C (3:07)

Thank you so much for this invitation and opportunity. Gregory.

B (3:10)

Thank you. James, why did you write this article and who is the target audience?

C (3:17)

Well, I've been thinking about writing this article for a number of years, and it's a topic of great interest to me personally, and it's been a hobby, more or less, over the last decade or so. But interestingly, during the pandemic, I had an opportunity to write an entire textbook on the subject, and that textbook has come out. It's called Space Radiation, Astrophysical Origins, Radiobiological Effects, and Implications for Space Travelers. But as you might have imagined, during the pandemic, my colleagues and friends who were going to contribute chapters got sidetracked by more pressing, urgent issues and didn't have time to contribute the chapters that they initially promised they would. So I wound up writing the whole book myself. And ever since then, I've been absolutely fascinated and engrossed in the whole subject. And I thought it would be fun to contribute less intense, shorter version of the topic, review on Space Radiation for American Scientists. So I did that along with my colleagues Andy Karam and Robert Peter Gale over the last year. And it's been a fun little effort.

B (4:44)

Perfect. And yeah, I think you nailed it. What I found about the article was just how fascinating it was. I mean, as I alluded to in the opening, radiation is not the first topic you think about when, for example, traveling to Mars or even beyond our solar system. But clearly, as you lay out so well, it is something that we need to confront, and it sounds like we're in the early stages of doing that, and I want to talk to you about that. But as you allude to in the article and I did in my opening, early human space missions essentially to the moon were relatively protected from radiation compared to, for example, a potential mission to Mars. Why was that?

C (5:26)

Well, the earliest space exploration missions were basically at high altitude, but within our atmosphere and certainly within our magnetosphere. The magnetosphere is the protective cocoon, so to speak, that shields us from charged particles, a certain type of radiation that can cause wreak havoc on human cells and human bodies if unmitigated. But the atmosphere and the magnetosphere will protect astronauts from relatively low altitude space missions. And the earliest space missions were well within the magnetosphere. And currently the International Space Station, for instance, orbits at low Earth orbit. And that is also within the protective cocoon of our magnetosphere. But it's beyond our atmosphere. And so once you get beyond both the atmosphere and the magnetosphere, you're at the mercy of the unmitigated radiation from above. And there's a lot of radiation that is out there that was not appreciated early on. And it was learned about during those early space missions that in addition to weightlessness, microgravity and the the absence of oxygen and all the other hazards that are obvious in space travel, there was an invisible hazard that was underappreciated.

B (7:07)

Excellent, James. Use a specific term, ionizing radiation, throughout the article. Could you briefly describe what that is and why that is dangerous to human being traveling beyond the moon or our atmosphere?

C (7:21)

Sure. So radiation is a general term for invisible energetic energies such as electromagnetic waves, which can include anything from radio waves, visible light, all the way to gamma rays and X rays and charged particles radiation. But not all of these forms of radiation are ionizing. Only if the radiation is energetic enough to knock an electron off an atom or a molecule does it qualify as ionizing radiation. And ionizing radiation has different biological consequences compared to lower energy non ionizing radiation such as visible light and microwaves and radio waves, et cetera.

B (8:20)

You introduced the term let linear energy transfer, low let and high let to distinguish between the two types of radiation energy. Could you briefly describe the difference between those two?

C (8:34)

Sure. So it's a little counterintuitive in the sense that initially I, for instance, always assumed that the higher the energy, the more biologically devastating the radiation would be. But it turns out that that is not necessarily the case. There's another parameter, and that is the linear energy transfer or let that you alluded to that's more relevant for quantifying the biological damage that radiation would cause. The let is the number of ionizing events, number of electrons that might get knocked off molecules along the path of the radiation. So per unit length, if there's a lot of ionization events, that's high let. If along a unit length there are relatively sparse and few numbers of ionizing events, that's low let radiation. The higher the let, the higher the biological impact.

B (9:44)

Perfect. Just another clarification question. You also talk about primary cosmic rays. What are they and how dangerous are they?

