. SPACE ENVIRONMENTAL EFFECTS ON MATERIALS
. SPACE ENVIRONMENTAL EFFECTS ON HUMAN - SOON
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SPACE ENVIRONMENTAL EFFECTS ON MATERIALS
The International Space Station provide a challenging Research Environment with its exposure to extreme heat and cold cycling, ultra vacuum, atomic oxygen and high energy radiation.
Big first, the micro-gravity alters many observable physical and life sciences phenomenas, which include surface wetting and inter-facial tension, multiphase flow and heat transfer as well as system dynamics, solidification, end fire phenomena and combustion.
But, placed in Low-Earth-Orbit at approximately 240 miles (400 kilometers) with an inclination of 51-degree and an orbit of 90-minute, which give an orbital path over 90 percent of the Earth's population, ISS offer a unique vantage point.
View of the Materials International Space Station Experiment (MISSE) 6A and 6B Passive Experiment Containers (PECs) on the European Laboratory/Columbus. Photo was taken during a fly around of STS-123 Space Shuttle Endeavor.
It is for all these reasons NASA test materials in space, by exposure them on the exterior of the station. Outside, they are subjected to many environmental threats that can degrade many of them. At the level of ISS, these threats are Vacuum, solar ultraviolet (UV) radiation, charged particle (ionizing) radiation, plasma, surface charging and arcing, temperatures extremes, thermal cycling, impacts from micro-meteoroids and orbital debris (MMOD) and environment-induced contamination.
In LEO (200-1000 km), we have a particularly harsh environment for most non-metallic materials, because single-oxygen atoms(AO) are present along with all other environmental components (Yang and de Groh, 2010)
Also, we have to think about the mission duration, the specific mission environment and its orbital parameters that one. That means, knowing the solar cycle and events, view angle of spacecraft surfaces to the sun and orientation of spacecraft surface with respect to the spacecraft velocity vector in LEO (Dever et al., 2005)
Materials Spaceflight experiments to evaluate the environmental durability of various materials and components in space have been conducted since the early 1970s, including 57 experiments on the Long Duration Exposure Facility (LDEF), which was retrieved in 1990 after spending 69 months in LEO (de Groh et al., 2011)
The Materials International Space Station Experiment (MISSE) is a series of materials flight experiments, the first two of which were delivered to the ISS during STS-105 in 2001.
Consisting of a pair of trays hinged together like a surface (Passive Experiment Container-PEC) and containing an array of individual experiment, the PEC is attached to the exterior of the ISS providing long-duration exposure to space radiation.
In the MISSE suite (MISSEs 1 through 8), 10 PECs (and one smaller tray), together containing thousands of samples, have been flown in various external locations on the ISS. With participants from NASA, DoD, industry and academia, MISSE is the longest running multi-organization technology development and materials testing project on the Station.
Published MISSE data (50 years of analyzed results of tests on Materials And Processes Technical Information System (MAPTIS)http://maptis.nasa.gov/
Aspects of the Space Environment
The hard VACUUM of space will cause out-gassing, which is the release of volatiles from materials, and then, go on the cold surfaces. This molecular contamination affect optical properties of vehicle, payload surfaces and, particularly, spacecraft performance of sensitive optics.
To mitigate this problem, the ISS has specified in NASA SSP 30426, Space Station External Contamination Control Requirements, what the limits are for molecular deposition, induced molecular column densities and the release of particulates.(To learn more about this, see ASTM E1559: Standard Test for contamination out-gassing characteristics of spacecraft materials)
So, a material known to out gas should be thermal vacuum baked for a minimum of 24 hours at a temperature above that expected in orbit or, if that is not known, at 100 °C. To be sure, assemblies may be thermal vacuum baked prior to flight.
Atomic Oxygen (AO)
AO is produced when short-wavelength UV radiation reacts with molecular oxygen in the upper atmosphere. Then, at the ISS altitude, this molecular is the more relevant for the material degradation. We know that, by Pippin et al., 2004, many materials have Polymers containing fluorine, like Teflon, increase they reactions to Atomic Oxygen when they are exposure to UV radiation for longtime.
Not limited to Teflon, AO oxidizes also many metals like silver, copper and osmium, reacts strongly with any material containing carbon, nitrogen, sulfur and hydrogen bonds, meaning that many polymers react and erode.
Image shows atomic oxygen undercutting degradation of the solar array wing blanket box cover on the ISS after one year of space exposure
The AO exposure, characterized by the fluence (atoms/cm2) to an experiment will depend not only on the orientation and spacecraft altitude but also on the solar activity at the time of flight. An experiment to determine AO reactivity, also known as erosion yield (cm3/atom), of materials needs an exposure that is long enough for measurable erosion to occur.
Software such as the Mass Spectrometer Incoherent Scatter (MSIS) Model can be used to estimate the AO fluence for a particular mission, which is then used to determine if the experiment will receive a high enough level of AO for the desired measurements and also whether the sample thickness is sufficient if the expected fluence is high.
AO in orbit is ~5.2-eV energy, principally as a result of the ISS orbital velocity. Plasma asher create AO but also produce heating, intense Lyman-alpha UV radiation and a significant percentage of ions rather than just neutral atoms. In addition, the arrival direction of AO may vary more than in space.
AO beam facilities may use laser-detonation or microwave sources to generate AO at energies close to that at ISS altitude. For polymers that do not contain fluorine, simulations in these facilities are generally close to flight, with the AO reactivity being ±10 percent. Polymers with fluorine are more sensitive to the UV generated simultaneously with the AO and consequently have a higher reactivity in the beam facility than observed in orbit.
