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unit7

By Martin Stevens,2014-01-21 17:21
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unit7

     astronaut n. 宇航员

     hatch n. ?门、地板上的?开口,活动板!舱口

     capsule n. 1.太空舱,宇宙飞船密封舱 2.胶囊

     aviation n. 航空! 航空学!飞行术

     weightless a. 没有重量的!失重的

     weightlessness n. 失重?状态?!没有重量

     comic a. 1.令人发笑的!滑稽的 2.喜剧的!有喜剧特点的

     clockwise ad.&a. 顺时针地?的?

     testify v. 证明!证实

     novelty n. 1.新奇!新颖!新鲜 2.新奇的事物

     numerical a. 数字的,数字表示的!数值的

     qualitative a. 质量的!性质?上?的!定性的

     analytic a. 分析的!善于分析的

     kidney n. 肾,肾脏

     bowel n.

     jerk v. ?使?猛地一动!急拉

    n. 猛拉!猛动

     stationary a. 静止的!不动的

     apparatus n. (pl. -tuses or -tus) 1.器官 2.仪器!装置! 器械

     partition vt. 1.分割!划分 2.用隔板隔离或隔开 n. 隔开物!隔墙!隔板

     calcium n.

     linear a. 线的,线状的

     orientation n. 方位,方向

     spontaneous a. 自发的!自动的

     spontaneously ad. 自发地,自动地

     shuttle n. 航天飞机

     astronomy n. 天文学

     invert v. ?使?倒转! ?使?反向!?使?颠倒

     literal a. 实实在在的,确确实实的

     supersonic a. 超音速的

     aggregate n. 总数!合计!总量

    vt. 总计,共计

     impair vt. 损害!削弱

     impaired a. 损害的!削弱的

     converge vi. 会聚,集中

     mediate v. 调解,斡旋

     visualize vt. 设想!想像

    Phrases and Expressions

     cut off 使停止运转

     throw up 呕吐

     lunar a. 月球的!月亮的

     loom vi. 隐约出现!显得重要或令人生畏

     dubious a. 1.引起怀疑的!不确定的,含糊的 2.觉得可疑的,怀疑的

     recede v. 变得模糊以至消失

     subsidiary a. 附带的,次要的,附属的

     eject vt. 1.扔出!喷出 2.强制离开!驱逐

     invaluable a. 非常宝贵的!无价的

     revelation n. 1.揭露,透露,揭示 2.被揭示的真相!水落石出的事实

     galaxy n. 星系

     chord n. 和弦,和音

     thesis n.(pl. theses) 1.论题!论点 2.毕业论文

     aerial a. 空气?中?的!空中的

     drainage n. 排水!排水系统! 排水装置

     tract n. 1.一大片?土地? 2.?动物体中的??器官?系统!管!

     redundant a. 多余的!没必要的

     oxide n. 氧化物

     ultraviolet a. ?光线?紫外的

     rig vt. 1.装置!配备 2.给?船只?装配船帆、索具等

     rotary a. 旋转的!转动的

     alloy n. 合金

     sieve vt. 筛?出?!滤?出?

    n. 格筛,筛子!滤网

     unanimous a. 1.一致?同意?的!无异议的 2.?决议等?得到一致赞同的

     polar a. ?南、北?极地的

     radius n. 半径

     automation n. 自动化?操作?

     intrigue vt. 激起?极大?兴趣

     intriguing a. 引起好奇心的!令人感兴趣的

     relay vt. 1.传达,传递 2.转播

     supervise vt. 监督!管理!指导

     shipment n. ?从海路、陆路或空中运的?一批货物

    Phrases and Expressions

     venture into 冒险进入

     strike a chord 使人想起某事!引起共鸣

     hang on 坚持

     bring up 带上来!提升

The Effects of Space Travel on the Human Body (Part 1)

     When a healthy Russian astronaut opened the hatch of his space capsule after a

    world-record 438 days on the Mir Space Station, he had demonstrated that humans could

    live and work in space for months at a time. It was not always clear that this would

    be the case.

     In 1951, more than 10 years before the first human space flight, an expert in aviation medicine tried to predict some of the medical effects of space travel and, in particular, of weightlessness. Some of the things he predicted, such as the motion sickness that often occurs at the beginning of a flight, have been observed in real life. Others, such as the comic notion that space travelers would suddenly start to spin clockwise during normal motion in space, have not.

