Thursday, August 17, 2017

The Perils of Cosmic Radiation: Are We Made for Deep Space Travel?

We know we are intrepid explorers by nature. Our pop culture reflects that fact. We believe that someday we will overcome all technological odds to travel to, and set foot on, distant but promising exoplanets, perhaps somewhere where an alien ecosystem has taken root. Maybe that life is biochemically similar to us. It's only a matter of time.

There is a problem, however, one that has not been given much play in our sci-fi movies and books. We are not made for outer space. We are extremely delicate organisms that have evolved within a pocket shielded from deadly solar wind and even more violent cosmic radiation. We live inside a thick envelope of gas surrounded by a powerful planetary magnetosphere, which in turn is enveloped in an even more powerful and far-reaching stellar magnetosphere. These powerful magnetic envelopes deflect most harmful radiation away and our atmosphere does a thorough job of absorbing what still makes it through. As a result, our bodies are exposed over our lifetimes to just a miniscule fraction of the fast atomic fragments that bombard every square meter of deep space.

Space is not the empty, cold, benign backdrop portrayed in most movies. It is a nuclear blast zone. Stars like our Sun are ongoing fusion reactions sloughing off electromagnetic radiation, protons, electrons, neutrinos, and small atomic nuclei in every direction. Almost infinitely more powerful stellar explosions and collisions, happening all the time across the universe, blast matter away at almost the speed of light. Nothing slows these deadly particles down as they fly across the light-years. An astronaut's suit or spaceship hull stands little chance against this constant cosmic onslaught. No currently available material is strong or dense enough to absorb or deflect cosmic radiation while also being lightweight enough to be launch-able. Any ship-wide magnetic deflection shield powerful enough to work will require an enormous supply of energy during the decades it will take to get to even a relatively nearby exoplanet is also a distant, if not impossible, technological dream.

Despite the inhospitable nature of space, we have inhabited it, at least close to home where we have some protection from cosmic radiation. The International Space Station (ISS) is an artificial biosphere that takes care of the physical needs of a few humans at least on the scale of many months. On its predecessor, Mir, Valery Polyakov, a Russian astronaut, spent 437.7 consecutive days in space during 1994/1995. What we might forget is that this is time spent in space orbiting close to Earth. Mir, for example, maintained a near-spherical orbit of between 352 km and 374 km above the surface of Earth, within the second most outer layer of Earth's atmosphere called the thermosphere, which is just above the ionosphere, which forms the inner edge of Earth's magnetosphere. At this altitude the density of air is extremely low so there is no atmospheric protection from solar or cosmic radiation. Air here is so diffuse that a single molecule of oxygen would have to travel on average one kilometer before it collided with another molecule. However, Mir was well within Earth's thick protective magnetosphere. Even at its narrowest region, where it is compressed by incoming solar wind on the side of Earth facing the Sun, the magnetosphere is about 60,000 km thick.

How does atmospheric cosmic and solar radiation protection work? Our atmosphere is transparent to low frequency electromagnetic (EM) radiation emitted from the Sun. Sunlight, for example travels through air to bathe us on the surface. High-frequency EM radiation from the Sun, such as X-rays and gamma rays, is absorbed by the plasma (consisting of electrons and electrically charged atoms and molecules) in Earth's ionosphere. In addition to EM radiation, the Sun also emits particles in all directions, most of which are protons, and they are traveling very fast, about 400 km/s. Earth's rotating metallic core generates a powerful magnetic field that deflects most of these charged particles away from the surface. Our atmosphere is also opaque to this radiation. It absorbs what isn't deflected. Consider Mars for a moment. It possesses neither a deflecting planetary magnetosphere nor a thick highly absorptive atmosphere. Its thin carbon dioxide rich atmosphere provides some radiation protection, but not much. It is roughly 2.5times less protective than Earth's very thin thermosphere through which ISS orbits. Although its surface at the equator approaches a livable temperature,  the continuous bombardment of radiation makes it more inhospitable than you might think.

While astronauts on Mir and now the ISS have very little atmospheric radiation protection, they are protected from solar wind by Earth's magnetosphere. Thanks to the Sun's magnetosphere they are also mostly protected from far more powerful radiation - gamma rays and particles with far higher kinetic energy than any solar wind particle. Yet even here close to Earth, astronauts can only stay on ISS for a limited time. A very small amount of cosmic radiation makes it through our solar magnetosphere, meaning that the astronauts do receive a low but cumulative dose of cosmic radiation. A  2014 study by radiation expert Francis Cucinotta indicates that ISS astronauts exceed their lifetime safe limit due to cosmic radiation in just 18 months for women and two years for men. 