C (9:54)

It's a very interesting concept, and it's confusing. Primary cosmic rays are those Cosmic rays that are above our atmosphere. And they come from the sun, they come from beyond our solar system. So they might be solar primary cosmic rays, they might be galactic cosmic rays, but either one of them are considered primary cosmic rays. But when those primary cosmic rays strike our atmosphere, well, our atmosphere could shield us from that radiation, but it could also multiply that radiation. And what I mean by that is a high energy particle from outer space. A primary cosmic ray might strike a nitrogen atom or molecule and cause that nucleus within the atom and molecule to shatter, undergo spallation, and the products of that interaction constitute secondary cosmic rays. So while a single cosmic ray might come in and strike an atmospheric molecule or atom, upon nuclear spallation, you might multiply that radiation a dozen fold in the form of electrons, neutrons, positrons, and subatomic particles. So those are secondary cosmic rays that are caused by the interaction of primary cosmic rays with our atmosphere.

B (11:41)

Perfect. And I wanted to ask about one of those particles, namely a muon, which is extremely short lived, but in some cases can still reach Earth's surface. How is that possible?

C (11:53)

Well, that's a fascinating example, and I say it's the classic example that proves that Einstein's theory of special relativity wasn't just nonsense. It's so mind boggling to think that time can slow down or length can change, or that mass can increase as you go faster and faster and faster. But muons prove that the special theory of relativity is the real thing. Specifically, muons are relatively short lived. They have a lifetime of about 2.2 microseconds, and it's an extremely short duration. But even at the speed of light or close to it, something that's only going to live 2 microseconds or so is only going to travel about 600 meters or so. Yet they're reaching us from 10 kilometers above the surface. How is that possible? So they're not traveling at faster than the speed of light, but instead, time slows down. And as time slows down, that 2.2 microsecond half life is dilated. They call it time dilatation. And this allows those muons to strike us on Earth's surface. And muons are perhaps the primary example of secondary cosmic rays that affect us here at sea level. At sea level, under ordinary circumstances, we say that about one muon per second per area, the size of our hand is penetrating our body. So we're constantly being inundated and penetrated by muons from outer space that would not reach us were it not for special relativity's time rotation. Phenomenon.

B (13:59)

Yeah, that is just fascinating, you know, the relativity and the impact that we're seeing in our daily existence. You referenced a term, relative biological effectiveness in the article, or rbe. Can you discuss the RBE of a given species of radiation and how the potential damage is not always straightforward or intuitive? I think you just touched on this, the difference between LET and high let and low let.

C (14:28)

Yeah. And so high energy does not necessarily mean high RBE or high biological effectiveness. Relative biological effectiveness is a mathematical way of quantifying just how much radiation damage occurs to a biological entity. And it depends on the specific endpoint that you're seeking. Damage to DNA, killing of cancer cells, causing a cataract, things of that sort. So RBE is not constant. It depends on the biological endpoint in question. But it's also not constant for different types of radiation. So although high energy does not guarantee high rbe, high let's high linear energy transfer does increase rbe. So it's not the energy, it's the let. And this means that there's a paradoxical inverse relationship between energy and let. So the higher the energy, the lower the let for a type of radiation. And this is a little bit counterintuitive, but high energy X ray might have a given rbe, but as that energy is increased to the gamma ray range or beyond, it does not have the same RBE and let. It actually goes down because the distance between ionizing events increases. And this means that it might whiz right through a cell without causing any ionization and therefore not cause any damage. Whereas if you decrease its energy, decrease its speed, it might cause more damage per unit length and strike the chromosomes in the DNA, thereby causing damage.

B (16:40)

Wow, that is again, fascinating and counterintuitive. I want to briefly talk about the NASA. I guess their notion of radiation during mission planning and what were its limits here? I'm talking about Mercury for Apollo planning and clarify, please. But it seems like it was primarily focused on minimizing travel time and choosing lower radiation routes. Was that the extent of the early NASA planning?

