Space Guardians: Radiation Shielding for Living among the Stars
Written by Ashvin Verma, President of New Delhi Space Society, and edited by Rudraneel Sinha, Vice President of New Delhi Space Society. Both have participated extensively in space settlement design contests, and want to share all they’ve learnt about.
Humanity will soon embark on a long, arduous journey towards living among the stars, starting with NASA’s Artemis mission to return to the Moon, this time to stay. But are there any challenges that make these missions substantially different from our past missions? After all, we’ve been to the Moon many times, and individuals have stayed in the ISS for years.
I consider space radiation protection to be such a challenge, along with the technology for complete self-sufficiency for space settlements, and avoiding medical consequences of living in non-Earth gravity. I’ll be looking at the problem of radiation here, on why we need radiation protection, old and new ways of accomplishing it, with a focus on modern and futuristic “passive” shielding methods, and challenges that lie ahead.
But first, what even is Radiation?
Put simply, radiation is energy that travels through space either as light (photons) or matter (electrons, neutrons, etc.). Though popular media and cultural situations have attached a terrifying connotation to the term, radiation is not really unfamiliar to us, we just often neglect the unharmful kinds around us. Your microwave oven uses “microwave” radiation to heat food up, your phone uses radio waves to communicate, and even the light from bulbs and screens is a form of radiation; but then why can we mostly use these devices safely in our daily lives, but not waltz out into space with a zorbing ball?
https://forms.gle/xwbRGFx9fGyASD5B9The key lies in the strength of the radiation; the radiation from our daily surroundings is mostly pretty weak and we don’t particularly need any protection, except some sunscreen for UV light, whereas the radiation in space is fairly strong and said to be “ionizing”, i.e. it can penetrate and rip apart atoms and materials (blocked for us by our friendly neighbourhood planetary magnetic field). These include X-rays, gamma rays, electrons, and neutrons. The central issue is that ionizing radiation causes irreversible damage to our cells and DNA, and drastically increases the odds of cancer, and other diseases [1].
What do we know about Radiation right now?
Currently, our knowledge about radiation comes from two major sources: 1) A-bomb survivors who received high radiation doses in a short period, and 2) astronauts who received low radiation doses over a longer period. Their effects range from radiation burns to cataracts and infertility to heart disease and, very often, various forms of cancer unfortunately. The chance of experiencing these effects, and often how severe it becomes, depends highly on the radiation dose received, measured in Sieverts.
Now, if we consider any spot in our solar system far from a planet, we find lots of ionizing radiation from nearby stars and galaxies (Galactic Cosmic Radiation) and high-energy particles from the sun (solar wind) acting as a source of constant bombardment, leading to an exposure of over 350 milliSieverts in a year for an average human[2]. For this reason, in missions with a time-span of years or even human lives, it is necessary to minimize the risk of developing radiation-related illnesses and injuries in space to a minimum or almost zero. This is achieved mostly through radiation shielding.
How do Radiation shields work?
Firstly, there are two types of radiation shields, firstly, we have active shields which use energy to repel radiation, and secondly, there are passive shields that use an assortment of materials to absorb and defend against radiation, though in real applications, only passive shields have proven to be practical.
Finally, coming to the ways passive radiation shields work, we first need to decide what it does exactly. A shield needs to stop a spectrum of different kinds of radiation while not producing much “secondary radiation” in the process (more on that later). Now, we can go through possible shielding mechanisms, discuss how they fulfill these objectives, and figure out the basics of radiation shielding design:
Conventional Method
Firstly, we have the intuitive approach, what you might picture when imagining how to stop something from coming through: a wall. And that is somewhat effective, true. We can use a thick layer of a material to block radiation, just like how concrete walls in our house block light and WiFi signals (partially). We have actually been doing something similar for decades by using lead–in medical facilities, nuclear power plants, and military equipment, and it works pretty great. Lead is an amazing absorber of high-energy radiation, namely X-rays and gamma rays, because of its packed and dense nucleus, and corresponding high atomic number. So why isn’t it the solution to our problems?