Protons (hydrogen nuclei) and, to a lesser extent, larger atomic nuclei (such as alpha particles and helium nuclei), traveling near the speed of light which is 300,000 km/s, pervade every square centimeter of deep space. The ISS would never be feasible in this constant blast zone. However, protected within the magnetospheres of the Sun and the Earth, an aluminum hull just a few millimetres thick shields about 95% of the radiation that strikes it. Even thick plastic stops this radiation, which consists mostly of EM radiation along with relatively low-energy solar protons (averaging 400 km/s) that manage to pass through Earth's magnetosphere. We have to say average velocity here because the Sun isn't static. Magnetic storms sometimes rage across its surface, accelerating particles up to 3200 km/s, in the case of exceptionally violent storms called coronal mass ejections. A coronal mass ejection is a mass of highly magnetized plasma, a chunk of the Sun, hurled off into space when a magnetic field snaps. Velocities up to 3200 km/s have been recorded by the LASCO onboard the SOHO satellite orbiting between the Sun and Earth. Even this accelerated plasma, which would devastate Earth's electrical and communications systems, has nowhere the particle-to-particle punch of cosmic radiation, with velocities close to 300,000 km/s.

As mentioned, materials are still in the process of being developed that are both light and effective at shielding protons traveling close to light speed. Most cosmic radiation comes from supernovae. It consists of stellar particles violently spewed in every direction when a high-mass star reaches the end of its life. The shock wave of the explosion is powerful enough to accelerate material to near light speed. Only in extremely powerful accelerators can particles on Earth approach such velocities.

Radiation itself can be a confusing subject but at its core the concept of radiation is simple: it is an emission or transmission of energy. It comes in four basic kinds: electromagnetic (EM) radiation (radio waves, visible light, gamma rays etc.), acoustic radiation (sound waves, seismic waves), gravitational radiation (gravitational waves) and particle radiation (on Earth we typically deal with alpha radiation – alpha particles, beta radiation – electrons, and neutron radiation – neutrons). In terms of cosmic radiation, we are especially interested in protons, alpha particles and, to a much lesser extent, larger atomic nuclei.

Radiation is either ionizing or non-ionizing. With the exception of microwaves, sonic devices and intense or prolonged light exposure that can cause photochemical burns, non-ionizing radiation tends to present a minimal hazard to human health. This radiation simply doesn't have enough energy to ionize atoms in living tissue. I will explain what "ionize" means in a moment. Generally, a particle or wave must carry more than 10 eV (electron volts) of energy to ionize atoms and therefore damage biochemical bonds in molecules, but the line is blurry because some atoms ionize more easily than others do and some chemical bonds break more easily than others do. Consider the energy of a visible green light photon of about 2 eV. 2 eV is harmless (unless those 2 eV photons are concentrated into a laser. Then they can burn your retinas and blind you). Radiation of 10 eV or more, however, has enough energy to strip electrons off of (ionize) atoms and molecules, breaking chemical bonds between them in the process. This radiation can be harmful and even lethal to humans. Short wavelength (very energetic) EM radiation such as X-rays and gamma rays can break chemical bonds in DNA for example, creating genetic damage that can eventually lead to cancer. It can also damage biological proteins and enzymes, impairing cell function. All particle radiation from radioactive materials or in the form of solar radiation and cosmic radiation is ionizing. It causes biological damage at the microscopic level in our bodies. That said, The National Cancer Institute in the United States explains why radiation can sometimes be good thing. Targeted at cancer cells, which are dividing uncontrollably, the ionizing radiation damages their cellular DNA so severely that the cells program themselves to die, shrinking the tumour.

Most cosmic radiation consists of ultra-fast protons, and most of them are blasted out of exploding stars. A proton, a particle normally confined inside an atomic nucleus, is an exceptionally tiny object, weighing less than 2 x 10-27 kg. Despite its miniscule mass, a proton blasted from its atom and accelerated to near light-speed (now we call it a relativistic proton) packs a catastrophic punch, around 1 GeV (G means giga or billion electron volts). Compare this to the low end of ionizing radiation at 10 eV. This force is roughly equivalent to a baseball being thrown at you hard but with all of that impact condensed into an area about one millionth of a nanometre wide (a proton is about 10-15 m in diameter).