In general, laboratory results are normalized to flight results by flying materials with known reactivity to AO in addition to the test samples. Kapton® H and Kapton® HN polyimide are the most common materials selected for this purpose; polyethylene, polypropylene, and pyrolytic graphite have also been used. Following ASTM E2089, Standard Practices for Ground Laboratory Atomic Oxygen Interaction Evaluation of Materials for Space Applications, can reduce variability in results.
Earth’s atmosphere filters out most of the sun’s damaging light, but ISS materials bear the brunt of solar photon damage. While AO may bleach materials, UV generally darkens them (see below), particularly in the presence of contamination. UV radiation also damages polymers by either cross-linking or chain scission. UV under high vacuum can also create oxygen vacancies in oxides, leading to significant color changes.
Pre-flight (left) and post flight (right) images of the Optical Properties Monitor shown with ultraviolet-darkened insulation after nine months of exposure on the Mir Space Station.
Particulate or Ionizing Radiation
The three main sources of charged particle radiation naturally occurring in space are galactic cosmic rays, solar proton events, and the trapped radiation belts. For most materials on the ISS, the effects of AO and UV can overshadow any effects by particulate radiation. Depending on the polymer, particulate radiation can result in cross-linking or chain scission, similar to damage by UV.
The plasma environment around the ISS is composed of approximately equal amounts of positively charged oxygen ions (O+) and free electrons and varies with solar activity and altitude. Because of the differences in spacecraft velocities, ion thermal energy and electron thermal energy, electrons can impact any spacecraft surface, while ions can only impact ram surfaces. This can lead to a negative charge buildup, sputtering ion and, bay the way, arcing and parasitic currents in solar arrays, as well as re-attraction of contamination (James et al., 1994).
A principal investigator for such an experiment would need to work in conjunction with the Spacecraft Charging Assessment Team at NASA’s Johnson Space Center (JSC). Specific information about the plasma environment around the ISS can be provided through operation of the Floating Potential Measurement Unit. ISS plasma conditions may be modeled in the laboratory with a hollow cathode plasma source to create a low-density (106/cm3), low-temperature (≤1‑eV electron temperature) plasma. Argon is often used, but some sources may use xenon, oxygen or helium.
Temperature Extremes and Thermal Cycling - (Coefficients of Thermal Expansion [CTE])
As the ISS moves in and out of sunlight during its orbit around Earth, the degree to which a material experiences thermal cycling temperature extremes depends on its thermo-optical properties (solar absorption and thermal emittance), its view of the sun, Earth, and other surfaces of the spacecraft, durations of time in sunlight and in shadow, its thermal mass and the influence of equipment or components that produce heat (Dever et al., 2005).
A rule of thumb for these cyclic temperature variations is -120 °C to +120 °C, but high solar absorption with low infrared emittance will contribute to greater temperature swings. Large areas of material with poor thermal properties may not be allowed on the ISS because of exceeding touch temperature limits for the astronauts’gloves.
Micro-meteoroid & Orbital Debris Impact
All areas of a spacecraft may be impacted by micro-meteoroids traveling as fast as 60 km/s. Surfaces facing the ram direction are more likely than those in the wake direction to be hit with space debris, traveling at an average velocity of 10 km/s.
Space debris varies with the solar cycle: as the Sun’s activity increases, the atmosphere heats up, increasing the drag on space debris in orbit. Large space debris is tracked so that the ISS can perform avoidance maneuvers, but there is no current way to avoid small debris impacts. Most of the impacts on returned experiments have been small, creating ≤0.5-mm diameter craters.
The first two Materials International Space Station Experiments (MISSE-1 and MISSE-2) averaged less than two impacts/ft2/year (Pippin 2006), with a much smaller shielding factor from the then-incomplete ISS structure. As more space debris is added to the environment, however, the impact risk changes.
A micro-meteoroid or orbital debris impact results in a 0.6-mm diameter crater and approximately 3-mm diameter spall (loss of the coating). 2.54 cm diameter Tiodize coating on titanium flown on the Long Duration Exposure Facility A0171 experiment.
NASA and the National Science Foundation fund a variety of research efforts through grants, fellowships, contracts, inter-agency transfers and cooperative agreements. The NASA Solicitation and Proposal Integrated Review and Evaluation System (NSPIRES; http://nspires.nasaprs.com/) lists the solicitations and research opportunities. A guidebook for responding to NASA Research Announcements (http://www.hq.nasa.gov/office/procurement/nraguidebook/) guides the reader through notice of intent to propose, preparation of proposals, conduct of the research itself and the sharing of new knowledge through publications, public outreach and education. Research proposals are peer reviewed and should meet national research objectives.
The orientation of materials experiments are typically in the ram, wake, zenith and/or nadir directions. Ram refers to the velocity vector of the vehicle and has the greatest fluence of AO. Zenith, which points into space in the opposite direction of Earth, has the most solar illumination. Wake and nadir are the opposing faces of ram and zenith, respectively. The wake direction is good for studying UV effects with typically an order of magnitude less AO as the ram direction, and some experimenters may wish to fly duplicate samples (ram- and wake-facing) to differentiate between AO and UV effects. A nadir orientation is desired for Earth-viewing experiments.
NASA launched the Long Duration Exposure Facility developed at NASA's Langley Research Center in 1984. It spent almost six years in space collecting data on how 57 experiments mounted in 86 trays withstood the harsh space environment, until it was retrieved on Jan. 12, 1990. Results from LDEF have helped improve spacecraft construction. This story was produced in October, 1990. Credit: NASA Langley Research Center