Outer space is the most noxious of substances: devoid of air and filled with a soup of deadly particles in the form of high-energy photons and energetic bits of atomic nuclei. The lack of gravity there affects every element of your being, as even the proteins in your body can’t figure out which way is up.
Books and magazine articles about space voyages often compare the adventure to setting off to new lands across treacherous oceans. Our ancestors paddled across the South Pacific in outrigger wooden canoes made by hand with primitive tools. They set off never expecting to return. They spent days, weeks, and months on the open waters, exposed to the elements, with precious little food and water. Many died along the way, but a few made it to their destination and started a new life. No doubt these early migrations tens of thousands of years ago were perilous, but it’s not as if each drop of water burns a hole in your DNA; the sea mist doesn’t destroy your brain cells; the choppy waves don’t cause fluid to build up in your eyes and cause permanent retinal damage. When you finally get to dry land, you can walk. You don’t need a team of doctors and engineers to carry you off the boat because your legs are too weak to support you. And, chances are, you will find food and water at your destination when you arrive.
In short, some things can actually live in water, and that which cannot live in water can cross it on driftwood. But space is both sterile and sterilizing. The challenges presented by every journey on Earth that has occurred in the centuries and millennia past, however arduous, pale in comparison to those of a journey in space beyond the Moon. Suggesting otherwise minimizes the sacrifices that the first generation of spacefarers will make. To be clear, space travel is technically feasible today from an engineering standpoint. We placed humans on the Moon fifty years ago, after all. We’ve sent probes clear out of the Solar System, and we have made soft landings of probes on the surfaces of Venus, Mars, Saturn’s moon Titan, the comet 67P / Churyumov–Gerasimenko, and several asteroids. But sending humans beyond the Moon is considered by many doctors to be so dangerous that it is tantamount to suicide.
The expected radiation exposure for a Mars excursion for an astronaut—a federal worker—far exceeds the levels permitted for terrestrial workplace activities by the US Occupational Safety and Health Administration (OSHA). To even consider going to Mars, NASA needs a special waiver from OSHA based on a principle called ALARA (As Low As Reasonably Achievable). The waiver, which NASA has, requires the agency to carefully assess the health risks to astronauts prior to launch.
Yet radiation exposure is only one danger. The NASA Human Research Roadmap has identified thirty-four known health risks and 233 “gaps” in our knowledge of risks. For example, four known health risks are associated with radiation: radiation poisoning from solar flare; brain damage; cardiovascular damage; and regular ol’ cancer. But among the gaps are questions about hereditary, fertility, and sterility effects from space radiation. So, it’s likely that there are more health risks than we realize. Here are the 34 known risks of space travel—risks that go beyond basic mechanical dangers, such as the rocket blowing up.
- concern about clinically relevant unpredicted effects of medication
- concern about intervertebral disc damage upon and immediately after reexposure to gravity
- risk of acute (in-flight) and late central nervous system effects from radiation exposure
- risk of acute radiation syndromes due to solar particle events
- risk of adverse cognitive or behavioral conditions and psychiatric disorders
- risk of adverse health and performance effects from celestial dust exposure
- risk of adverse health effects due to host-microorganism interactions
- risk of adverse health events due to altered immune response
- risk of adverse health outcomes and decrements in performance due to in-flight medical conditions
- risk of an incompatible vehicle / habitat design
- risk of bone fracture due to spaceflight-induced changes to bone
- risk of cardiac rhythm problems
- risk of cardiovascular disease and other degenerative tissue effects from radiation exposure and secondary spaceflight stressors
- risk of decompression sickness
- risk of early-onset osteoporosis due to spaceflight
- risk of impaired control of spacecraft / associated systems and decreased mobility due to vestibular / sensorimotor alterations associated with spaceflight
- risk of impaired performance due to reduced muscle mass, strength, and endurance
- risk of inadequate design of human and automated / robotic integration
- risk of inadequate human-computer interaction
- risk of inadequate mission, process, and task design
- risk of inadequate nutrition
- risk of ineffective or toxic medications due to long-term storage
- risk of injury and compromised performance due to extravehicular activity (EVA) operations
- risk of injury from dynamic loads
- risk of orthostatic intolerance during reexposure to gravity
- risk of performance and behavioral health decrements due to inadequate cooperation, coordination, communication, and psychosocial adaptation within a team
- risk of performance decrement and crew illness due to an inadequate food system
- risk of performance decrements and adverse health outcomes resulting from sleep loss, circadian desynchronization, and work overload
- risk of performance errors due to training deficiencies
- risk of radiation carcinogenesis
- risk of reduced crew health and performance due to hypobaric hypoxia
- risk of reduced physical performance capabilities due to reduced aerobic capacity
- risk of renal stone formation
- risk of spaceflight-associated neuro-ocular syndrome
Of these 34 risks, three are potential showstoppers: radiation, gravity (or lack thereof), and the need for surgery or a complicated medical procedure.