C (17:10)

Well, in a way, yes. Some of the earliest missions were the ones that discovered this previously unrecognized radiation out there. And like I said, some of the hazards of space travel are quite obvious. The absence of oxygen, low gravity, et cetera. But ionizing radiation was not always at the forefront or the front of mind regarding radiation. Regarding space travel hazards, the earlier missions did discover that there were two large donuts wrapped around the Earth. These donuts were full of radiation. They're now called the Van Allen belts. So having mapped out the Van Allen belts in the Explorer mission, subsequent space travel missions aimed to avoid those Van Allen belts. And for the Apollo missions, going to the moon aimed to minimize the extent that astronauts would be traveling through those Van Allen belts. So if the Man Allen belts are like donuts wrapped around our planet at the equator, well, instead of exiting right out from the equator, maybe you go through the north or south pole regions and avoid the radiation to some extent that way. And so this is one of the consequences and advantages of what was gleaned from the earlier missions, such as Mercury and an explorer that discovered this radiation. And Apollo capitalized on it by minimizing that radiation exposure to the manned missions.

B (19:08)

And I should say you have some very nice illustrations in the article, particularly around the outer and inner Van Allen belts, as well as others that do a nice job of elucidating or explaining these concepts. James, you reference an August 1972 solar storm and suggest it was almost a near miss or the astronauts got lucky for the the Apollo astronauts. Why do you say that?

C (19:34)

Yeah, this was something that I do think was a very lucky break that that happened during the Apollo missions. And history could have been very different if one of the Apollo missions took place in August of 1972. Specifically, August 7, 1972 was the date of the so called seahorse solar flare, I think it was called. And it was a very powerful solar flare that was spewing out vast amounts of radiation. And even though it didn't directly strike Earth, it was on the level of the carrington event of 1859, which did cause a direct hit and caused all kinds of havoc to the technology back in those days, such as telegraph systems. If something like that were to cause a direct hit today, it would really wreak havoc on these electronics. And if the astronauts were up there and exposed to the unmitigated full fury of that radiation, it could have led to a dose that might have been fatal. So I think that the Apollo astronauts were very fortunate to not be en route to the moon in August 1972 because they could have suffered dire consequences.

B (21:16)

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C (21:57)

Well, I don't want to underplay the hazards of going to the Moon. There's plenty of radiation out there, and. And the Moon is beyond both our atmosphere and our magnetosphere. So the Moon is a high radiation environment. But traveling to Mars, which is much, much further away, Means that we'll be in deep space for a much, much longer time. And it's not necessarily on the surface of Mars that the radiation is most problematic. It's in the deep space, the interplanetary space, that the radiation is most intense. And in fact, on Mars, there is a thin atmosphere. Even though there's no magnetosphere to protect human colonists and astronauts. There is a thin atmosphere that does attenuate radiation to some degree, Perhaps more so than on the lunar surface. But nevertheless, if it takes a year or 18 months to get there and back, There's a lot of radiation exposure en route. And unlike a trip to the Moon, which might be over and done with in a week or two, if we're talking about 18 months of radiation, that's a lot more cumulative dose.

B (23:25)

Hmm. Interesting. How can a spacecraft. I want to go back to the Van Allen belts for a minute. But how can a spacecraft launched from the US Or Earth. Reduce exposure when passing through the Van Allen belt? I thought this was interesting because you talk about the magnetic poles.

