“Space radiation.” For the average person, the term might conjure images of sci-fi battles and strange, extraterrestial worlds. For space exploration agencies and technology companies, though, it’s a big deal. In fact, it poses a very real threat to the humans and the sensitive technologies that get us to space in the first place.
Today’s space tech developers actively seek and develop advanced materials for spacecraft, CubeSats, spacesuits, habitats, vehicles, and more. The critical concern is a material’s resistance to space radiation. There’s a lot of it, and in order for space technology to advance and thrive, it has to be mitigated. Fortunately, today’s advanced materials address space radiation in a number of innovative ways.
What is Space Radiation?
The two radiation sources that pose the most serious threat to astronauts and space equipment are solar particle events (SPEs) and galactic cosmic rays (GCRs).
Solar radiation is exhausted from the sun primarily as a steady stream of protons. During low flux, a spacecraft can almost completely shield its contents from solar radiation. Occasionally, however, radiation occurs in large bursts via solar explosions (e.g., solar flares or coronal mass ejections). These explosive, high-flux events can create lethal doses of radiation over a matter of days (or even hours). In both cases, solar radiation modifies the deep space environment and has the potential to create harmful effects for humans over months and years.
The other, more concerning radiation source are GCRs. These are particles that travel close to the speed of light from other stars and galaxies. Like SPEs, GCRs are primarily protons, though they also can include high atomic number ions without electrons. When these heavy particles crash into materials, they create nuclear byproducts in the form of secondary neutrons and other products harmful to humans. GCRs are especially harmful to humans’ central nervous system cells and other biological tissues.
GCRs penetrate much deeper layers of shielding than, for example, X-rays. While one may assume that more shielding is better, this isn’t feasible for space missions as increased mass leads to increased fuel and costs. In fact, the lower the atomic number of a material, the better its radiation shielding capabilities for GCRs and SPEs.
The advanced materials industry is heavily intertwined with space agencies and tech companies in the search for optimal materials with structural integrity, thermal integrity, and adequate radiation shielding. Here are four radiation shielding innovations and investigations in today’s advanced materials industry:
1. Hydrogenous Composites
It turns out that typical radiation shielding materials (e.g., lead) produce more secondary radiation (i.e., secondary neutrons and other nuclear byproducts) than lighter elements such as carbon and hydrogen. Due to this, more attention has been placed on hydrogenous-rich composite materials. For example, polyethylene (PE) contains a large number of hydrogen nuclei and has a history of use as radiation shielding.
The low melting point of PE (around 100°C) prevents its use in spacecraft. However, recent findings show that custom hydrogenous-rich, high-tensile PE composites can be designed with effective radiation shielding, temperature, and mechanical properties. The radiation shielding properties of hydrogenous-rich composites exhibit improved radiation shielding abilities over aluminum, which is currently in use on the International Space Station (ISS). This discovery is promising: PE is cheap, recyclable, and in abundance (often found in the form of water bottles and plastic shopping bags). However, PE’s thermal properties and weight today remain hurdles for its adoption in spacecraft structures.
2. Boron Nitride Nanotubes
Boron nitride nanotubes (BNNTs)—particularly hydrogenated BNNTs—have attracted increased attention for use as radiation shielding due to boron’s effective absorption of secondary neutrons (i.e., neutrons excited as a result of contact with GCR and SPE particles). The added effect of hydrogen slows down high-energy secondary neutrons. The nanotube material is strong and has great thermal properties and chemical stability. In fact, NASA has experimented with BNNT-polymer composite materials for neutron shielding. BNNTs have even been made into yarn and exhibited enough flexibility to be woven into fabrics for spacesuits. The key elements of BNNTs—boron and nitrogen (and optionally hydrogen)—have smaller atomic numbers than aluminum, which is currently in use on the ISS. BNNTs were difficult to construct until recently when NASA developed a relatively new synthesis method called the “pressure vapor condenser (PVC) method.” NASA has reported interest in the ability of BNNTs incorporated into high hydrogen polymers and used as combination matrix resins for structural composites.
3. Living Composites
Certain fungi thrive in high-radiation environments. One example is Cladosporium sphaerospermum, which has been found growing in the Chernobyl nuclear power plant. Such fungi utilize radiosynthesis, which uses melanin pigments to convert gamma-radiation into chemical energy.
A recent study sent a specimen of C. sphaerospermum to the ISS to observe radiation attenuation effects for 30 days. The 1.7mm layer of fungus reported 2% less radiation compared to the control. While this is a small result, this shows the promising use of radiotrophic fungi in thicker layers for Martian settlements and more. Linear estimations suggest that a 21cm thick layer of C.sphaerospermum could negate the annual dose-equivalent of the radiation environment on the surface of Mars.
Interestingly, melanized biomass and pure melanin have ranked among the most effective radiation attenuators, emphasizing the great potential they hold as components of radiation shields to protect astronauts from GCRs and SPEs. “Living composites,” comprised of radiotrophic fungi and Martian or Lunar regolith, may be the key to forming adaptive, self-healing composites suitable for space missions. In fact, NASA has investigated the use of fungi for Martian settlements. The melanin content within these composites is limited due to the density of the fungal cultures; however, metabolic engineering and purification of melanin may work to increase the melanin content in such composites and enhance their radiation shielding effects.
4. PMMA Nanocomposites
Compared to high-density PE, PMMA has much more attractive mechanical and thermal properties. It scores slightly lower on radiation shielding; still, pure PMMA provides improved radiation shielding compared to most other materials (most importantly aluminum). Nanofillers can enrich a polymer’s mechanical properties and radiation shielding effectiveness, forming a polymer nanocomposite. Nanofiller materials, such as bismuth oxide (Bi2O3) and carbon nanotubes (CNT), in combination with PMMA, have recently been under investigation. The addition of CNTs as a nanofiller in a PMMA matrix reduces secondary neutron generation compared to pure PMMA. Furthermore, recent research shows that pure PMMA and PMMA/multi-walled CNT nanocomposite matrixes can be significantly lighter than aluminum with the same proton stopping power and improved secondary neutron protection. Similarly, a study focused on Bi2O3 nanofillers found that a PMMA- Bi2O3 composite presented promising gamma radiation shielding properties and increased hardness compared to pure PMMA. In addition, the composite was manufactured with a fast ultraviolet curing method, making it an ideal material for rapid manufacturing.
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