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Solar Flares - Massive coronal mass ejections of protons and helium nuclei from the Sun can be evaded by sending astronauts into shielded compartments, where hydrogen-rich materials such as polyethylene or water may be used to absorb such radiation (since collision with the protons of heavier atoms can dislodge neighboring neutrons and cause dangerous radiation cascades). Accurate monitoring of the Sun to warn of such events has improved with the availability of the Solar and Heliospheric Observatory (SOHO).
Cosmic Rays - Such radiation include heavy ions (i.e., heavier than helium nuclei) flung from supernovae at near light-speed. Such radiation tends to disrupt cellular structure and DNA in a branching pattern from the creation of secondary particles of radiation, leading to tissue damage and genetic mutation. Currently, there is no effective shielding against such high energy (speed) radiation. For a thousand-day stay in space (i.e., a voyage to Mars and back), the exposure may be a thousand times the dose of a year's stay on Earth at sea level. (See NASA's effort to assess radiation exposure on the International Space Station through a dummy torso.)
First "Alpha" (Expedition 1) crew of the
International Space Station in late 2000.
From left: Sergei Konstantinovich
Krikalev, Yuri Pavolich Gidzenko,
and Bill (William) M. Shepherd.
Vacuum Pressure and Impact Shielding
In the vacuum of space, space structures have to be air tight to be habitable for Earth-type life. A major force pressing outward from such structures is the pressure of the air inside a space vehicle or station, at about 15 pounds of force on each square inch (2.54 by 2.54 cm). While structures on Earth also have this internal pressure, the external pressure of the atmosphere balances it out.
In order to help contain internal air, each of the modular components of the current Space Station have an outer shell of lightweight aluminum, rather than heavier steel. This shell and a protective layer of advanced materials such as impact-resistant Kevlar and ceramic fabrics provide some protection from impacts by tiny meteoroids and man-made debris. Because the Station orbits the Earth at about 27,000 km (about 17,000 milles) per hour, even dust-sized grains puncture a module and cause an air leak. Man-made debris, a drifting legacy of past space exploration, poses an even greater threat.
NASA -- larger image
To ensure the safety of crew
aboard from small impacts, the
Space Station is covered with
a "bullet-proof vest," made of
layers of Kevlar, ceramic fabrics,
and other advanced materials.
Tested with high-velocity guns
on Earth, these layers of
protective shielding form a
blanket up to 10 cm (3.9 inches)
thick around each module's
aluminum shell. Windows are
made of four panes of glass from
1/2 to 1-1/4 inches thick, with
exterior shutters made of
aluminum like the walls (more).
Bone and Muscle Loss - Weightlessness triggers bone mineral loss, where the density of weight-bearing bones (e.g., pelvis and leg bones) declines at the rate of one to two percent per month. Mimicking osteoporosis, the process begins with the atrophy of large weight-bearing muscles in the legs and reduces the torsion and compression on attached bones. The result is greatly reduced bone renewal and possibly a greater risk of forming kidney stones. Moreover, the heart will also lose muscle mass and weaken.
On Earth, the bone in our bodies is continually being renewed. Old bone is being absorbed, but new bone is formed. Without the stress of gravity, however, bone renewal is greatly reduced in space so that some astronauts have lost 20 percent of hip bone density. Accumulated bone loss may level off at about 40 percent, if the experience of paraplegics on Earth is applicable to prolonged exposure to microgravity.
Over four and a half months on the Mir Space Station, astronaut David A. Wolf lost 40 percent of his body's muscle mass, 12 percent of his bone mass, and 23 pounds. After Wolf returned to Earth, it took six months to recover the lost strength and a year to get back the lost bone mass. Two other astronauts that stayed over over four months still have bone deficits after more than two years on Earth.
Countermeasures include: vigorous exercise (e.g., two hours per day), including working out on a treadmill housed inside a pressure chamber; drugs used to promote bone growth and/or slow mass loss; vibrations to stimulate muscle and bone interaction to promote bone growth (Clinton Rubin of SUNY at Stony Brook; and devices to provide frequent doses of pseudogravity through rotation -- from a short-radius centrifuge or rotating vehicles at high rpm to very large habitats providing Earth gravity ("1 g") at low rpm. (See current research being pursued at the National Space Biomedical Research Institute, or more information on "space bones" from NASA.)
NASA (Advanced Space Transportation)
Two residential modules linked
by a long tether, rotating about
a spacecraft, provide pseudo-
gravity for long voyages.