Let’s explore the gravity issue.
Some science fiction writers in the mid-20th century speculated that zero gravity would be life-giving: blood would flow more easily; arthritis would be a thing of the past; back pain would be cured for good; and aging itself would slow down. So, bring grandma along for the ride. We had hints from early in the space program that such a rosy scenario wasn’t true. Astronauts returned from just a few days of weightlessness feeling weak. But they recovered; and many thought, well, maybe it isn’t so bad. Then we spent more time in space. Russians on the Mir space station for months appeared to have some serious, prolonged health issues on their return. The Russians were tight-lipped about the health of their cosmonauts, though, so we never knew for sure. Many of these cosmonauts, championed as heroes, were rarely seen in public after their return. It was the ISS missions that drove home the message: long-term exposure to zero gravity is detrimental to human health on many levels. Kudos to NASA for that.
Before I continue, I should first define some terms. Zero gravity, however visually convenient, can be a misnomer in the context of near-earth activity. The astronauts on the ISS are not living in the absence of gravity. Rather, they are in free-fall, forever falling over the horizon and missing the Earth. The ISS and other satellites are not floating in space because they have escaped the pull of Earth’s gravity; they stay up there because of their terrific horizontal speed. The ISS is moving at 17,500 miles per hour. If, somehow, it came to a complete stop, it would fall straight down to Earth, and down would come astronaut, cradle and all. The Earth’s gravitational force, in fact, keeps the moving satellites in orbit as a perfectly balanced counterforce, in a downward motion, to the lateral motion set in place during the launch. Without the Earth’s gravitational force (if the Earth suddenly, magically, disappeared), the satellites would shoot off in a straight line. Therefore, more accurate terms for describing the lack of sensation of gravity aboard the ISS are microgravity and weightlessness. Yet, even these terms are neither perfect nor synonymous. Astronauts on the ISS have weight, about 90 percent of their weight on Earth, which is only about 200 miles below their feet. They’d be much lighter on the Moon, actually, at just about 16 percent of their weight. Absolute zero gravity is not attainable, because gravity is the force of attraction between any two objects. But in deep space, far from the gravitational tug of any moon, planet, or star, gravity is attenuated to almost zero. I tend to use the terms zero gravity, micro-gravity, and weightlessness interchangeably in the context of space travel.
Our understanding of gravity’s effect on the body has only two data points: one and zero. On Earth, we live with a gravitational force of 1G. On the ISS, astronauts live in 0G. We really don’t know about anything in between. Air force pilots might accelerate their jets so quickly that they experience forces of 5G or higher, which sometimes causes them to black out. That’s five times the force of normal Earth gravity, which pushes blood out of their brains. But such forces typically last only a few seconds; the pilots aren’t living in a hyper-gravity environment. And anyway, we don’t care too much about forces greater than 1G because every place we want to go in our Solar System—L2 orbit, the Moon, Mars, and so on—has a gravitational force less than 1G.
