My last post briefly discussed the anticipated radiation dose from Galactic Cosmic Rays. More specifically, NASA calculates that future travelers to Mars will receive a dose of 660 milliSieverts during their one-year return trip. Since we colonists have no intention (or means) of returning, we can expect to receive a 330 milliSievert dose on our six-month one-way trip. That represents one-third of NASA's recommended lifetime dose, which they estimate will increase one's cancer risk by 3%. It's a dose that we should avoid to the greatest extent possible, but it's not a show-stopper by any means compared to all the other risks our colonists will face.
But then there are solar flares, possibly accompanied by a Coronal Mass Ejection and/or a Solar Particle Event. If we do encounter one of these events, what can we expect? Here's a link to an interesting article by the US National Institute of Health, where researchers tormented many small pigs to determine radiation health effects, and which peripherally discusses expected dose rates based on historical events. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3831828/#R5 If we encounter an event similar to the last three 'extremely large' events, we can expect up to 32 Grays outside the ship and 5 Grays inside the ship. Grays and Sieverts can be quite different depending on the energy of the individual particles, but in a Solar Particle Event the particle energies are in a range that makes the two scales virtually identical. We can substitute 'Sieverts' instead of 'Grays' and we won't be far off.
The article doesn't specify mortality rates, but 32 Sieverts will kill you, period. Don't go outside. 5 Sieverts inside is a very interesting number, right in the middle of the presumed 50% human mortality range. We're all going to get very sick. Half of us will live, and the other half will die. Here's a set of graphs depicting dose-rate mortality at Chornobyl and on a lab-rat study, taken from "Radiation and Reason. The Impact of Science on a Culture of Fear" by Wade William Magill Allison.
The lower graph indicates that the first Chornobyl worker died at 3 Sieverts, everyone died at 7 Sieverts, and the 50% mortality range was 5.5 Sieverts. The lab rats show a virtually identical mortality curve but shifted 2 Sieverts to the right, with the first rat dying at 5 Sieverts, all of them dead at 9 Sieverts, and the 50% mortality range at 7 Sieverts. So rats are better suited to this space travel thing. If something needs to be done outside the ship, send a rat. Tell them there's food. They'll go.
Below is a table that I liberated from an MSc (Aerospace Engineering) thesis at CalTech by Nathan Gehrke. Nathan's thesis is entitled "Utilizing Permanent On-board Water Storage for Efficient Deep-Space Radiation Shielding", so it is very pertinent to this discussion.
This table describes the reduction to radiation inside a spacecraft during a "Carrington-Class" solar super-storm, a coronal mass ejection similar in severity to the one in 1859 during Solar Cycle 10 that remains the largest-ever recorded event. The author estimates that the addition of 12 cm of water would reduce Solar Particle Event radiation by 55%, and if you put the water in plastic containers you would get an extra 5% reduction. That would reduce our 5-Sievert dose down to 2 Sieverts, meaning that nobody dies, but you still have a spacecraft full of sick people. Of course, you could add more water, or something else like polyethylene or lithium hydride blocks, but if that becomes a limiting factor because of mass or space requirements, we can look inside the spacecraft to see if there's anything we can do with those squishy pink humans to reduce the radiation impact. And there is.
Firstly, instead of armoring the spacecraft, we can armor the people. NASA is working with a private company named StemRad to develop anti-radiation vests, and the technology is very promising. The company currently manufactures vests for the nuclear industry, and they are modifying them to be more suitable for space travel. I have included a graph below depicting the impact of their vests on mortality based on the Chornobyl and Fukushima events.
Several interesting things here. They peg 50% mortality at 4 Sieverts, with the caveat that the survival rate depends on the energy of the individual particles. This varies quite a bit from the 5.5 Sieverts quoted in the Chornobyl-only reference above, but the variation probably stems from the variation in individual particle energies in Chernobyl, Fukishima, and Solar Particle Events (Wikipedia lists human LD50 at 4.5 Grays). Note that there is no mortality when wearing the vests, even at 6 Sieverts! The numbers for predicted cancer incidence reflect very significant reductions, based on modeling of how many Sieverts reach the organs.
Vests. Who knew?
The Astrorad Variation of Stemrad, Currently Under Development.
We don't have hard numbers for the performance of the new vest (the Astrorad), but we will soon. 'Matroshka' mannequins developed by the German Aerospace Center mimic the human body and contain thousands of radiation sensors throughout, and two of them were on the first Orion flight around the Moon in November 2022, one with an Astrorad vest and one without. We're all waiting for the data to be published! The technology is very promising, so if it ends up not being this vest it will be another one. Check out the Astrorad website at https://stemrad.com/astrorad-4/
Helga and Zohar with the Astrorad vest that went around the Moon.
And then there are drugs.
Entolimod (CBLB502), under development by Cleveland Biolabs, is both a pivotal ant-radiation drug as well as an anti-cancer drug. Animals received lethal whole-body radiation doses both before and after the drug was administered, with the following result: "These studies demonstrate that a single administration of entolimod either before or after lethal total-body irradiation leads to significant improvement in animal survival. Entolimod has been shown to reduce radiation damage to both hematopoietic and gastrointestinal tissues and improve tissue regeneration."
Another drug, Ex-Rad (recilisib sodium), developed by Onconova Therapeutics, is both a protection and a treatment to reduce radiation effects by limiting DNA damage. "A study in mice demonstrated the efficacy of Ex-Rad by increasing the survival rate of mice exposed to typically lethal whole-body irradiation. The study tested oral and parentarel administration of Ex-Rad for both pre- and post-exposure radiomitigation."
Both drugs have been dose-tested on humans, and are well-tolerated within specific dosage ranges. It appears we have at least two new drugs that can significantly reduce the adverse effects of ionizing radiation and also reduce cancer risk.
In summary, Coronal Mass Ejections are still really scary, but even the worst of them are survivable. Use the water in your spacecraft as strategic shielding, wear radiation vests (probably at all times), and when a solar flare indicates an upcoming Solar Particle Event take your anti-radiation drugs. We can't know what the final radiation dosage will be under those circumstances, but according to my back-of-the-napkin calculations, these measures will reduce a 5-Sievert dose by at least an order of magnitude (10X) and possibly quite a bit more.
In closing, let's quickly go back to the beginning of this post where we briefly discussed the danger of Galactic Cosmic Rays. That is predominantly a long-term cancer risk, and a 330 milliSievert exposure might represent a 1-2% increase in the lifetime risk of cancer mortality based on the Linear No-Threshold Model. I have serious reservations about that model, (I am not going to make any further argument here, but I encourage you to explore the subject yourself. Here is one entrance to the rabbit hole
https://www.sciencedirect.com/science/article/abs/pii/S0009279722002691) but even if we accept the model without question, that is a pretty insignificant risk to a colonist facing all the dangers and rigors of a Mars colonization project. Having said that, we should take all reasonable measures to reduce that risk as much as possible, and it turns out they are the same measures we take to reduce the risk from the Sun. Strategically locate our water and methane to act as shielding, wear anti-radiation vests at all times, and take anti-radiation pills as prophylaxis. Again, the cumulative effect of these measures cannot be known, but a 2/3 reduction is a conservative estimate, bringing exposure down to perhaps 0.1 Sieverts for the one-way trip. Radiation is a real risk that must always be respected, but we have the knowledge and the tools to reduce exposure, and to turn it into a manageable risk.
That's it for space travel for now. It's time to start talking about the interesting stuff, surviving and thriving on Mars! As always, thanks so much for reading along!
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