     As most doctors can testify, it is difficult to predict what will happen when the novelty of a brand-new challenge is presented to the human body. Time and again, space travel has revealed the body's marvelous and sometimes subtle ability to adapt. But only in the last few years have scientists begun to understand the body's responses to weightlessness, as both numerical and qualitative data have grown tremendously. Pursuit of this analytic knowledge is improving health care not only for those who journey into space but also for those of us stuck on the ground. The unexpected outcome of space medicine has been an enhanced understanding of how the human body works right here on Earth.

     Although many factors affect human health during periods in space, weightlessness is the dominant and single most important one. The direct and indirect effects of weightlessness lead to a series of related responses. Ultimately, the whole body, from bones to brain, kidneys to bowels, reacts.

     When space travelers grasp the wall of their spacecraft and jerk their bodies back and forth, they say it feels as though they are stationary and the spacecraft is moving. This is due to our reliance on gravity to perceive our surroundings.

     The continuous and universal nature of gravity removes it from our daily notice, but our bodies never forget. Whether we realize it or not, we have evolved a large number of silent, automatic reactions to cope with the constant stress of living in a downward-pulling world. Only when we decrease or increase the effective force of gravity on our bodies do our minds perceive it.

     Our senses provide accurate information about the location of our center of mass and the relative positions of our body parts. Our brains integrate signals from our eyes and ears with other information from the organs in our inner ear, from our muscles and joints, and from our senses of touch and pressure.

     The apparatus of the inner ear is partitioned into two distinct components: circular, fluid-filled tubes that sense the angle of the head, and two bags filled with calcium crystals embedded in a thick fluid, which respond to linear movement. The movement of the calcium crystals sends a signal to the brain to tell us the direction of gravity. This is not the only cue the brain receives. Nerves in the muscles, joints, and skin particularly the skin on the bottom of the feet

respond to the weight of limb segments and other body parts.

     Removing gravity transforms these signals. The inner ear no longer perceives a downward tendency when the head moves. The limbs no longer have weight, so muscles are no longer required to contract and relax in the usual way to maintain posture and bring about movement. Nerves that respond to touch and pressure in the feet and ankles no longer signal the direction of down. These and other changes contribute to orientation illusions, such as a feeling that the body or the spacecraft spontaneously changes direction. In 1961 a Russian astronaut reported vivid sensations of being upside down; one space shuttle specialist in astronomy said "when the main engines cut off, I immediately felt as though we had inverted 180 degrees." Such illusions can recur even after some time in space.

     The lack of other critical environmental cues also confuses the brain. Although flight around the Earth is a literal free fall the only difference from "normal"

    falling is that the spacecraft's supersonic forward velocity carries it around the curve of the planet space travelers say they do not feel as if they are falling. The perception of falling probably depends on visual and wind cues along with information from the organs that sense gravity directly.

     The aggregate of the changes in brain signals produces a motion sickness that features many of the same symptoms as motion sickness on Earth: headache, impaired concentration, loss of appetite, and even throwing up. Space motion sickness may affect half or more of space travelers, but usually does not last beyond the first three days or so of weightlessness.

     At one time, scientists attributed space motion sickness to the unusual pattern of inner ear activity, which conflicts with the brain's expectations. Now it is clear that this explanation was too simple. The sickness results as a variety of factors converge, including the alteration of the patterns and levels of muscle activity necessary to control the head itself. A similar motion sickness can also be elicited by computer systems designed to create virtual environments, through which one can move without the forces and nerve signals present during real motion.

     Over time, the brain learns to mediate between conflicting signals, and some space travelers visualize "down" as simply where their feet are. This process probably involves physiological changes in nerve-cell patterns. Similar changes occur on the ground during children's growth and during periods of major body-weight changes. The way we control our balance and avoid falls is an important and poorly understood part of medical science. Because otherwise healthy people returning from space initially have difficulty maintaining their balance but recover this sense rapidly, studies of returning astronauts may allow doctors to help others who suffer a loss of balance on Earth, such as the elderly.

The Argument for Going to Mars

     For centuries, explorers have risked their lives venturing into the unknown for reasons of economic benefit and national glory. Following the lunar missions of the early 1970s, Mars now looms as humanity's next great, unknown land. But with dubious prospects for short-term financial return and with international competition in space a receding memory, it is clear that imperatives other than profit or national pride will have to compel human beings to leave their tracks on the planet's red surface. Could it be that science, which has long been a subsidiary concern for explorers, is at last destined to take a leading role? This question naturally invites a couple of others: Are there experiments that only humans could do on Mars? Could those experiments provide insights profound enough to justify the expense of sending people across such a large distance?