C (23:45)

Yeah, yeah. So, ideally, you avoid the Van Allen belts. And you choose path that would go through the whole of the doughnut. Rather than through the bulk of the doughnut, which is full of the radiation. But if you must be exposed to the radiation of the Van Allen belts, well, then the equipment has to be hardened so the electronics can be. They call it hardening of electronics against radiation. And this is a very interesting subject in and of itself. Because the specialists who aim to harden electronic instruments for NASA, for example, use radiation therapy facilities to test their products and determine the sensitivity or resistance of their products at the proton center, at the neutron center that I worked at. To see if the changes they've implemented have made a difference. And those changes could be simply increasing redundancy of a circuit. So that if one goes down, well, you still got a couple more that will get the job done. Or to specifically harden the electronics. So that they're inherently more radiation resistant. And will not break down if exposed to high intensity radiation. Or to improve the shielding around that electronic equipment. So even though as a physician, I focus primarily on the biological consequences, the medical consequences of Ionizing radiation. You have to keep in mind that if radiation can cause damage to a human cell and the DNA within that cell, it can cause damage to other things. And those other things, if they're in sensitive electronic equipment, are going to affect the functioning of that electronic equipment. So that's a long winded way of saying that you should avoid the Van Allen belts. But if you have to go through the Van Allen belts, better make sure that your equipment can tolerate it.

B (26:03)

Perfect. And I want to circle back towards the end of the article. You do talk about ways to shield ourselves from this radiation, both the human beings and the equipment. But before that, I want to talk about, I guess, sources of radiation in our solar system and then outside our solar system for the former. I think the sun is the first example. Can you present in the article, can you talk about the type of radiation the sun emits?

C (26:32)

Sure. The sun is the primary source of radiation in our solar system, and it comes in the form mostly of energetic protons. And there are electrons, helium nuclei, or alpha particles as well. But for the most part, solar radiation is proton radiation, and that is emitted in a constant stream called the solar wind, which is relatively modest energy and intensity. But solar radiation is punctuated by phenomena called solar energetic particle solar particle events, in which energetic solar particles, or SEPs, solar energetic particles, mostly protons, are emitted during solar flares and coronal mass ejections. So solar flares, coronal mass ejections, and the solar wind are forms of radiation that come from the sun, with the solar flares and coronal mass ejections being the ones that punctuate that radiation in high intensity solar energetic particle events.

B (27:57)

How do solar maximum and solar minimum affect radiation exposure?

C (28:01)

Well, that's a very interesting concept because it's counterintuitive. While I talked about the solar wind and coronal mass ejections and solar flares, these are more intense, more frequent during solar maximum. Solar maximum occurs every 11 years or so and coincides with more sunspot activity. So sunspots are really just visible manifestation of intense solar activity. But ironically, it is during solar max that radiation exposure to astronauts is lower. And that is because the solar wind will be blowing hotter and counteracting the incoming galactic cosmic ray radiation. So if we had to choose between solar radiation and galactic cosmic rays from beyond the solar system, which one is worse? The consensus is that the galactic cosmic rays from way out there are more dangerous, more hazardous. And the solar radiation during solar max is blowing that radiation back out of the solar system and away from our Astronauts. And for that reason, during Solar Max, there's a little bit less general radiation than during solar minimum. But it's kind of a double edged sword because although the radiation from the sun will be blowing the galactic cosmic rays away and protecting us from that. Well, don't forget that during Solar Max you have more solar flares and you have more coronal mass ejections, which can provide overwhelming doses of radiation to the astronauts and be more dangerous than the galactic cosmic rays are. So basically there's no safe time for an astronaut to be out there when it comes to the radiation.

B (30:18)

Yeah, no, you get that sense. You always need to be on your guard. You reference galactic cosmic rays. Now we've talked about cosmic rays from solar energetic particle events, but there's another type of cosmic rays. Again, these cosmic rays. Could you talk about the source and why they are dangerous?

C (30:40)

Sure, I could talk about it, but I don't know about it and nobody does know about it for, for a fact. But the presumption is that galactic cosmic rays, which are highly energetic atomic nuclei, again mostly protons, but also helium nuclei and heavier atomic nuclei ranging all the way up to uranium nuclei. These are products of supernova explosions many light years away from us. And it's not clearly understood how they gain their phenomenal energy. But these atomic nuclei are not in the KEV and MEV range that we're familiar with radiation on, on Earth, but gev, tev, PEV and beyond. The energies are absolutely astounding. And therefore if they are heavy charged atomic nuclei with that much energy, they're going to be highly biologically effective and have high. Let's again causing some confusion because I said before that high energy might mean lower let. Well, all bets are off when you're talking about galactic cosmic rays because they have high charges. So carbon ions and oxygen ions, silicon nuclei, sulfur nuclei, your iron nuclei, these are so positively charged that despite their intense energy and tremendous velocity, they nevertheless are highly and densely ionizing and very biologically effective.

