Blood concrete, water shields, and thin atmospheres; Can a Mars mission overcome the hurdle of radiation?
- Cosmic Arc
- Jul 14, 2024
- 7 min read
Charged particles on the Martian surface captured by the Curiosity rover during a solar storm earlier this year (Credit: NASA)
During the early days of manned spaceflight, little was known about what happens to the human body in long-duration missions or even if a mammal could survive the alien environment of outer space. This was just one of the many mysteries the USSR and the US had to uncover as they began to send people to Low-Earth orbit and beyond. For the first voyages to orbit on Vostok and Mercury, there wasn’t much information to work with regarding the effects of extended space travel on the human body, It was only with the advent of large space stations like MIR and the ISS that we gained the opportunity to examine what future may lie ahead for space explorers.
The effects of microgravity on the human body are somewhat well known now with the key data we acquired from ISS and MIR experiments, such as the twins study, where astronaut Scott Kelly spent a year aboard the ISS while his twin brother, Mark Kelly, remained on Earth so they could be compared once Scott Kelly returned to Earth. However, what’s less known about long-duration spaceflight is the effect of radiation on astronauts.
Astronauts on the ISS for a 6-month stay usually receive an ionizing radiation dosage of about 50 to 2,000 milli-Sievert (mSv) [1]. For comparison, 0.3 mSv is roughly the amount you get from a chest X-ray and, according to the World Nuclear Association, 1.5 to 3.5 mSv is the annual dose someone here on Earth gets from natural background radiation [2]. As with many things in space, they aren’t as simple as they seem, as astronauts on interplanetary journeys to Mars would be traveling beyond the protective magnetosphere of Earth, bringing up new challenges involving different forms of radiation.
There are 2 main types of radiation interplanetary crews would face on their journey, solar radiation and galactic cosmic rays (GCR).
Solar radiation refers to periods of intense activity on the sun, such as solar flares and coronal mass ejections (CME) that are very difficult to predict. This is not only a danger to the crew but can also damage electronic systems on any spacecraft we send, not an unlikely scenario given that something similar has happened before with Earth’s power grid, triggering disruption and blackouts. The second form of radiation is much more prevalent and omnidirectional; galactic cosmic rays are particles originating in interstellar space from other stars, they’re high-energy and could easily pass through a spacecraft's hull.
The main thing we worry about with such missions isn’t the possible scenario of a crewmember immediately succumbing to the effects of radiation sickness halfway through the mission, with mere days to live, but the increased risk of developing cancer that could arise perhaps decades after the mission [3], this risk becomes even more severe if you would like a future of hundreds of thousands of ordinary people traveling between Earth and Mars regularly. Not every human body can tolerate the same levels of ionizing radiation.
Solar flare captured by the Solar Dynamic Observatory (Credit: NASA).
Radiation shielding on Earth is actually not that complicated, a large enough mass of matter can stop most forms of radiation from reaching you. We see this with solid slabs of lead metal used to store radioactive elements or even very thick layers of concrete could do the trick. However, we can't just add as much mass as we like to our vehicle, most spacecrafts are very lightweight, allowing for more important payloads like people, and adding more mass means adding more fuel which also contributes to our mass. So we turn to more efficient rather than more massive forms of shielding, which use mass already onboard the spacecraft.
Water is a necessity for the crew, constantly filtered and replenished to keep a stable supply throughout the journey, which is exactly what we do on the ISS today, extracting water from astronaut waste or even humidity in the cabin air. Water is commonly used as radiation shielding here on Earth, especially against neutron radiation, a form of radiation we will encounter on any Mars mission.
Water is mostly hydrogen, a single proton element, which is almost the exact same weight and size as a neutron, absorbing its energy and protecting the crew inside [3], lining the inner walls of the crew quarters would be many tanks of water, used up and recycled over the course of the mission. It's possible that if our Mars mission architecture includes in-situ (extracting resources already on Mars) utilization of water on the Martian surface, we could replenish the shield for the journey back to Earth.
Illustration of a spacecraft using water shielding.