     With Mars the scientific benefits are perhaps higher than they have ever been. The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by accumulating evidence that Mars once had abundant liquid water and by the controversy over suggestions that fossils of bacteria rode to Earth on a rock ejected from Mars during its early history. A definite answer about life on Mars, past or present, would give researchers invaluable data about the range of conditions under which a planet can generate the complex chemistry that leads to life. The revelation that life arose independently on Mars and on Earth would provide the first concrete clue in one of the deepest mysteries in all of science: how prevalent is life in our galaxy?

     One of the reasons why the idea of sending people to Mars strikes a chord in so many people is that it is already possible the US has the money and the fundamental

    technology needed to do it. More important, recent discoveries about the planet's environment in the distant past have presented a clear and compelling scientific incentive for sending people: to search for evidence of life. The thesis that liquid water was once stable on Mars has been strengthened by aerial photographs taken last year that showed what appeared to be a drainage channel cut deeply by water flowing for hundreds if not thousands of years.

     A thorough hunt for any life on Mars that might be hanging on would also have to be undertaken by humans, according to some experts. Such life will be hidden and probably tiny. "Finding it will require surveying vast tracts of territory," one expert explains. "It will require the ability to cover long distances and adapt to different conditions." Robots might be up to the task sometime in the distant future, making human explorers redundant, he concedes. But relying on them to survey Mars during periodical missions to the planet would take a very long time "decades if

not centuries," he believes.

     Another reason why humans may have to be on site to conduct a thorough search for life stems from the fact that if any such life exists it is probably deep underground. Mars' atmosphere contains trace quantities of a chemical agent that destroys organic compounds, turning them into inorganic oxides. So most strategies for bacteria hunting involve digging down to depths where life or organic matter would be shielded from this chemical agent as well as from extremely high levels of ultraviolet light.

     Future probes will be rigged with rotary drills made of a special steel alloy that can bore several centimeters into rocks or dig a few meters down into the soil. But barring any discoveries sieved from those shallow depths, researchers will have to bring up samples from hundreds of meters below the surface before they can declare Mars dead or alive. Drilling for samples at such depths most likely will require humans.

     Researchers are unanimous in saying that a human mission to Mars would advance our understanding of the planet. The points of contention have to do with the cost of human missions in comparison with robotic ones. The problem is that so little is known about several key factors that any analysis must depend on some largely arbitrary assumptions.

     Then, too, it is difficult to predict the capacities of robots even 5 or 10 years from now. Today the kind of robot that can be delivered to another planet is not really up to the demands of a game of cards, let alone those of fossil hunting in a complex environment that is colder than the polar regions of Earth. The kinds of robots used so far on Mars have been terribly limited: the last robot delivered to Mars traveled within a radius of just 106 meters from the landing site. And the best computer brains for robots can't even match the intellectual ability of an insect, making the automation of the explorer's task difficult indeed.

     One intriguing option is to send a robot that would be controlled by human operators on Earth. Unfortunately, the round-trip time to relay communications from Mars is up to 40 minutes long. "You can't yet do it this way," an expert says. "At best, you can do something like supervise an independent robot, and I don't think that would be good enough to do serious scientific work." One fact everyone agrees on is that human space missions are costly. Estimates of the cost of a human mission to Mars range from $20 billion to about $55 billion.

     Although a human mission would be more expensive, it would also be more cost-effective, some advocates of human missions insist. One expert concedes that sending astronauts to collect rock and soil samples and bring them to Earth would cost about 10 times more than sending robots. But by his calculations the human mission

    would return with a shipment of 100 times more material, gathered from an area 10,000 times larger. With its enormous territory, amazing landscape and difficult climate, Mars will surely be conquered only by a combination of people and machines.

The Effects of Space Travel on the Human Body (Part 2)

     During weightlessness, the forces within the body undergo dramatic change. Because the spine is no longer compressed, people grow taller (two inches or so). The lungs, heart, and other organs within the chest have no weight, and as a result, the rib cage and chest relax and expand. Similarly, the weight of the liver, kidneys, stomach, and bowels disappears. One astronaut said after his flight: "You feel your guts (内脏) floating up. I found myself tightening my belly, sort of pushing things back."