What’s so special about 1G? This is simply the force we evolved with. Our bones are as thick as they are because of this precise level of gravitational force. Without the pervasive force of gravity all around them, sending constant signals to the cells, bones begin to demineralize and weaken. Muscles, too, expect a certain resistance when contracting. Without the grip of gravity, muscles atrophy and lose their tone. You can exercise in space. Astronauts on the ISS are required to exercise for two hours each day to minimize bone loss and minimize muscle loss. This works to some degree. But nevertheless, in zero gravity, bones lose density at a rate of 1 or 2 percent per month on average, compared to the rate of an elderly person on Earth losing 1 percent per year. To visualize how bad that bone loss is, consider the fact that the major obstacle to fully recycling urine into drinking water on the ISS was that the filters got clogged regularly with calcium deposits. That calcium is leached from the bones into the urine; this leaching also puts the astronaut at short-term risk for kidney stones and long-term risk for kidney disease.
And for all that muscle exercise on special treadmills, astronauts still find it difficult to walk or even hold a cup after returning from several months in space. Worse for the muscles is the fact that most can’t be exercised. Workouts focus on the major skeletal muscles that move the limbs and torso. But there are hundreds of other muscles—cardiac, involuntary, smooth, and other skeletal—that cannot be exercised. Gravity is their workout on Earth, and on the ISS, they aren’t getting it. All those tiny muscles in the face and fingers get weaker. Tendons and ligaments also begin to fail in zero gravity. The spine lengthens, and astronauts become one or two inches taller in space, which causes back pain. The Space Medicine Office of the European Astronaut Centre, run by the European Space Agency (ESA), is designing a high-tech, highly tight-fitting “skinsuit” to help astronauts overcome back problems in space. Let’s just say the outfit is very, uh, European.
In the body, much more is going on at a cellular level that depends on 1G. Normally, blood pools in the feet because of gravity. Our circulatory system evolved to push blood upward to the brain, a rather important organ. Without gravity, the circulatory system pushes blood upward like a geyser, unharnessed, leaving your head with a pounding feeling. Your heart starts beating faster to pump blood to lower parts of the body. Your body starts thinking there’s a fluid surplus, asking, where is all this blood coming from? So your kidneys go into overdrive to remove excess water via urine. But now you are dehydrated, and your blood starts to thicken. This, in turn, triggers the body to stop making red blood cells, and thus you slowly become anemic, sluggish, short of breath, and prone to infection. And so on and so on. It’s a holistic medicine nightmare.
The eyes are particularly vulnerable to all this unnatural sloshing of fluids. More than two-thirds of astronauts report having deteriorated eyesight after spending several months in orbit. The fluid pressure flattens the back of the eyeballs, inflames optic nerves, and damages fragile blood vessels. NASA astronaut John Phillips was among the first to report the problem. Gazing out the window, he thought Earth looked blurrier and blurrier with each passing month. NASA tested his sight upon his return and found that his vision had deteriorated from 20 / 20 to 20 / 100 after six months in orbit. The implication is that a crew to Mars needs to pack eye-glasses with various prescriptions to help with each phase of their gradual, inevitable, and permanent vision loss. NASA considers the vision issue to be an astronaut’s top immediate-term health risk.
Like the eye, the entire brain floats in fluid. A study of 34 astronauts for whom MRI images of their brains were captured before and after their missions found microgravity-induced changes that could be permanent: essentially, compression as their brains shifted upward and a narrowing of the brain’s central sulcus, a groove in the cortex near the top of the brain that separates the parietal and frontal lobes. These are the parts of the brain that control fine movement and higher executive function; the longer the time spent on the ISS, the worse these brain changes were.
On Earth, and on the ISS, we are shielded from most cosmic radiation, also called galactic cosmic rays or high-mass, high-charged (or HZE) particles. Occasionally a few slip in, smashing into the upper atmosphere and causing a cascade of secondary and tertiary particles. What usually happens is that the cosmic rays collide with nitrogen and oxygen, the two most abundant atoms in our atmosphere, and break them open, releasing neutrons, electrons, and more exotic stuff such as muons, pions, alpha particles, and even X-rays. But there’s a lot of atmosphere to clear, so the radiation tends to decay or be absorbed before it reaches the surface. In fact, cosmic rays were not detected conclusively until 1912, when Austrian physicist Victor Franz Hess carried electrometers on a high-altitude balloon flight. Jet pilots and, by extension, flight attendants have an elevated exposure to radiation compared with the general population. Most of this is cosmic radiation.
