Every decade, a new promise is made to send humans deeper into space to establish colonies, and at the end of every decade, that promise is broken.
There are quite a few reasons for this. Some may point to challenges in funding, and although that is true to a degree, there is a much more obvious factor that is often overlooked: space is hard. It is a completely foreign environment that is hostile to humans in every way imaginable, and no amount of money will allow us to immediately solve the challenges that it throws at us. From extreme temperature fluctuations to microgravity-induced health issues, the list encompasses every field of science, and strangely enough, the most dangerous of these threats might just come in the form of tiny, subatomic particles that have a devastating power to destroy life: radiation.
Historically, human spaceflight missions have placed little importance on radiation because they have all been within the protective environment of Earth's magnetic field. For instance, on the ISS, there is only a baseline level of radiation protection built-in to the standard module walls because it just isn't necessary. The danger to astronauts is limited, and scientists have found that as long as ISS astronauts don't exceed a certain amount of flight time, their risk of long-term radiation-associated health conditions like cancer will be minimally impacted. In the worst-case scenario of a solar flare, astronauts would have a small amount of time to reach more heavily shielded sections of the station because a robust network of space-weather satellites is already in place in regions near Earth.
The only exception to the trend of manned missions that stay within Earth's safety net is the Apollo program, but in that case, radiation wasn't considered for a different reason. Since the Apollo missions were less than two weeks each, the radiation exposure to astronauts was minimal to the point where it was largely insignificant. However, if there had been a solar flare, that would be another story. In fact, there was a close call in 1972 when a large sunspot erupted repeatedly for 10 days, but luckily, that occurred in between the Apollo 16 and 17 missions. Unfortunately, luck isn't something we can rely on for future missions beyond Earth orbit.
There are quite a few reasons for this. Some may point to challenges in funding, and although that is true to a degree, there is a much more obvious factor that is often overlooked: space is hard. It is a completely foreign environment that is hostile to humans in every way imaginable, and no amount of money will allow us to immediately solve the challenges that it throws at us. From extreme temperature fluctuations to microgravity-induced health issues, the list encompasses every field of science, and strangely enough, the most dangerous of these threats might just come in the form of tiny, subatomic particles that have a devastating power to destroy life: radiation.
Historically, human spaceflight missions have placed little importance on radiation because they have all been within the protective environment of Earth's magnetic field. For instance, on the ISS, there is only a baseline level of radiation protection built-in to the standard module walls because it just isn't necessary. The danger to astronauts is limited, and scientists have found that as long as ISS astronauts don't exceed a certain amount of flight time, their risk of long-term radiation-associated health conditions like cancer will be minimally impacted. In the worst-case scenario of a solar flare, astronauts would have a small amount of time to reach more heavily shielded sections of the station because a robust network of space-weather satellites is already in place in regions near Earth.
The only exception to the trend of manned missions that stay within Earth's safety net is the Apollo program, but in that case, radiation wasn't considered for a different reason. Since the Apollo missions were less than two weeks each, the radiation exposure to astronauts was minimal to the point where it was largely insignificant. However, if there had been a solar flare, that would be another story. In fact, there was a close call in 1972 when a large sunspot erupted repeatedly for 10 days, but luckily, that occurred in between the Apollo 16 and 17 missions. Unfortunately, luck isn't something we can rely on for future missions beyond Earth orbit.
The issues with radiation all revolve around just how difficult it is to stop. We've already mentioned that scientists have a basic space weather network at their disposal, but the problem with this is that forecasting only applies to solar radiation, which is generally less dangerous than the other major form of radiation: galactic cosmic rays. This type of radiation is much more energetic, and because it is produced by stars and galaxies all around the solar system, it would pose a constant threat to any mission leaving Earth's protective reach. If the sheer thickness of walls was used as a protective barrier against cosmic radiation, an absurd amount of material would be required, and consequently, the spacecraft's mass and cost would rise dramatically. Therefore, the only option left is to use materials that are more efficient at blocking high-energy protons and neutrons. As it turns out, the best way to fight fire is with fire, or in this case, fight protons with protons.