B (32:45)

And in the article you talk about, or I guess speculate on several sources of these cosmic galactic cosmic rays, including supernovae, which is the death of massive stars, hypernovi, the collapse of supermassive stars, and kilonovae, which is the merging of two neutron stars or the collision of a neutron star in a black hole. And these two, I think the latter two events, namely the hypernovi and the kilonovae, would give rise to gamma ray bursts and potentially the birth of new black holes.

C (33:20)

Is that fair?

B (33:20)

There's some Correlation or connection with black holes and gamma rays?

C (33:24)

Yes, that's the understanding at this time. Gamma ray bursts are phenomena in which an intense blast of gamma rays is detected in space. These were initially discovered during the VELA mission back in the 1960s, where there was a nuclear test ban treaty in place so that exploding nuclear bombs was prohibited. And there was a verification system in place known as VERA to determine if there were any gamma rays coming from COVID nuclear explosion operations. But practically every day gamma rays were being detected. They were not coming from the Earth's surface, however, they were coming from outer space. And this led to a lot of confusion. And it was only years later that the scientists recognized that the radiation was not coming from atomic bomb testing or anything man made, but coming from deep space, coming from so called gamma ray bursts that were many, many light years away. And in fact not even from our own galaxy, but from other galaxies. And for the gamma rays to be reaching us from galaxies beyond our own Milky Way meant that the intensity, the energy of these gamma ray bursts was beyond anything previously imagined. A We divide gamma ray bursts into short duration and long duration gamma ray bursts based on whether it's longer than two seconds or shorter than two seconds. Those that are longer than two seconds are believed to originate from the so called collapsar model, where a supermassive giant star collapses in on itself, not to a neutron star, but into a black hole and creates a long duration gamma ray burst. That's called the hypernova phenomenon. And then the Kilonova phenomenon is what is believed to lead to short duration gamma ray bursts. When two neutron stars, for example, might merge and lead to the creation of a new black hole. And during that process emit gamma ray bursts coming from the north and south axial pole. And if those pole polar gamma rays are pointed towards us, we see a short duration gamma ray burst. Wow, fascinating stuff.

B (36:30)

Absolutely. You write a sufficiently high acute penetrating whole body dose of radiation as might occur with gamma rays or X rays, can cause acute radiation syndrome. What is acute radiation syndrome?

C (36:44)

Acute radiation syndrome is a potentially very serious medical phenomenon in which penetrating radiation, meaning gamma rays or X rays as opposed to alpha particles or beta particles from outside, are which do not penetrate the body very deeply, cause trouble. So if the radiation is total body high dose and penetrating it can cause acute radiation syndrome. And the radiation syndrome can be fatal, depending on whether it's a hemological sub syndrome, gastrointestinal or GI syndrome, or the CNSCardiovascular sub syndrome. The latter two are uniformly fatal. But it's the hematological or hematopoietic subsyndrome that might be salvageable if appropriate medical intervention is provided.

B (37:51)

That's a great segue to my next question. Why are, I guess, Earth based medical treatments not feasible or practical during deep space missions?

C (38:00)

Well, it turns out that for severe acute radiation syndrome in the form of the hematological or hematopoietic sub syndrome is encountered, patients may be salvaged by intense medical efforts. And those medical efforts might require blood transfusions, platelets and red blood cells. For instance, may be injections of colony stimulating factors that promote the growth of white blood cells that prevent infection. But it also might require a bone marrow transplant. And a bone marrow transplant is something that requires intensive care in a hospital for a prolonged period of time and is currently beyond anything that could be even imagined in a today. However, cognizance of the radiation environment out there means that someday we'll have to be able to provide such care. If long term colonization of the moon or Mars and interplanetary travel becomes more routine someday we're going to need to be able to address acute radiation syndrome and be able to provide such intense medical care in space. Something we can do today though?