Apollo astronauts have seen the effects of cosmic radiation—quite literally. Frequently a cosmic particle has zipped through their eye sockets, producing a flash. This has since been named cosmic ray visual phenomena. What’s happening at a biological level is not clear. A cosmic ray might be colliding into an optic nerve or perhaps passing through the gel-like vitreous humor, creating a cascade of subatomic particles akin to what happens in the atmosphere. The Apollo astronauts, who traveled beyond the magnetosphere on their way to the Moon, sensed the flashes at a rate of about one every three to seven minutes. The astronauts described the flashes in a variety of ways, which may indicate different physical interactions. The reported shapes of the flashes were spots or dots, stars, streaks or stripes or comets, and blobs or clouds, in order of commonality. Closing your eyes won’t help. The astronauts reported that the flashes occurred even when they were trying to sleep.
Of course, the eyes are just a tiny part of the body. The existence of cosmic ray visual phenomena implies that the entire body is being pinged by cosmic radiation around the clock; thousands of rays would pass through you every second. Physicist Eugene Parker of the University of Chicago has said that a third of your DNA would be sliced up by cosmic rays every year you spend in interplanetary space. This is far too much damage to be controlled by the body’s own DNA repair mechanisms. We also must remember that we won’t be alone in space. We are carrying billions of bacteria, viruses, and fungi, in the form of our microbiome that plays an important role in maintaining health. Microflora in our gut help digest our food, for example. Cosmic radiation could kill or otherwise cause mutations in our microbial passengers, presenting unknown dangers. Only very thick shielding or some kind of mini-magnetosphere around the craft or base (which I explore below) can stop these cosmic rays from passing through your body in space. This has major ramifications not only for spaceflight but also space living. Bases on the Moon, Mars, and nearly anywhere we set up camp beyond our magnetosphere, regardless of how distant they are from the Sun, will be inundated with cosmic rays unless properly shielded. When outside, you would be forced to live with flashes in your eyes, causing untold damage, let alone with the other ramifications of this radiation exposure. Contrary to the silliest of science fiction tropes, cosmic rays don’t engender super-human strength.
The research results from experiments on rodents and space radiation have been ambiguous. Charles Limoli, a professor of radiation oncology at the University of California, Irvine, School of Medicine, led a NASA-sponsored study that exposed laboratory mice to a level of radiation similar to that expected on a six- month one-way trip to Mars. His team found that the radiation caused significant long-term brain damage, including cognitive impairments and dementia, a result of brain inflammation and damage to the rodents’ neurons. The mice’s brain cells showed a sharp reduction in features called dendrites and spines, like a tree losing its leaves and branches, disrupting the transmission of signals among neurons. The radiation also affected part of the brain that normally suppresses prior unpleasant and stressful associations, a process called fear extinction which, if disabled, can cause anxiety. “This is not positive news for astronauts deployed on a two- to three-year round-trip to Mars,” Limoli told me at the time of his 2016 study.
However, as often seen in animal studies, the dose rate in this experiment—bursts between 0.05 and 0.25 Gy / min—was much higher than what would be expected in a human mission to Mars, in which the estimated total dose for a six-month mission is 1 Gy, or 100 rad, evenly dispersed over time. The scientists weren’t able to place mice in a habitat with constant exposure to space radiation for six months. Instead, mice were bombarded in great bursts of radiation from a particle accelerator at the NASA Space Radiation Laboratory at Brookhaven National Laboratory and then *observed* for six months. But dose rate matters. Drinking six beers in one hour might get you drunk; drinking six beers in six hours, maybe not. Same exposure, different rate. Better-designed studies would be needed to truly test whether astronauts will be sane or “punch drunk from radiation” when they arrive at Mars.