You might expect a single proton at such high velocity to fly right through your body, causing minimal damage along its sub-microscopic course. After all, a general rule in physics is that the higher the kinetic energy of a particle, the smaller the fraction of its kinetic energy tends to get deposited in the material. The problem is that the proton will, more than likely, glance off atoms in your body along the way. A homemade analogy here might be that while a massless "slender"  gamma photon elegantly dances through the atoms, a massive proton acts like a bull in a china shop. Those atoms in the proton's path will be ionized, and they will ionize other atoms and so on, depositing energy along an ever-widening path of damage in the body. This process is technically called linear energy transfer (LET), which I will explain in a moment. An astronaut onboard the ISS might by hit by only a few cosmic protons during a months-long mission but new research indicates that even low-dose infrequent cosmic radiation exposures can cause significant long-lasting damage, particularly to the brain. An astronaut on route to Mars or on the surface of Mars, with only the Sun's magnetosphere as protection (Mars doesn't have one), will receive a much higher exposure to cosmic radiation than an ISS astronaut. Of most concern is the cumulative damage to our fragile and very slow to heal brain.

Cosmic radiation is not a new discovery. Scientists have known about it for many decades. In fact, in 1909, Theodor Wulf discovered that the rate of ion production inside a sealed container of air was higher at the top of the Eiffel Tower than at its base, refuting the then-current theory that radiation originated from radioactive elements in the ground. A few years later Victor Hess launched ionization-measuring electrometers on a balloon during a near-total solar eclipse. His results ruled out the Sun as the source of this radiation and confirmed Wulf's data.

Primary cosmic radiation, originates outside the solar system and some of it comes from outside the Milky Way galaxy. About ¾ of cosmic radiation consists of protons (hydrogen nuclei). About a quarter consists of heavier alpha particles (helium nuclei) and about 1% consists of still heavier nuclei, mostly lithium, beryllium and boron nuclei. These heavier so-called HZE (high atomic number and energy) ions, though far less abundant, are especially damaging. Though very sparse even in deep space where there is no magnetic shielding, these infrequent impacts would contribute significantly to an astronaut's overall radiation dose. 

A radiation dose is measured as an absorbed dose. The SI unit is the gray (Gy). It measures the radiation's energy deposited in the body. Radiation is also measured by the action upon the matter of the body, by its linear energy transfer (LET). This means it measures the amount of energy lost per distance traveled. Two equivalent absorbed doses can do vastly different amounts of damage in the body depending on their LET values. A high LET means that the radiation leaves behind more energy and therefore causes more atoms to ionize in its wake. A higher mass particle, such as an alpha particle, will leave a track of higher ionization density than a proton, if both are going the same velocity when they strike the body. A gamma photon will have a far lower LET value yet. The chemical make-up of our cells also plays a significant role determining ionization damage. Low LET radiation, such as X-rays or gamma rays, ionizes water molecules inside cells. It breaks them up into H+ and OH- ions (also called radicals). It does this damage over a long track through the tissue so it tends to leave one event per cell. The often single H+ and OH- ion pair simply recombines to form water once again, releasing some energy in the process. When ionization occurs over a shorter wider track, many H+ and OH- ions can form within each cell. A pair of OH- ions near each other can recombine into H2O2 (peroxide) instead. This molecule causes additional oxidative damage to proteins, lipids and to DNA in the cell on top of the ionization damage to various biological molecules. Dense clusters of high LET ionization damage in the body means that cosmic particle radiation is extremely dangerous to astronauts, especially to their brains as we will discuss below in more detail.