Of course, one factor is that lead is toxic, but that’s not really an important concern as long as it's sandwiched between other materials and there’s no direct exposure. There is another catch though: these same properties make it a poor shield for other types of radiation, including high-speed electrons (beta radiation), which rip the shield apart and actually create more radiation than what had hit the shield to begin with, through a domino effect. This “secondary radiation” produced through the collision of radiation with the shield is called bremsstrahlung radiation, high-energy photons zipping through space.
On the other hand, light and low atomic number elements and compounds like water—the stark opposite of lead—are fantastic at avoiding secondary radiation and absorbing neutron and electron radiation. They’re not good at absorbing gamma and X-rays, though, so they aren’t a complete solution to our problems either.
Concrete is actually a quite economical and commonly available material that functions as a decent radiation shield, having water, cement, and heavy metal ions, but its weight makes it prohibitive to use in space applications.
Why use only one material though, when you need the properties of multiple to truly be safe in space?
After all, as the renowned scientist Rosalie Bertell once said,
there is no safe level of exposure to ionising radiation, and the search for quantifying such a safe level is in vain.
A low-cost but heavy alternative for usage on our moon, Luna, maybe the lunar regolith, or lunar ground material a few meters deep, which has a fine mix of compounds which can be made into lunar concrete, “lunarcrete”, and act as a simple radiation shield. This has been explored as a viable option for future lunar bases.[3]
But we must have an efficient and practical solution for spacecraft and settlements which orbit or travel through space, which is exactly what engineers and scientists have come up with.
Modern Developments
After decades of experimentation, scientists figured out a way to combine the properties of materials, inventing the Graded-Z shield: a stack of materials in decreasing order of their atomic numbers. This provides balanced protection across types of radiation, while the secondary radiation produced by the first few elements is absorbed by successive low atomic number elements. It’s up to 60% more effective than single-material shielding.[4]
Usual combinations start with heavier elements like lead, to copper and aluminium, and sometimes light hydrogen compounds, including boron carbide and potentially Boron Nitride Nanotubes.
It’s not perfect though; it’s somewhat thick and requires many layers, increasing manufacture and transportation complexity. But we can do better.
An Advanced Approach
But there’s another clever way to combine the properties of different materials: mixing them together in the same layer.
Scientists are doing this through a process of “doping” higher-Z materials in an epoxy resin matrix of low-Z materials. Phew, that was a mouthful. But all that really means is that they are dispersing particles of heavier elements (such as tungsten) in the spaces between the polymer solids of lighter elements (such as high-density polyethylene). These are lightweight, and have amazing physical and radiation protection properties.[5]
These composite-Z, and more advanced graded-composite-Z, shields currently need a whole lot more development and testing, but they seem promising for our future in space.
These incremental developments of technology push us faster towards a brighter future, when after waking up, we can safely look out and see a world different from the one we were born on. And, perhaps, you can play a part in making that happen.
Feel like joining a like-minded group of space enthusiasts and scientists? Join New Delhi Space Society here: https://forms.gle/xwbRGFx9fGyASD5B9
Citations
[1] Christensen, D. M., Iddins, C. J., & Sugarman, S. L. (2014). Ionizing Radiation Injuries and Illnesses. Emergency Medicine Clinics of North America, 32(1), 245–265. https://doi.org/10.1016/j.emc.2013.10.002
[2] Globus, A. (2017). Radiation Paper. Al Globus. http://space.alglobus.net/papers/RadiationPaper.pdf
[3] Meurisse, A., Cazzaniga, C., Frost, C., Barnes, A., Makaya, A., & Sperl, M. (2020). Neutron radiation shielding with sintered lunar regolith. Radiation Measurements, 132, 106247. https://doi.org/10.1016/j.radmeas.2020.106247
[4] Fan, W. C., Drumm, C. R., Roeske, S. B., & Scrivner, G. J. (1996). Shielding considerations for satellite microelectronics. https://doi.org/10.1109/23.556868
[5] More, C. V., Alsayed, Z., Badawi, M. S., Thabet, A. A., & Pawar, P. P. (2021). Polymeric composite materials for radiation shielding: a review. Environmental Chemistry Letters, 19(3), 2057–2090. https://doi.org/10.1007/s10311-021-01189-9
