Deep Space Radiation Effects on Inflatable Habitat and Spacesuit Materials, Year 1 (MRAD)

How well will spacesuit and inflatable habitat materials survive the extreme cosmic radiation experienced during Mars missions? This three-year project provided risk reduction data to assess galactic cosmic ray (GCR) and solar particle event (SPE) space radiation damage in materials used in Martian surface and deep space missions. Long duration (up to 50 years) space radiation damage was measured for materials used in inflatable structures (1st priority), and space suit and habitable composite materials (2nd priority). The data collected has relevance for nonmetallic materials, including polymers and composites used in NASA missions where long duration reliability is needed. Risk reduction data are needed to assess space radiation damage and therefore qualify materials for their intended service application. High volume and low mass habitable areas achieved via inflatable structures are likely to play an enabling technology role for deep space exploration. Close to 1000 specimens were irradiated over the course of this project. Background A manned expedition to Mars is the culminating goal of U.S. human spaceflight, with missions anticipated in the next 15 to 20 years (by the mid-2030s). But more than a half-century after the dawn of the Space Age, shielding and sheltering measures to protect crews from space radiation still need to be developed. This requires breakthroughs in the lightweight materials needed to make long duration space missions possible. Space travel has always sought lighter, more cost-efficient vehicles. As mission durations lengthen during humankind's journey into deep space, lighter mass/volume efficient structures are needed to allow these spacecraft to carry more fuel and supplies. Inflatable habitable structures provide a viable solution to address this issue. One such structure is currently deployed aboard the International Space Station (ISS): the Bigelow Expandable Activity Module (BEAM). This inflatable habitat deployed on May 2016 for a two-year in-space technology demonstration. To send an expedition to Mars today, NASA would have to knowingly expose astronauts and vehicles to high levels of space radiation due to the long-durations required for Mars missions. During a Mars round-trip, the vehicle would spend a majority of its time in deep space, or on the Martian surface, facing exposure to the following types of space radiation. Solar particle events (SPE) generated by solar flares or coronal mass ejections from the sun. Such events consist primarily of protons with energies ranging from tens to hundreds of MeV. Galactic cosmic rays (GCR) from outside the solar system, but generally from within our Milky Way galaxy. GCRs consist of omnidirectional protons mostly, but also silicon, iron and other nucleons (nuclei stripped of electrons) with particle energies on the order of 1 GeV or greater. Secondary radiation resulting from primary radiation interacting with spacecraft structures (for example, the x-rays, gamma rays, and secondary particles produced by the interaction of GCRs with the aluminum hull of the ISS), the surface of a planetary body (for example, the neutron albedo produced by the interaction of GCRs with the lunar regolith), or the atmosphere of a planetary body (for example, the neutron albedo produced by the interaction of GCRs with the Earth's atmosphere). Radiation degradation or hardening of spacecraft materials may pose a serious threat to crew depending on materials functioning in an identical manner as they did on Earth. If material properties change significantly while exposed to large levels of radiation, the structure could fail unexpectedly resulting in loss of life and/or mission. For example, in multifunctional composite habitat designs the radiation resistance of the adhesive bond line between the composite face sheet and sandwich core used is a key consideration in assessing the reliability of such structures in their intended space radiation environment. Objectives The primary goal of this project was to observe the effects of simulated cumulative 50-year (maximum) total ionizing dose (TID) due to SPE and GCR radiation for inflatable and composite habitat spacecraft materials NASA may use. This timeframe reflects the staging of equipment and prolonged usage that may occur, while also providing some margin for error. A cumulative 10-year (maximum) Martian mission TID due to SPE and GCR radiation was also simulated for spacesuit materials. By determining the effect of the TID on material performance after irradiations in appropriate atmospheres, such as inert gas (simulating vacuum) and air (simulating spacecraft cabin environments at a worst case), conservative service lifetimes are established. The layers of most concern in an inflatable habitat are the restraint and bladder layers. The layer of most concern in a multifunctional composite habitat is the adhesive bond line between the composite face sheet and the underlying micro meteoroid and orbital debris (MMOD) arresting layer. The layer of most concern in a spacesuit is the outer orthofabric, or rip-stop. Accordingly, the radiation resistance of these layers will be the main focus. HZETRN, a one-dimensional transport code used to calculate the TID in materials, was used to calculate the 50-year dose for ISS composite overwrapped pressure vessels (COPVs) in low Earth orbit, the 50-year dose for inflatable and composite habitats used on the Martian surface and in the deep space mission environment, and the 10-year dose for spacesuit materials used on the Martian surface. For example, HZETRN results show that doses from GCR