Radiation Units: A Frustrating Labyrinth

Articles about radiation can be very difficult to grasp because so many unfamiliar units are thrown around seemingly at random and sometimes interchangeably. A large part of this confusion stems from the use of both "old" American units and SI (standard or metric international) units. Here in North America the switch over to SI has been particularly slow in the field of nuclear science in part because the stakes are so high. Even a small conversion error can lead to dangerous radiation exposures. In addition, a number of different units must be used to accurately describe radiation as it travels from its source, through the atmosphere or through space and then into our bodies. The rate of emission of radiation from its source is measured in curies (old) or becquerels (SI). Sometimes radiation emission is measured in terms of emission energy instead of rate. In this case it is measured in electrovolts (eV) or joules (J, an SI-derived unit). I used eV earlier to compare ionizing to non-ionizing radiation energy. Once the radiation is emitted and is now ambient, its ambient concentration is measured in roentgens (old) or coulombs/kg (SI). Once ambient radiation strikes a living body or other object, the raw amount that object absorbs is measured in eitherrads (or just rads; old) or grays (SI). I gray is equivalent to 100 rads. Rems (old) and Sieverts further complicate the picture. These units measure the effective dose, or dose equivalent, in other words the biological harm caused by radiation in living tissue. Rather than describing a radiation disaster in terms of rads, for example, seiverts or rems can offer a better description of how the radiation exposure will affect human health. This measurement reflects the different LET values of different kinds of radiation as well as the kind of tissue receiving the dose. Some body tissues are more sensitive to radiation than others. This measurement of dose equivalent is still an inexact science, however, as there are so many not entirely known biological factors to consider. Randomized experiments that test radiation damage to living human tissues are, of course unethical. Animal study results are sometimes difficult to extrapolate to humans because their bodies, tissues, and physiologies are different. We also still don't know much about how our human tissues react to various kinds of radiation exposures, especially from cosmic radiation.

The Edge of a Mystery: The Brain and It's response To Radiation

Scientists once assumed that brain tissue is less sensitive to radiation damage than other tissues because brain cells tend to multiply at a much slower rate than other cells in the body, such as gut and skin cells do, for example. Cells that divide less often spend less of their time in the process of division. The DNA in a quiescent cell is tightly coiled into a dense structure called chromatin. This structure is very stable and resistant to radiation damage and it offers a very small target as well. When a cell starts the process of dividing, its DNA must unravel so the machinery of DNA synthesis can replicate the entire genome. The DNA in this state is highly susceptible to damage. Luckily, cells have mechanisms that detect and repair DNA damage when it occurs, but if the radiation dose is high enough, the dividing cell can't fix all the damage before it completes replication. It will then either program itself to die off or it will pass on the DNA damage as mutations.

Based on instances of acute radiation exposure studied after radiation accidents and war, tissues in which cells multiply most rapidly such as those in the gastrointestinal tract, the spleen and bone marrow, tend to present symptoms of radiation damage (radiation sickness) first. These assumptions are based on whole-body one-time exposures to an absorbed dose of between 6 and 30 Gy. Neurological symptoms (including cognitive defects) typically only manifest after a dose higher than 30 Gy occurs. This is a catastrophic dose; the victim will die within two days. There is a fascinating, if sobering, chart of whole-body dose effects here. Scientists are discovering that the effects of acute exposure cannot be extrapolated to the effects of sustained low-dose exposure, especially long-term effects. The results don't take into account cellular damage that takes weeks, months or years to manifest. More importantly, different tissues in the body deal uniquely to long-term radiation exposures with different LET values in complex ways that are still poorly known. The study we are most interested in here (explored in detail below) considers both Gy and LET values as indicators of damage, in this case, to brain cells. The results are based on both behavioural changes and structural tissue changes in mice exposed to radiation that mimics cosmic radiation.

Until a few years ago, NASA and other space agencies only suspected that long-term cosmic radiation caused cognitive impairment in astronauts. This suspicion was largely based on clinical data from cranial radiotherapy and radiation treatment for brain cancer and then comparing that data to before/after cognitive test results of astronauts. The extrapolation from clinical data to cosmic radiation exposure, as they knew, was problematic, much like comparing apples to oranges. A typical daily dose during cranial radiotherapy is about 2 Gy. Compare this to a far lower roughly 1/5000th daily dose of radiation of around 0.48 mGy expected for an astronaut during a round-trip and stay on Mars. Adding that exposure up for a 300-day trip, for example, still adds up to just 1.44 Gy, less than the dose of a single cranial radiotherapy treatment. Travel to Mars shouldn't be a problem right? The trouble with comparing the two doses is that they have vastly different LET values. In the clinic, X-rays and gamma rays are most often used. These energetic EM photons are not nearly as densely ionizing as cosmic particle radiation because a massless photon particle has far less momentum than a particle of mass traveling at nearly the same velocity.