Since protons and neutrons are similarly sized, they can both be stopped with the same materials; the higher the concentration of protons and neutrons in the shielding material, the more effective it will be. One such element is hydrogen, the most abundant element in the universe. Though it may seem counter-intuitive to use such a small particle, it makes sense from a subatomic perspective because hydrogen atoms are just pairings of a proton and electron, and the limited count of electrons means that less “radiation-blocking volume” is wasted per atom. However, it would not be feasible to make spacecraft walls out of hydrogen, so scientists have narrowed their search to compounds that contain the element in large ratios.
A simple solution that has been proposed is to use water. If a spacecraft was designed with water tanks that wrap around its hull, not only would it serve life support purposes, it would also be a highly effective shield against radiation because of its 2:1 hydrogen to oxygen ratio. Unfortunately, incorporating water tanks into spacecraft hulls poses some serious logistical issues. For one, water levels in the tank would need to remain relatively constant, so a highly effective life support system capable of pumping purified wastewater back into the tanks would be required. Additionally, the structural stresses imposed on the hull by a wraparound tank would also need to be studied much more closely. The primary benefit, however, is that scientists already have plenty of experience managing water in space, so any remaining issues would largely be optimization problems.
Another possibility is to use polyethylene, which is the same type of plastic used in grocery bags. This material ticks the boxes of “easy to produce”, “high hydrogen count”, and “inexpensive”, but it also comes with its own set of structural challenges. Polyethylene alone would not be strong enough to replace a metal wall, and if it was used as an insulative layer, spacecraft mass would go up tremendously. One situation where this material could be used is in windows; NASA has already made the switch to acrylic windows in its Orion crew capsule, so incorporating polyethylene into plastic windows in the future is not out of question.
Even though both water and polyethylene are quite promising, some scientists have decided to focus on developing custom materials that would maintain radiation-blocking properties while also being stronger, lighter, and more thermodynamically capable than aluminum, the current standard for spacecraft construction. For instance, NASA scientists are currently working on a material known as hydrogenated boron nitride nanotubes (BNNTs). We have already heard the promise that nanotubes will make fantastical structures like space elevators feasible, so there is no question about the strength of BNNTs, and by impregnating them with hydrogen, it will also be able to block radiation. In fact, hydrogenated BNNTs have been turned into yarn, so the material could also be used in spacesuits. This is a significant development because future Martian expeditions will require astronauts to spend lots of time outside protective shelters, so shielding will not only be needed in habitats but also in the spacesuits themselves.
While all of the materials discussed above are being studied for use in the relatively near future, some thought has already been put into large-scale radiation shielding that will be required on permanent colonies in space. For instance, the forms of shielding that would apply to a 6 person, 3 year round trip to Mars wouldn’t necessarily be scalable to a martian city of thousands of people. In theory, the entire city could be built with materials like hydrogenated BNNTs, but that would be limiting in terms of the design and size, so scientists have looked to Earth for inspiration.
Our planet’s magnetic field is essentially a massive force field that deflects charged particles coming from space, and it is thought that without it, complex life may not have evolved. However, the idea of creating an artificial, localized magnetic field isn’t completely in the realm of science fiction as you might think. Scientists are already working on miniaturized versions that would generate a small protective bubble, and from there it is only a matter of scale. However, this technology is not likely to be ready anytime soon, but luckily, large-scale cities in space won’t come so quickly either.
An alternative to artificial magnetic fields that traces its roots from ancient times instead of sci-fi involves a physical barrier of regolith, which is just extraterrestrial soil. You might even say that this solution is dirt cheap! For the foreseeable future, this might be the only feasible way to protect astronauts in permanent colonies because it is such a simple idea: habitats would either be built underground in naturally existing structures like lava tubes, or they would be covered with soil on the surface. Either way, the effect would be the same. If enough regolith is used, not only would astronauts be safe from solar radiation, but they would also be protected against high-energy cosmic rays. So in the end, the only way to go up into space may be to go down underground!
Regardless of how the issue of radiation is tackled for spacecraft and colonies, it will undoubtedly require a great deal of creativity and innovation. It's possible that none of the mitigation methods discussed above will be used, but it is also highly likely that a combination of methods will be used. After all, both proactive measures like shielding and reactive measures like medication will need to be developed because there is no telling what space might throw at us. Just like how a tiny virus is ravaging our planet right now, even smaller particles may be our strongest enemy in space, and unless we can create a “vaccine” for radiation, humanity may be stuck in the confines of near-Earth space.
Sources & Further Reading
NASA solar flares
NASA radiation mitigation