Radiation in space

“The best way to stop particle radiation is by running that energetic particle into something that’s a similar size. Otherwise, it can be like you’re bouncing a tricycle off a tractor-trailer.”

- Jonathan Pellish,
Radiation effects and analysis team leader, NASA.

Radiation is simply waves or sub-atomic particles that transport energy to another body (e.g. astronauts or spacecraft components). Energetic particles can be dangerous to humans because they pass right through the skin, depositing energy and damaging cells or DNA along the way i.e. an increased risk for cancer later in life or, acute radiation sickness during the mission. In order to send humans beyond the low earth orbit for exploration purpose, we will have to shield them against radiation. Following are some of the techniques that are being considered for this purpose, but we should also keep in mind that the technology that will actually be used is more likely to be a combination of different radiation shielding techniques rather than being just one single technology.

I] NANO MATERIALS TO STORE HYDROGEN:

Hydrogen, because it exists as a single proton and an electron, blocks protons and neutrons extremely well because they are similar in size. Nanotubes like Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) have been favored to store hydrogen because they have greater surface areas and higher hydrogen binding energies. Also, BNNTs are more efficient than CNTs because BNNT bonds, due to their ionic character, offer a 40% higher hydrogen binding energy than CNT bonds. How it works? Well, the point charges on the tube’s wall induce a dipole on the hydrogen molecule, resulting in more efficient binding. The diffusion of hydrogen is slower in small diameter than larger diameter BNNTs. Therefore, the hydrogen desorption temperature of small-diameter BNNTs is much higher, which is beneficial for use in high-temperature environments. Small-diameter tubes can also provide smaller pores among the bundles, allowing for more effective storage of hydrogen with a higher heat of adsorption. Experimentally, multiwalled bamboo-like BNNTs exhibited up to a 2.6% (by weight) hydrogen storage and collapsed structure BNNTs could store up to 4.2% (by weight) hydrogen, even at room temperature, which is significantly higher than for CNTs. The majority (95%) of the adsorbed hydrogen is safely stored up to 300°C–450°C. Another advantage is that defects on the BNNT wall can improve the hydrogen storage because the defects offer lower charge densities that allow hydrogen molecules to pass through the BNNT wall for storing them. CNTs and BNNTs can also improve other materials properties and make the resulting nanocomposite multifunctional, for example, CNTs can improve electrical conductivity and provide electrostatic control to mitigate spacecraft charging. Hyper-hydrogenated BNNTs, theoretically, can be created with hydrogen coverage up to 100% of the individual atoms. Therefore, higher hydrogen contents can be achieved by this approach however, a systematic experimental hydrogenation study has not yet been reported. Also, this approach might sacrifice, the other attractive structural and thermal characteristics. Therefore, only an optimum degree of hydrogenation can be practically achieved. Some more challenges of this method are, because the interaction between hydrogen and the host material is due to weak van der Waals forces, only a small amount can be stored under ambient conditions. If hydrogen storage using chemical hydrides and metal hydrides is considered for getting hydrogen into materials systems, it would produce fragmentation products during interactions with the incident radiation because they are heavy.

II] WATER:

Water is dense (1 g/mL), therefore you can pack a lot of atoms in between whatever you want to shield (e.g. astronaut’s body) and the out space exposed to radiation.

And, it is cheap. There is significantly less development cost when water is to be used for shielding purposes. Hence, a system that exploits water, a well-proven radiation shielding material, and a resource always available on board, to support exploration missions while achieving significant mass reductions is considered. How it works? Well, it consists of two main components, a radiation protection suit, selectively shielding the most radiosensitive organs, and a system of bags constituting a movable and modular protective water wall, which also acts as water storage. To save mass, all these elements are made of light polymeric materials, which can be drained and compacted when not used. The shielding elements are designed to sustain over-pressurization due to potential specific water loading interface conditions, to minimize the risks of water leakage inside the habitat. The design is based on a set of these polymeric bags, all interconnected, able to contain water and to maintain the desired thickness of shielding around the most radio-sensitive body parts. After the suit has been filled and used, it can be drained and the discharged water recycled, for conservation of resources. This technique offers an innovative shielding approach, complementary to whole habitat shielding, keeping in mind the necessity to optimize the on-board available resources. Personal shielding, combined with modular and movable water walls, offers promising perspectives in terms of radiation protection effectiveness and resources optimization. Also, it is a viable way for astronauts to perform emergency operations outside the safe shelter, preventing the onset of acute radiation effects, and at the same time mitigating long-term detrimental health effects. With the ASI funded PERSEO project, it was possible to test this concept for the first time in a weightlessness environment, showing that the mass of water is minimally impacting the astronauts’ movements. However, the challenge with these types of suits are in the domain of ergonomics and flexibility. Also, the suit does not cover all the required portions of the upper body, including arms, hips and thighbones. These types of suits are not yet tested beyond low earth orbits and hence require extensive data acquisition before determining the risk and reliability conclusions.

III] DIETARY COUNTERMEASURES:

Nutrients like vitamins C and A help against radiation by soaking up radiation-produced free-radicals before they can do any harm and hence prevent the radiation damage. Pectin fiber from fruits and vegetables, and omega-3-rich fish oils may be beneficial countermeasures to damage from long-term radiation exposure. Strawberries, blueberries, kale, and spinach prevent neurological damage due to radiation. Some drugs stimulate the immune system to “restore and repopulate” bone marrow cells after radiation exposure. Other drugs appear to reduce gene mutations resulting from radiation exposure. Drugs such as Radiogardase (also known as Prussian blue) that contain Ferric (III) hexacyanoferrate (II) are designed to increase the rate at which radioactive substances like cesium-137 or thallium are eliminated from the body. Although these types of drugs (radioprotectants) are now used to treat people exposed to radiation contamination on Earth, they may be good candidates for use on long duration space missions. One major challenge of this method is that when administered in effective concentrations, some radioprotectants also have limiting negative side effects such as nausea, hypotension, weakness, and fatigue. Since the Space habitats will most likely be shielded to keep out harmful amounts of ultraviolet radiation, normal vitamin D production in an astronaut’s skin is inhibited. To compensate, the astronauts will also require vitamin D supplements. Human epidemiology can be applied to space exposures; however, there are uncertainties related to the quality of radiation in space that is known to produce both qualitative and quantitative differences in their effect on the human body. Also, reducing the uncertainties in risk assessment is required before a deep space exploration mission is undertaken and these large uncertainties in risk will only be reduced by improving basic understanding of the biological processes and their disruption by space radiation. It is unlikely that the radiation risk problem for space exploration will be solved by a simple countermeasure, such as shielding or radioprotective drugs. Hence, a combination of different techniques will have to be considered for practical purposes.

REFERENCES:

https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars

https://www.cambridge.org/core/journals/mrs-bulletin/article/nanomaterials-for-radiation shielding/61DC5477B66C8EEE46E9B89518D3D114 https://www.cambridge.org/core/journals/mrs-bulletin/article/nanomaterials-for-radiation- shielding/61DC5477B66C8EEE46E9B89518D3D114

https://three.jsc.nasa.gov/articles/Shielding81109.pdf

https://ttu-ir.tdl.org/handle/2346/74180 

https://www.reddit.com/r/askscience/comments/18wyzs/why_is_water_such_an_effective_radiation_shield/https://www.nasa.gov/pdf/284275main_Radiation_HS_Mod3.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160004369.pdf