Other researchers have found that proton radiation causes attention deficits and poor task performance in rats in a simulated space environment and that HZE particles caused an increase in amyloid beta plaque growth associated with Alzheimer’s disease. From clinical studies, we know that people who undergo certain kinds of radiation treatment for brain cancer can be cured but have notable declines in their cognitive function. The term is radiation-induced cognitive decline. Upward of half of all patients who receive cranial radiation treatment and who survive their cancer for at least six months will be left with progressive cognitive impairment, particularly in the domains of processing speed (thinking quickly) and memory. But again, this may not translate directly to space: the patients are receiving intense radiation over a period of a few months, whereas in space the radiation exposure on a trip to Mars would be spread out across nearly three years.
What can be done to mitigate the risks? Shielding, lots and lots of shielding. Cosmic radiation is more energetic than solar radiation. Basically, it’s moving faster; and some of those atomic bits, such as iron nuclei, are far heavier than the protons and electrons in the solar wind. An iron nucleus would be hundreds of times more energetic than a hydrogen nucleus, which is what a proton is. Flimsy shielding is worse than nothing at all because of the cascade of secondary particles, like shrapnel. A thin layer of spacecraft metal merely scatters the impact of a cosmic ray, turning one fast bullet into scores of only slightly slower bullets. The ship needs thick shielding, and how thick is a simple matter of physics—and economics.
A few centimeters of lead would do the trick. But that would add hundreds of tons to the mission and hence billions of dollars. Water can act as an effective shield. And we need to bring water anyway. So, engineers are playing around with the idea of a hull encompassing the entire craft filled with water. You’d need a lot of it, though, to protect a spacecraft large enough to take a crew to Mars—that is, much more water than you’d need to drink. You can also use trash as extra protection. Still not enough material, but it helps. One very effective shield with low mass would be hydrogen gas, but you’d need a pressurized chamber to hold it, bringing too much mass back into the equation.
The answer to the shielding problem may be a combination of ideas that makes materials serve double-duty. In this regard, hydrogenated boron nitride nanotubes, or hydrogenated BNNTs, show great promise. These tubes are made of carbon, boron, and nitrogen. They are extremely light, hold up to heat and pressure, and are strong enough to serve as load-bearing primary structures for the entire spacecraft. The tubes could be filled with hydrogen gas or water as a primary radiation shield. Boron is an excellent absorber of secondary neutrons, minimizing the radiation cascade effect. As with carbon nanotubes, BNNTs are prohibitively expensive for now but may come down in price in the near future. If the entire craft can’t have such a shield, perhaps just the sleeping chambers could, which would effectively reduce radiation exposure by a third if the crew spent eight hours a day sleeping or resting. Perfect protection, as we have on Earth, might not be feasible, but partial protection might reduce the health risks enough to relieve everyone’s worries.
Researchers at CERN in Switzerland are working on a magnetic force field to serve as a mini-magnetosphere to naturally deflect the cosmic rays. In 2014, CERN broke a record by creating a current of 20,000 amps at a temperature of 24 Kelvin (about −249°C) in an electrical transmission line comprising a pair of twenty-meter-long cables made of a magnesium diboride (MgB2) superconductor. While that bodes well for cheaper and more reliable power transmission on Earth, CERN also joined the European Space Radiation Superconducting Shield project to apply the technology to a spacecraft and space habitat. The goal is to create a magnetic field 3,000 times stronger than Earth’s own magnetic field, with a ten-meter diameter protecting astronauts within or directly outside a spacecraft. CERN is working on a way to reconfigure the electrical coil for space with MgB2 superconducting tape. All of this—the magic materials and the force field—are years away from application. There’s no solution to the cosmic radiation problem in the near future aside from the hope that it isn’t as bad as the laboratory studies are predicting.
Excerpt adapted from Spacefarers: How Humans Will Settle the Moon, Mars, and Beyond, by Christopher Wanjek, published by Harvard University Press. Copyright © 2020 by the President and Fellows of Harvard College. Used by permission. All rights reserved.
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