Disorientation - Disorientation, motion sickness, and a variety of illusions are commonly experienced by more than two-thirds of all astronauts upon reaching orbit, including veteran test pilots. Nausea usually fades after a few days as the body and mind adapts to the lack of gravity stress and related visual and other sensory information. During and after returning to Earth, however, astronauts whose "sense of balance" (i.e., have adapted to weightlessness or pseudogravity through habitat rotation may have trouble standing up, keeping a steady their gaze, walking, turning corners, and "running into doors," and otherwise maintaining a sense of balance. Research is underway to develop countermeasures that can help Humans adjust more rapidly to various gravitational fields (see current research at the National Space Biomedical Institute).
Impaired Healing of Injuries - Astronauts have to be very careful when they're in space because minor injuries typically don't heal until they land back on Earth. The reason for this impairing healing is not well understood. It is known, however, that a cell's mitochondria -- its energy generators -- do not appear to function very well in microgravity.
One proposed countermeasure is boost the efficiency of mitochondria by irradiating them with red and infrared light. This should stimulates them to produce key chemicals called cytochromes. Indeed, fibroblasts and muscle cells have been found to grow five times faster when cultured under such light by Harry Whelan, a neurologist at the Medical College of Wisconsin in Milwaukee. The question is how to generate the light. According to Ronald Ignatius, the president of Quantum Devices, the firm has worked with NASA to develop large and highly efficient LEDs to bathe an area sufficient to irradiate Human beings. Combined with treatment in high-pressure oxygen chambers and perhaps various growth-inducing chemicals, Whelan hopes that the LED-based therapy will provide a way for astronauts to heal their wounds, and even prevent muscle and bone loss during extended spaceflight. (See New Scientist article.)
Sleep Deprivation and Weakened Immune System - Human sleep patterns become disturbed with prolonged exposure to microgravity, the loss of a normal 24-hour light cycle, confined spaces, and overwork during extended space flight. Chronic restriction of sleep to an average of six hours a day has been found in manned space flight. Experiments on Earth suggests that people restricted to four to six hours of sleep per day suffer from accidents, slowed thought processes, and fatigue. Moreover, they may also suffer from a weakened immune system, the reactivation of dormant viruses already present in the body, and a drop in red blood cell levels leading to anemia. Current research is directed at taking actions that promote longer sleep or by adding in a pre-planned (preemptive) nap that breaks up an otherwise, long period of wakefulness. (See current research at the National Space Biomedical Research Center or the problems of being "wide awake in outer space" or on Mars from NASA.)
Motion restraints and a dark
mask will help this floating
astronaut to fall asleep
aboard the brightly-lit, freely
falling space shuttle (more).
Adapting to the Coriolis Effect - If a spaceship could spin its crew comparments, artificial or pseudo-gravity would be created by centripetal forces to keep Human bodies strong and make everyday living easier. Unfortunately, spinning spaceships also generate a strong Coriolis effect so that moving objects and parts of bodies veer in a direction not experienced on Earth. As it turns out, however, Humans in spinning environment (e.g., in spinning spaceships) may be able to adapt to the Coriolis effect better than originally expected. Despite experiments done in the 1960s which seemed to show that Humans did not adapt well to rotation, recent experiments (by James Lackner and Paul DiZioby under the sponsorship of NASA's Office of Biological and Physical Research) involving subjects being given specific motion goals (such as reaching out to touch a target) were able to learn to move accurately after only 10 to 20 attempts.
(John Frassanito & Associates, NASA
Recent experiments suggest that Humans can adapt to the Coriolis effect experienced in spinning space habitats and spaceships -- more).
Apparently, when a goal is present, the Human brain dictates the desired motion to the muscles more precisely so that deviations from that intended motion are detected more readily by sensory feedback to the brain. The hypothesis is that Human bodies and brains may have evolved, to a degree, to deal with the Coriolis effect because every time Humans turn and reach for something simultaneously, they experience a brief Coriolis experience on Earth. Even more interesting, after rotating for a while, subjects no longer perceived the Coriolis effect, and the veering pull on their arms and legs seemed to vanish because their brains had compensated for it and so their minds no longer took notice of it. Indeed, when test-subjects return to a non-rotating environment, they report feeling a Coriolis-pull in the opposite direction, but after another 10 to 20 attempts at a goal-oriented motion, their brains readjust to the non-rotating world and the phantom effect goes away. As a result, most Humans may be able to adapt to rotational speeds as fast as a carnival-like ride of 25 rpm, which is about as fast as most can turn their bodies in daily life. By comparison, a spinning spaceship would likely rotate more slowly, perhaps only 10 rpm, depending on the size and design of the craft (more from NASA).
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