每日阅读2014年11月.doc

TouchdownComets are leftovers from the birth of the solar system. For the first time, a space ship has landed on oneNov 12th 2014 | From the print edition SPACE exploration is a serious scientific business. But ever since the beginning of the Space Age in the 1950s, it has been accompanied by a hefty dose of glitz and PR. Two years ago, Earthlings watched with bated breath as a one-tonne, nuclear-powered, laser-armed robot rover fizzed through the Martian atmosphere, before being deposited gently on the surface by a rocket-powered “skycrane”. The distance between Mars and Earth meant that the missions controllers had to wait seven agonising minutes to find out whether the rover had survived the journey. Their fingernail biting was broadcast live by NASA. When news arrived of a successful landing, they whooped, hugged and lit cigars.Now, it is the turn of the European Space Agency (ESA) to put on a show. In August, after ten years blazing a circuitous trail through the solar system, including three fly-bys of Earth, one of Mars, two trips through the asteroid belt and a two-and-a-half year hibernation in the chilly void beyond Jupiter, its spacecraft, Rosetta, caught up with a 4km-wide comet called 67P/Churyumov-Gerasimenko. It will spend a year in orbit around this, hitching a ride as it plunges towards the sunAnd Rosetta is not alone. At 08.35 GMT on November 12th, the craft launched Philae, a boxy lander the size of a washing machine, which then began a careful, nerve-jangling descent towards the comets surface. Seven and a half hours later, Philae reported that it was downbut perhaps not safely.Philaes landing marks the first time peopleor, rather, their robotic representativeshave made a soft landing on an astronomical body other than a planet or a moon. It raises the number of bodies on which a landing has been accomplished to five (the others are Earths Moon, Venus, Mars and Titan, a satellite of Saturn).Engineers are reasonably comfortable with sending probes to big targets like planets and moons, which have useful features like an atmosphere to slow their descent, and gravity strong enough to ensure that once a probe reaches the surface, it stays there. Comets have neither, and that makes things difficult. Philae was nudged gently away from Rosetta at a speed of about 80 centimetres a second, and left to drift towards the comet under the influence of that objects feeble gravity.Ensuring the probe landed on the relatively smooth area of the comet chosen by ESAs scientists was tricky. Philae had no ability to control its descent, which meant that everything depended on the accuracy of Rosettas initial shove. In the event, the probe landed almost exactly on target.Once it reached the surface, though, things went less smoothly. The comets gravity is so low that even a slight bounce could have sent the probe careering back into space. A pair of harpoons, designed to anchor it in place, failed to fire. Philaes mission controllers do not know why. In the absence of the harpoons, they think the landers flexible legs managed to absorb most of the impact energy, but not enough to prevent a small bounce.Philae, in other words, may have landed twice. Even without the harpoons, a system of screws in the base of its legs may offer it some purchase. But it was unclear whether these had been deployed, and therefore (as The Economist went to press) how securelyor otherwisethe probe was attached to the comet. ESA would say only that the mission was a “success”, and that plenty of science data had been collected already. That is good, for the mission is more than just an exercise in risky deep-space engineering. It is designed to drill into the comet, taking samples of its crust and attempting to work out exactly what it is made of. Is it, for instance, mostly rock and ice with a few pockets of gas, or is it a loose pile of rubble held together by gravity?Dusty old historyResearchers are interested in comets because they are space-going fossilsthe builders rubble left over from the solar systems construction 4.6 billion years ago. That means they should contain information about how the solar system came together. One theory, for instance, holds that cometary impacts seeded the newly formed Earth with much of its water. Another suggests that comets are the source of the carbon-based molecules that were the building blocks of the first life. Instruments aboard Rosetta and Philae should be able to address those questions. After ten years, the nail-biting engineering is over, and it is time for the science to begin.MyopiaLosing focusWhy so many Chinese children wear glasses Nov 8th 2014 | BEIJING | From the print edition SPARKLY, spotted or Hello Kitty: every colour, theme, shape and size of frame is available at Eyeglass City in Beijing, a four-storey mall crammed only with spectacle shops. Within half an hour a pair of prescription eyeglasses is ready. That is impressive, but then the number of Chinese wearing glasses is rising. Most new adoptees are children.In 1970 fewer than a third of 16- to 18-year-olds were deemed to be short-sighted (meaning that distant objects are blurred). Now nearly four-fifths are, and even more in some urban areas. A fifth of these have “high” myopia, that is, anything beyond 16 centimetres (just over six inches) is unclear. The fastest increase is among primary school children, over 40% of whom are short-sighted, double the rate in 2000. That compares with less than 10% of this age group in America or Germany.The incidence of myopia is high across East Asia, afflicting 80-90% of urban 18-year-olds in Singapore, South Korea and Taiwan. The problem is social rather than genetic. A 2012 study of 15,000 children in the Beijing area found that poor sight was significantly associated with more time spent studying, reading or using electronic devicesalong with less time spent outdoors. These habits were more frequently found in higher-income families, says Guo Yin of Beijing Tongren Hospital, that is, those more likely to make their children study intensively. Across East Asia worsening eyesight has taken place alongside a rise in incomes and educational standards.The biggest factor in short-sightedness is a lack of time spent outdoors. Exposure to daylight helps the retina to release a chemical that slows down an increase in the eyes axial length, which is what most often causes myopia. A combination of not being outdoors and doing lots of work focusing up close (like writing characters or reading) worsens the problem. But if a child has enough time in the open, they can study all they like and their eyesight should not suffer, says Ian Morgan of Australian National University.Yet China and many other East Asian countries do not prize time outdoors. At the age of six, children in China and Australia have similar rates of myopia. Once they start school, Chinese children spend about an hour a day outside, compared with three or four hours for Australian ones. Schoolchildren in China are often made to take a nap after lunch rather than play outside; they then go home to do far more homework than anywhere outside East Asia. The older children in China are, the more they stay indoorsand not because of the countrys notorious pollution.AdvertisementSince poor sight is associated with higher incomes and more schooling, it is less prevalent in rural areas of China. In the countryside a third of primary-school students are myopic, compared with nearly half of urban children, according to the heal