To really understand how long-term cosmic radiation will affect astronauts during trips to Mars or on possible future deep space missions, you need direct experimentation, using relativistic particles. How would you design an experiment to do this? Charles Limoli, a neuroscientist and radiation biologist at the University of California School of Medicine, has taken some of the first steps in answering the question. His research findings to date can be found in the February 2017 issue of Scientific American. It's an excellent read that inspired me to write this article. He explains not only the radiation problems that current astronauts face based on current research findings, but he also outlines the challenges of getting the data scientists will need to plan future deep space travel. A reference paper for this article called What Happens to Your Brain on the Way to Mars by Vipan Parihar et al. provides a good background for this research. Here I try to provide a glimpse into his work, and offer some insight into the challenges and excitement of designing new experiments, into the process itself. The findings are preliminary so far but they are quite stunning. Undoubtedly a lot of future work will build upon it.

In this case, mouse brains are used as living models for astronaut brains in deep space. There are three significant challenges to this approach. First we have to ask if a mouse brain is similar enough to a human brain in terms of structure and function to make a valid model. We also have to ask if a mouse brain reacts to radiation the same way a human brain does. Rats are used as extensively as mice in neurological studies. There is a mountain of neurological data using either species, suggesting that they are reliable and useful models. I wondered, though, if rats would make a better model in this case. That turned out to be an interesting question. A 2016 research paper by Bart Ellenbroek and Jiun Youn explores the differences between rats and mice and how reliable they are as models for human brain behaviour. The species differ in their behaviour, as anyone who has worked with both species knows. For example, rats tend to be more comfortable with lots of human physical contact while mice can be stressed by similar attention. Unexpected stress on an animal could affect the results of behavioural tests. The researchers also point out that, while both rodent brains are very similar to human brains, there are fundamental differences between them that could affect the reliability of test results. Limoli's research presents a unique physical limitation on what kind of animal model you can use – it has to fit into the accelerator target area.

An additional thing to keep in mind is that the brain is still not fully understood. Neurology, neuroscience and psychiatry are very active fields of research. Still, at least one basic fact about the brain seemed to be firmly established: the brain contains two basic types of cells with neutrons performing the twin star roles of function and structure and with glial cells playing a supporting role. This is called the neuron doctrine. Research over the last few years calls these assumptions into question. For example, a  2013 Scientific American blog post by Douglas Fields points out an unexpected but key difference between mouse brains and human brains that suggests that glial cells are far more involved in function than researchers realized.  It was assumed that glial cells can't do any electrical signalling. Until now they've been thought of simply as physical and physiological support cells for neurons. The researchers transplanted human glial cells into mouse brains and discovered that these mice soon significantly surpassed their untreated siblings in both memory and learning. Somehow human glial cells imparted an improved, perhaps more humanlike, cognitive ability into a mouse mind. A specific type of cultured human glial cells called astrocytes are in fact much larger and have a more variable morphology than mouse astrocytes. It is a clue that these cells might be involved in the evolution of human intellect. Researchers now know that glial cells not only propagate calcium signals over long distances but they also form electrically coupled synchronized units through gap junctions (similar to how heart cells are synchronized to contract during a heartbeat).

The second question is how do we reliably test cognitive function in mice before and after radiation exposure? Fortunately mice have been bred and used extensively for various kinds of cognition and memory research for decades. There is lots of data to draw from as well as a large collection of reliable cognitive and memory test protocols available to use. This 2015 compilation paper by SM Holter et al. provides an overview of those tests. Third, how do we expose our test animals to specific doses of relativistic particles? A tremendous amount of energy must be used to accelerate particles to nearly the speed of light. Few natural mechanisms outside of stellar explosions can do the job. The only way to do this in a lab is to expose the mice to radiation inside a particle accelerator. Fortunately, NASA Space Radiation Laboratory, commissioned in 2003, is designed for exactly these kinds of experiments. Radiation consisting of a variety of particles with a range of very high energies is specifically designed to resemble cosmic radiation.

Out of practical time constraint limitations, the mice received a single dose of radiation equivalent to many months to years of actual cosmic radiation exposure according to Limoli's article. The article states a dosage of either 30 cGys or 0.3 Gy was used, which is very low, approximately 50 times lower than an expected round trip to Mars radiation dose. Would an astronaut's brain, exposed to the same amount of radiation but spread out over many months, have enough time to repair the damage between intermittent particle exposures? Researchers are not sure but the results of this experiment are not promising as we will see in a moment. This research is an essential first step to find out just how big a problem long-term cosmic radiation exposure could be for future astronauts.