B (39:34)

No. Fascinating. Why is effective radiation shielding difficult to implement on spacecraft?

C (39:42)

Well, the energy of the radiation in space vastly exceeds what we are familiar with here on Earth. So the gamma rays, alpha particles and beta particles that come from radioactive decay, or the X rays that we use in diagnostic radiology or radiation oncology are effectively shielded with materials and thicknesses that we are capable of managing. This energy of the space radiation puts this energy to shame and would require much thicker shielding. And if you've ever been to a radiation oncology center and seen the vault, the walls are about a meter thick and the doors are about a meter thick out of lead. They literally weigh many tons. And this starts to become impractical in terms of creating spacecrafts that have such heavy thick shielding. So more innovative ways of shielding against space radiation are required. And since space radiation is largely charged particles, at least the idea of magnetic shielding as one alternative or supplement to the heavy thick shields that we are familiar with here on Earth.

B (41:18)

In the closing part of the article, you offer a few, I guess, strategies that we're pursuing for radiation mitigation for space travel, including one you just mentioned, electromagnetic deflection. Could you briefly talk about that? As well as storm shelters and something I found fascinating again, radiotrophic fungi, if I have that right, sure.

C (41:41)

Well, for a colony on Mars, there are several Ideas that are being bandied about, such as conventional shielding, that's just beefed up in terms of its thickness and effectiveness, but that means also its weight. So there are disadvantages to that approach alone. Additionally, like we talked about primary radiation and secondary radiation, we have to keep in mind that while primary cosmic rays cause secondary cosmic rays because of interaction with our atmosphere, something similar is going to happen on Mars where primary cosmic rays and primary cosmic radiation strike the shield and cause secondary radiations. And the secondary radiations may be better or more effectively shielded by a different type of material. So alternating materials may be an attractive solution for space radiation, so that something with high density or high electron density, such as lead or tungsten, may be on the surface. But then neutrons that are produced as a form of secondary radiation may be more effectively shielded by material that has high hydrogen content. So paraffin wax could be on the inside. And then a third layer that would shield against tertiary radiations, and fourth layer, so on and so forth to protect us from the primary radiation, the secondary radiation, tertiary radiation, et cetera. And the radiotrophic fungi is an interesting concept that remains to be proven, but it's a fascinating topic of conversation. And it stems from the observation that in places like Chernobyl, where you would expect no life, you do see life. And one of the forms of life is growth of fungus that is darkened in appearance. And that darkened appearance is called melanotic changes. It means that the fungus growing there has a lot of melanin in it. The melanin seems to protect it from radiation or allow it to thrive in a high radiation environment. And there's speculation that certain fungi may actually be using the radiation as a source of energy, just like plants use light from the sun in photosynthesis. Is it possible that some fungi are using radiation as a source of energy for radiosynthesis? This is something that's quite fascinating. Hasn't been proven yet. But if it is, it leads to another potential wild speculation, which is that, well, if you're in deep space, once you have a fungus garden that would be so thick that it serves as shielding, and it's growing on its own because of the radiation air in outer space. And if we're lucky, it tastes good and you can just grab a little bit, saute it up and have something equivalent of nice mushroom stew. But this is beyond any anything that has data to support it at this point. It's just fun speculation.

B (45:40)

No, but, you know, again, very, very interesting. That concludes our interview. Again, the article is weathering space in the January February issue of the American Scientists, by James Welsh, Robert Peter Gale, and Andrew Karam James, thank you so much for your time today and again writing such an interesting and fascinating article. I'm sure our readers will enjoy it.

C (46:03)

Thank you so much for the opportunity, Greg. It's been great speaking to you. A lot of lot of fun.

B (46:08)

Likewise.

C (46:15)

Foreign.

A (46:22)

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