radiation are in the vicinity of 700 cGy (700 rad) for an inflatable bladder or restraint layer over a 50-year deep space duty cycle. While this dose may seem small, it must be realized that GCR nucleons have highly penetrating energies on the order of 1 GeV and greater, and contain a spectrum of heavy ions that can be very damaging due to their larger nucleon size. The SPE exposure is much greater with absorbed doses up to 15,900 Gy (1.59 Mrad) in the outer deployment system, 117 Gy (11,700 rad) in the restraint layer, and 103 Gy (10,300 rad) in the bladder layer of an inflatable habitat over a 50-year deep space mission. A few mission or service lifetime durations less than 50 years were also considered, as appropriate, for the material under investigation. The experimental portion of the project involves exposing inflatable test specimens to a terrestrial radiation source that approximates GCR and SPE space radiation at a ground-based test facility. The first choice is the Brookhaven National Laboratory NASA Space Radiation Laboratory (BNL NSRL). The facilities at BNL NSRL allow both separate and combined SPE and GCR effects to be investigated. Once results are acquired on inflatable materials (1st priority), irradiation of habitable composite structures and space suits materials (2nd priority) will be considered. The Martian atmosphere and breathable cabin air can also inhibit or accelerate property changes in irradiated parts. These effects were investigated to a limited extent. Controls were implemented, or the experimental design altered, such that artifacts such as radiation-induced oxidative degradation, were controlled or eliminated. For example, test specimens can be conditioned, irradiated and stored in the anticipated atmosphere (air or CO2), or conditioned, irradiated and stored in an inert atmosphere (Ar or N2) to simulate exposure to vacuum. Knowledge Gained Materials characterization work was performed at NASA JSC (Valle, Shariff, Peters, Hussain and Litteken) and JSC's White Sands Test Facility (Waller and Nichols) to provide materials characterization data. Properties evaluated include vacuum permeation rate, tensile strength, as well as room temperature, -30 °F, and -50 °F flexural data. Identical tests were performed on irradiated materials to measure changes in performance. Other results to date include hypervelocity impact test data showing degradation in the ballistic performance of irradiated MMOD protecting layers used in inflatable habitats. Spectra®, which is an ultra-high molecular weight gel-spun polyethylene, is the likely choice for the outer layer in future NASA space suits due to its high strength-to-weight ratio, abrasion and cut resistance, and dimensional stability. To ensure space suits can withstand mission radiation doses, it is important to characterize the physical and mechanical property changes caused by scissioning and cross-linking reactions accompanying irradiation. However, Spectra® is difficult to break in conventional tensile tests. To overcome this difficulty, the team collaborated with Honeywell, and the Crew and Thermal Systems Division at JSC to examine the tensile properties of Spectra® and other space suit materials by blunt probe puncture testing and single fiber tensile testing after exposure to particle radiation simulating GCR and SPE space radiation at BNL NSRL. The doses chosen were based on a simulation performed by the Project Management and Integration Office at JSC for the expected space suit duty cycles in a Mars reference mission. Spectra® fabric exposed to a radiation equivalent of 2x, 10x, and 20x a Mars mission duty cycle allowed a wide range of potential radiation effects to be evaluated. A paper was presented to the International Conference on Environmental Systems in FY17 detailing the results of this work, making recommendations about material suitability or the need to implement engineering controls for space suit pressure garment materials in both nominal and worst case duty cycles. The team also released a draft protocol for Qualification of Spacecraft Materials for Space Radiation Environments and its final report to the NASA JSC Technology Working Group review committee who funded this task. Another conference publication was presented in FY17 detailing the effects of particle radiation on NanoSonic inflatable materials-of-construction, with and without a self-healing gel. Results discussed will include cryo-puncture tests, tensile tests (in a temperature-controlled environmental chamber), rheology measurements, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Future Work Transport code modeling procedures, irradiation protocols, material test methods, and industry-government contacts have become well-defined over the course of this effort. This provides a new capability for integrated material qualification that will provide a direct benefit for spacecraft, especially those operating outside the relative safety of Earth's magnetosphere. A wide variety of novel candidate materials and material test methods were evaluated or considered by the team, some of which could not be evaluated due to scheduling and/or funding constraints. All considered, the team is in an excellent position to accommodate follow-on work that is likely to result in FY18 and beyond. Now that testing is complete, the need to implement any engineering controls to assure mission safety should be assessed. Future work may include developing strategies to reduce cabin dose by investigating multifunctional shielding materials, reformulation of polymers and composites to enhance their radiation resistance, and examining the combined effect of radiation and physical aging. More »