     Meanwhile muscles and bones come to be used in different ways. Our muscles are designed to support us when standing or sitting upright and to move body parts. But in space, muscles used for support on the ground are no longer needed for that purpose; moreover, the muscles used for movement around a capsule differ from those used for walking down a hall. Consequently, some muscles rapidly weaken. This doesn't present a problem to space travelers as long as they perform only light work. But preventing the loss of muscle tissue required for heavy work during space walks and preserving muscle for safe return to Earth are the subject of many current experiments.

     Bone physiology, too, changes substantially. One of the strongest known biological materials, bone is a dynamic tissue. Some cells have the job of producing it, whereas others destroy it. Both types usually work together to maintain bones throughout life.

     Bone contains both organic materials, which contribute strength and stability, and inorganic materials, which make the bones stiff and serve as a reservoir of minerals within the body. For example, 99 percent of the calcium in the body is in the skeleton. Stable levels of calcium in the body's fluids are necessary for all types of cells to function normally.

     Studies have shown that astronauts lose bone mass from the lower spine, hips, and upper leg at a rate of about 1 percent per month for the entire duration of their time in space. Some sites, such as the heel, lose calcium faster than others. Studies of animals taken into space suggest that bone formation also declines.

     Needless to say, these data are indeed cause for concern. During space flight, the loss of bone elevates calcium levels in the body, potentially causing kidney stones and calcium crystals to form in other tissues. Back on the ground, the loss

    of bone calcium stops within one month, but scientists do not yet know whether the bone recovers completely: too few people have flown in space for long periods. Some bone loss may be permanent, in which case ex-astronauts will always be more prone to broken bones.

     These questions mirror those in our understanding of how the body works here on Earth. For example, elderly women are prone to a loss of bone mass?骨质疏松症?.

    Scientists understand that many different factors can be involved in this loss, but they do not yet know how the factors act and interact; this makes it difficult to develop an appropriate treatment. So it is with bone loss in space, where the right prescription still awaits discovery.

     Many other body systems are affected directly and indirectly. One example is the lung. Scientists have studied the lung in space and learned much they could not have learned in laboratories on Earth. On the ground the top and bottom parts of the lung have different patterns of air flow and blood flow. But are these patterns the result only of gravity, or also of the nature of the lung itself? Only recently have studies in space provided clear evidence for the latter. Even in the absence of gravity, different parts of the lung have different levels of air flow and blood flow.

     Not everything that affects the body during space flight is related solely to weightlessness. Also affected, for example, are the immune system (the various physical and psychological stresses of space flight probably play roles in weakening the immune system in astronauts) and the multiple systems responsible for the amount and quality of sleep (light levels and work schedules disrupt the body's normal rhythms). Looking out the spacecraft window just before going to sleep (an action difficult to resist, considering the view) can let enough bright light into the eye to trigger just the wrong brain response, leading to poor sleep. As time goes on, the sleep debt accumulates.

     For long space voyages, travelers must also face being confined in a tight volume, unable to escape, isolated from the normal life of Earth, living with a small, fixed group of companions who often come from different cultures. These challenges can lead to anxiety, depression, crew tension and other social issues, which affect astronauts just as much as weightlessness perhaps even more. Because these factors operate

    at the same time the body is adapting to other environmental changes, it may not be clear which physiological changes result from which factors. Much work remains to be done.

     Finally, space flight involves high levels of radiation. An astronaut spending one year in a low-Earth orbit would receive a radiation dose 10 times greater than the average dose received on the ground. A year's stay on the moon would result in a dose seven times higher still, whereas a flight to Mars would be even worse. A sudden surge in radiation from the sun, as occurred in August 1972, can deliver a dose more

    than 1,000 times the annual ground dose in less than a day. Fortunately, such events are rare, and spacecraft designers can guard against them by providing special shielded rooms to which astronauts can retreat.

     Obviously, the radiation hazard to long-duration space travelers and the

    consequent cancer risk is a major problem. The problems of space radiation are

    difficult to study because it is nearly impossible to duplicate on Earth the radiation environment of space, with its low but steady flow of high-energy particles. Even so, researchers generally believe that with proper radiation shields built into the spacecraft and protective drugs, the risks can be brought within satisfactory limits.

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