Another plan calls for the use of recycled polyethylene, a plastic from the ship's trash, as a form of radiation shielding [3]. It contains high amounts of hydrogen as a polymer of ethylene, with the chemical formula of C2H4 and it's a relatively lightweight and cheap to produce material. The major downside is its inability to make large durable structures, so this is only a material you could panel on a hull made of regular rigid aerospace-grade materials. There's also the option to heavily reinforce shielding on the walls of the crew's beds, where they will be stationary and occupy a small space for a decent duration of their transit. Coupling plastic and water shielding, would dramatically reduce the risk of the crew developing cancer later on in their lives, although you'd need to make sure the specific plastics in the plating you 3D print out won't react with radiation and become toxic or degrade over time.
Once you reach the Martian surface, the threat of radiation still exists but is far lesser than with the vulnerable vacuum of interplanetary space. Mars doesn't possess a magnetic field as strong as Earth, despite this, Mars’ thin atmosphere still blocks out a good portion of radiation coming from space. Case in point, the Curiosity rover’s Radiation Assessment Detector (RAD) instrument found that an astronaut would usually receive 0.7 mSv per day of radiation (~250 per year) which may not be too different than the environment of the ISS [4]. Adding to Mars’ natural protection, we'd ideally build the first Martian camps underground in expansive lava tubes or surround the habitats inside a dome of Marscrete (Martian concrete).
Martian regolith simulant is the closest thing we have on Earth to the real thing without actually going there and collecting it. We've been able to create a concrete material out of the data we have from surface missions, which will come in handy for shielding habitats from dust storms but also radiation, especially in periods of high solar activity, where the crew could quickly flee to the most heavily shielded portion of the base (usually its center). Several concepts have been proposed, some involving using trash, such as parachute fibers, to bind the concrete together or even using astronauts' bodily fluids as an ingredient. AstroCrete is a concept from 2021 that uses the human serum albumin protein, found in human blood, as a strong binder to Marscrete, even urea extracted from urine could be used to increase the compressive strength of Marscrete to be 3 times greater than normal Marscrete [5].
The Martian moons of Phobos and Deimos appear to be C-type asteroids, which could contain water ice, useful for general usage by people and as fuel for spacecraft [6]. Their likely porous structures would be thick enough to protect a space station embedded within the moon, possibly even situating itself in a large deposit of water ice (if such deposits do exist) as another layer of protection from radiation. Rocks left over from ice mining operations and trash could be compressed down into compact blocks acting as shielding from radiation and small debris impacts, which would be numerous due to base activity on their surfaces, perhaps even warranting covering large sections of the moons with blankets to keep dust from being ejected. Deimos is small enough that you could realistically hollow out its entire volume once you have thousands of people working on and around Mars. Compacting the loose shell with trash, covering it under the before-mentioned blanket, and stretching the inside into a sphere with a large pressurized compartment placed inside, you would have effectively turned Deimos into a mini-Earth. Spinning on its axis to produce “spin gravity” on an inhabited equator surrounded by latitudes of different plants which may grow more efficiently at certain lower accelerations protected from radiation by the shell.
Phobos over Mars as seen by Mars Express (Credit: ESA)
In the far future, when we would like to have millions living on the red planet, we'd use technologies unavailable to us at this current time. Efficient fusion engines could dramatically cut travel times to a few weeks, reducing total exposure time in interplanetary space, perhaps such crafts may repurpose their engine's magnetic field as a shield in transit, although this may prove power intensive. Aldrin cyclers, massive networks of hundreds of space stations encountering Earth and Mars periodically via gravity assists, could have hulls as thick as needed to protect their colonists. Mars may also become “semi-terraformed” where the atmosphere is thickened enough to shield against even more radiation, whether or not true, complete terraforming of the entire planet is possible is up to debate.
A more out-there proposal has been the idea of genetically engineering astronauts or even inventing some highly potent cancer cure to completely bypass the issue of radiation itself. This is outside of our technological reach as of now and the first crewed Martian missions wouldn't use such methods on their crews, but the question remains. However, if Martians on timescales of thousands of years would eventually genetically augment themselves to survive the harsh environment of Mars, still it's speculation. Whatever oppressive conditions we face out there in the many different worlds of the solar system, humanity will find a way through them, one way or another.
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