly amorphous materials as well as single-crystal particles. A replication-type, lift-off technique was developed to remove reaction products and debris from the specimen surfaces in order to isolate them from the background substrate without creating microchemical or microstructural artefacts or alterations. This resulted in surface features resting on a carbon support film which was virtually invisible to observation by electron microscopy and non-dispersive X-ray analysis. Characterization of these surface features involved observations by optical metallography, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray analysis; including an analytical transmission electron microscope with a STEM attachment. The results illustrate a wide variety of materials phenomena which must be addressed in the evaluation of materials exposure in space, and the formidable materials characterization effort which will be necessary to understand these features.

Space Qualification Of Optical Instruments Using The NASA Long Duration Exposure Facility

The NASA Long-Duration Exposure Facility (LDEF), designed for the in situ testing of optical instrumentation in the space environment is described. The LDEF is basically a reusable structure which is three-axis gravity-gradient stabilized when flying free in space. The structure weighs about 8,000 lbs. and accommodates experiment trays weighing about 10,000 lbs. A vacuum exposure control canister is being designed for controlled exposure experiments. The LDEF will have an experiment power and data system and a shuttle bay environmental measurements system is being designed for measuring parameters such as acoustics, vibration, acceleration. The induced environment contamination monitor will evaluate the contamination levels of shuttle payloads. Experiments currently under development include the evaluation of active optical components and the study of fiber optic data transmission.