crew of expedition 10 took shelter as a precaution in 2005 in a more heavily shielded part of the station designed for this purpose. however, beyond the limited protection of earth's magnetosphere, interplanetary human missions are much more vulnerable. lawrence townsend of the university of tennessee and others have studied the most powerful solar flare ever recorded. radiation doses astronauts would receive from a flare of this magnitude could cause acute radiation sickness and possibly even death. there is scientific concern that extended spaceflight might slow down the body's ability to protect itself against diseases. radiation can penetrate living tissue and cause both short and long-term damage to the bone marrow stem cells which create the blood and immune systems. in particular, it causes 'chromosomal aberrations' in lymphocytes. as these cells are central to the immune system, any damage weakens the immune system, which means that in addition to increased vulnerability to new exposures, viruses already present in the body—which would normally be suppressed—become active. in space, t-cells (a form of lymphocyte) are less able to reproduce properly, and the t-cells that do reproduce are less able to fight off infection. over time immunodeficiency results in the rapid spread of infection among crew members, especially in the confined areas of space flight systems. on 31 may 2013, nasa scientists reported that a possible human mission to mars may involve a great radiation risk based on the amount of energetic particle radiation detected by the rad on the mars science laboratory while traveling from the earth to mars in 2011–2012. in september 2017, nasa reported radiation levels on the surface of the planet mars were temporarily doubled, and were associated with an aurora 25-times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month. weightlessness astronauts on the iss in weightless conditions. michael foale can be seen exercising in the foreground. following the advent of space stations that can be inhabited for long periods of time, exposure to weightlessness has been demonstrated to have some deleterious effects on human health. humans are well-adapted to the physical conditions at the surface of the earth, and so in response to weightlessness, various physiological systems begin to change, and in some cases, atrophy. though these changes are usually temporary, some do have a long-term impact on human health. short-term exposure to microgravity causes space adaptation syndrome, self-limiting nausea caused by derangement of the vestibular system. long-term exposure causes multiple health problems, one of the most significant being loss of bone and muscle mass. over time these deconditioning effects can impair astronauts' performance, increase their risk of injury, reduce their aerobic capacity, and slow down their cardiovascular system. as the human body consists mostly of fluids, gravity tends to force them into the lower half of the body, and our bodies have many systems to balance this situation. when released from the pull of gravity, these systems continue to work, causing a general redistribution of fluids into the upper half of the body. this is the cause of the round-faced 'puffiness' seen in astronauts. redistributing fluids around the body itself causes balance disorders, distorted vision, and a loss of taste and smell. a 2006 space shuttle experiment found that salmonella typhimurium, a bacterium that can cause food poisoning, became more virulent when cultivated in space. on april 29, 2013, scientists in rensselaer polytechnic institute, funded by nasa, reported that, during spaceflight on the international space station, microbes seem to adapt to the space environment in ways "not observed on earth" and in ways that "can lead to increases in growth and virulence". more recently, in 2017, bacteria were found to be more resistant to antibiotics and to thrive in the near-weightlessness of space. microorganisms have been observed to survive the vacuum of outer space. motion sickness main article: space adaptation syndrome bruce mccandless ii floating free in orbit with a space suit and manned maneuvering unit. the most common problem experienced by humans in the initial hours of weightlessness is known as space adaptation syndrome or sas, commonly referred to as space sickness. it is related to motion sickness, and arises as the vestibular system adapts to weightlessness. symptoms of sas include nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. the first case of sas was reported by cosmonaut gherman titov in 1961. since then, roughly 45% of all people who have flown in space have suffered from this condition. bone and muscle deterioration aboard the international space station, astronaut frank de winne is attached to the colbert with bungee cords a major effect of long-term weightlessness involves the loss of bone and muscle mass. without the effects of gravity, skeletal muscle is no longer required to maintain posture and the muscle groups used in moving around in a weightless environment differ from those required in terrestrial locomotion. in a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up. those muscles then start to weaken and eventually get smaller. consequently, some muscles atrophy rapidly, and without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days. the types of muscle fibre prominent in muscles also change. slow-twitch endurance fibres used to maintain posture are replaced by fast-twitch rapidly contracting fibres that are insufficient for any heavy labour. advances in research on exercise, hormone supplements, and medication may help maintain muscle and body mass. bone metabolism also changes. normally, bone is laid down in the direction of mechanical stress. however, in a microgravity environment, there is very little mechanical stress. this results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip, and femur. due to microgravity and the decreased load on the bones, there is a rapid increase in bone loss, from 3% cortical bone loss per decade to about 1% every month the body is exposed to microgravity, for an otherwise healthy adult. the rapid change in bone density is dramatic, making bones frail and resulting in symptoms that resemble those of osteoporosis. on earth, the bones are constantly being shed and regenerated through a well-balanced system which involves signaling of osteoblasts and