A healthy brain neuron consists of a soma (cell body) which contains the nucleus and other organelles, dendrites (branched projections) and an axon (a long slender electrically conductive projection). See the diagram below. It connects to other neurons through specialized connections called synapses. 


(Wikipedia public domain)
One neuron can contact another neuron's dendrite, soma or, less commonly, its axon via a synapse. At the synapse an electrical signal is converted into a chemical signal by the release of neurotransmitter molecules into the gap between two cells. The neurotransmitter initiates an action potential in the connecting neuron. See the enlarged box in the diagram above. One neuron can connect to many other neurons to form complex neural networks in the brain.

To start, a healthy mouse explores toys in a box. Over a period of hours and days, its brain physically changes as it learns and forms memories. The neurons in its brain form new dendritic branches and trees and create new synaptic connections. Dendritic branching can be very extensive and complex. A single neuron can receive as many as 10,000 dendritic inputs from other neurons.

The toys in the box form part of a task that evaluates a mouse's cognition and memory abilities. In this task, called the novel object recognition task, a mouse is placed in a box containing toys to explore. After exploration for a fixed amount of time, the mouse was then removed from the box and the locations of the toys are changed and some are replaced with new toys. The mouse is returned minutes, hours or days later to the box to explore the novel landscape. A healthy mouse is curious - it will quickly notice changes and spend extra time exploring those changes. The time spent on checking out new things compared to overall time present in the box is that mouse's discrimination index, a reliable measure of its memory and learning ability. Memory and learning takes place primarily within the brain's prefrontal cortex and hippocampus. Limoli and his group discovered that a single dose of low-level cosmic-like radiation exposure greatly reduced the mice's discrimination index values. The deficit stood out in stark relief when irradiated mice were given the same follow-up tasks as healthy mice. Their curiosity was greatly diminished. They didn't seem to recognize changes made to their environment.

Six weeks after a single exposure of either 5 or 30 cGy of radiation, both very low doses representing sparse particle impacts, the performance of the mice dropped on average by a whopping 90%, regardless of dose. Furthermore, they also found that these impairments lasted 12, 24 and even 52 weeks after the exposure, suggesting that the damage to the mouse's brains didn't heal, at least within a year after being damaged.

The researchers confirmed the physical damage to the brain by imaging sections of the medial prefrontal cortex of healthy non-radiated mouse brains and of irradiated mouse brains. The imaging data revealed significant reduction in dendritic branching as well as a significant loss of dendritic spines. A dendritic spine is a tiny protrusion from the main shaft of the dendrite that contains the synapse that allows the dendrite to receive signals. If dendrites are the branches on the brain "tree," then spines are the tree's leaves, as Limoli describes it in his article. These structures are very plastic. They undergo constant turnover in a healthy brain. The growth of dendritic spines reinforces new neural pathways as an anatomical analogue of learning. They also maintain memories. Environmental enrichment (providing lots of learning opportunities) leads to increased dendritic branching, increased spine density and an increase in the number of synapses in the brain. Cosmic radiation exposure not only undid the changes associated with learning and new memory formation. It severely impaired the normal (baseline) cognitive function of the brain as well.

Only the medial prefrontal cortex, a region known to be associated with learning and memory, was imaged but it seems reasonable to assume that the radiation attacks the physical and functional integrity of synaptic connections across the entire brain.

Conclusion

This is disconcerting news to those of us who dream of mankind eventually traversing the universe to explore other planets and moons in person. Imagine the spectre of our best and most brilliant men and women gradually losing their cognitive abilities, losing their memories and themselves as they travel through deep space. Cosmic radiation would impair astronauts in the most critical way. The skills and mental acuity required to deal with maintenance issues and sudden problems as they arise during a long-term space flight are what set astronauts apart from the rest of us.  Stasis would be no solution, unless it is inside some as of yet undiscovered material that can block out cosmic radiation. The fact is, we evolved inside a layered magnetic cocoon where the threat of cosmic radiation doesn't exist. We'll have to rely on another facet of our evolution, our ingeniousness, to get past this hurtle.

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