osteoclasts. these systems are coupled, so that whenever bone is broken down, newly formed layers take its place—neither should happen without the other, in a healthy adult. in space, however, there is an increase in osteoclast activity due to microgravity. this is a problem because osteoclasts break down the bones into minerals that are reabsorbed by the body. osteoblasts are not consecutively active with the osteoclasts, causing the bone to be constantly diminished with no recovery. this increase in osteoclasts activity has been seen particularly in the pelvic region because this is the region that carries the biggest load with gravity present. a study demonstrated that in healthy mice, osteoclasts appearance increased by 197%, accompanied by a down-regulation of osteoblasts and growth factors that are known to help with the formation of new bone, after only sixteen days of exposure to microgravity. elevated blood calcium levels from the lost bone result in dangerous calcification of soft tissues and potential kidney stone formation. it is still unknown whether bone recovers completely. unlike people with osteoporosis, astronauts eventually regain their bone density. after a 3–4 month trip into space, it takes about 2–3 years to regain lost bone density. new techniques are being developed to help astronauts recover faster. research on diet, exercise, and medication may hold the potential to aid the process of growing new bone. to prevent some of these adverse physiological effects, the iss is equipped with two treadmills (including the colbert), and the ared (advanced resistive exercise device), which enable various weight-lifting exercises which add muscle but do nothing for bone density, and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment. astronauts use bungee cords to strap themselves to the treadmill. astronauts subject to long periods of weightlessness wear pants with elastic bands attached between waistband and cuffs to compress the leg bones and reduce osteopenia. currently, nasa is using advanced computational tools to understand how to best counteract the bone and muscle atrophy experienced by astronauts in microgravity environments for prolonged periods of time. the human research program's human health countermeasures element chartered the digital astronaut project to investigate targeted questions about exercise countermeasure regimes. nasa is focusing on integrating a model of the advanced resistive exercise device (ared) currently on board the international space station with opensim musculoskeletal models of humans exercising with the device. the goal of this work is to use inverse dynamics to estimate joint torques and muscle forces resulting from using the ared, and thus more accurately prescribe exercise regimens for the astronauts. these joint torques and muscle forces could be used in conjunction with more fundamental computational simulations of bone remodeling and muscle adaptation in order to more completely model the end effects of such countermeasures, and determine whether a proposed exercise regime would be sufficient to sustain astronaut musculoskeletal health. fluid redistribution the effects of microgravity on fluid distribution around the body (greatly exaggerated). the beckman physiological and cardiovascular monitoring system in the gemini and apollo suits would inflate and deflate cuffs to stimulate blood flow to lower limbs astronaut clayton anderson observes as a water bubble floats in front of him on the space shuttle discovery. water cohesion plays a bigger role in microgravity than on earth in space, astronauts lose fluid volume—including up to 22% of their blood volume. because it has less blood to pump, the heart will atrophy. a weakened heart results in low blood pressure and can produce a problem with "orthostatic tolerance", or the body's ability to send enough oxygen to the brain without the astronaut's fainting or becoming dizzy. "under the effects of the earth's gravity, blood and other body fluids are pulled towards the lower body. when gravity is taken away or reduced during space exploration, the blood tends to collect in the upper body instead, resulting in facial edema and other unwelcome side effects. upon return to earth, the blood begins to pool in the lower extremities again, resulting in orthostatic hypotension." vision in 2013 nasa published a study that found changes to the eyes and eyesight of monkeys with spaceflights longer than 6 months. noted changes included a flattening of the eyeball and changes to the retina. space traveler's eye-sight can become blurry after too much time in space. another effect is known as cosmic ray visual phenomena. nasa survey of 300 male and female astronauts, about 23 percent of short-flight and 49 percent of long-flight astronauts said they had experienced problems with both near and distance vision during their missions. again, for some people vision problems persisted for years afterward.— nasa since dust can not settle in zero gravity, small pieces of dead skin or metal can get in the eye, causing irritation and increasing the risk of infection. long spaceflights can also alter a space traveler's eye movements (particularly the vestibulo-ocular reflex). intracranial pressure main article: visual impairment due to intracranial pressure because weightlessness increases the amount of fluid in the upper part of the body, astronauts experience increased intracranial pressure. this appears to increase pressure on the backs of the eyeballs, affecting their shape and slightly crushing the optic nerve. this effect was noticed in 2012 in a study using mri scans of astronauts who had returned to earth following at least one month in space. such eyesight problems could be a major concern for future deep space flight missions, including a crewed mission to the planet mars. if indeed elevated intracranial pressure is the cause, artificial gravity might present one solution, as it would for many human health risks in space. however, such artificial gravitational systems have yet to be proven. more, even with sophisticated artificial gravity, a state of relative microgravity may remain, the risks of which remain unknown. taste one effect of weightlessness on humans is that some astronauts report a change in their sense of taste when in space. some astronauts find that their food is bland, others find that their favorite foods no longer taste as good (one who enjoyed