pale blue dot -carl sagan-第32章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
s； far below。 The Vega 2 balloon had an almost identical profile。 The atmosphere of Venus has also been used for aerobraking—changing the Magellan spacecraft's orbit by friction with the dense air； this is a key future technology for converting flyby spacecraft to Mars into orbiters and landers。
A Mars mission； scheduled to be launched in 1998； and led by Russia； includes an enormous French hot air balloon—looking something like a vast jellyfish； a Portuguese man…of…war。 It's designed to sink to the Martian surface every chilly twilight and rise high when heated by sunlight the next day。 The winds are so fast that； if all goes well； it will be carried hundreds of kilometers each day； hopping and skipping over the north pole。 In the early morning； when close to the ground； it will obtain very high resolution pictures and other data。 The balloon has an instrumental guide…rope； essential for its stability； conceived and designed by a private membership organization based in Pasadena； California； The Planetary Society。
Since the surface pressure on Mars is approximately that at an altitude of 100；000 feet on Earth； we know we can fly airplanes there。 The U…2； for example； or the SR…71 Blackbird routinely approaches such low pressures。 Aircraft with even larger wingspans have been designed for Mars。
The dream of flight and the dream of space travel are twins； conceived by similar visionaries； dependent on allied technologies； and evolving more or less in tandem。 As certain practical and economic limits to flight on Earth are reached； the possibility arises of flying through the multihued skies of other worlds。
IT IS NOW ALMOST POSSIBLE to assign color binations； based on the colors of clouds and sky； to every planet in the Solar System—from the sulfur…stained skies of Venus and the rusty skies of Mars to the aquamarine of Uranus and the hypnotic and unearthly blue of Neptune。 Sacre…jaunt； sacre…rouge； sacre…vert。 Perhaps they will one day adorn the flags of distant human outposts in the Solar System； in that time when the new frontiers are sweeping out from the Sun to the stars； and the explorers are surrounded by the endless black of space。 Sacre…noir。
CHAPTER 11 EVENING AND MORNING STAR
This is another world which is not of men。
—LI BAI； 〃QUESTION AND ANSWER IN THE MOUNTAINS〃 （CHINA； TANG DYNASTY； CA。 730）
You can see it shining brilliantly in the twilight； chasing the Sun down below the western horizon。 Upon first glimpsing it each night； people were accustomed to make a wish （〃upon a star〃）。 Sometimes the wish came true。
Or you can spy it in the east before dawn； fleeing the rising Sun。 In these two incarnations； brighter than anything else in the sky except only the Sun and the Moon it was known as the evening and the morning star。 Our ancestors did not recognize it was a world； the same world； never too far from the Sun because it is in an orbit about it interior to the Earth's。 Just before sunset or just after sunrise； we can sometimes see it near some fluffy white cloud； and then discover by the parison that Venus has a color； a pale lemon…yellow。
You peer through the eyepiece of a telescope—even a big telescope； even the largest optical telescope on Earth—and you can make out no detail at all。 Over the months； you see a featureless disk methodically going through phases； like the Moon： crescent Venus； full Venus； gibbous Venus； new Venus。 There is not a hint of continents or oceans。
Some of the first astronomers to see Venus through the telescope immediately recognized that they were examining a world enshrouded by clouds。 The clouds； we now know； are droplets of concentrated sulfuric acid； stained yellow by a little elemental sulfur。 They lie high above the ground。 In ordinary visible light there's no hint of what this planet's surface； some 50 kilometers below the cloud tops； is like； and for centuries the best we had were wild guesses。
You might conjecture that if we could take a much finer look there might be breaks in the clouds； revealing day by day； in bits and pieces； the mysterious surface ordinarily hidden from our view。 Then the time of guesses would be over。 The Earth is on average half cloud…covered。 In the early days of Venus exploration； we saw no reason that Venus should be 100 percent overcast。 If instead it was only 90 percent； or even 99 percent； cloud…covered； the transient patches of clearing might tell us much。
In 1960 and 1961； Mariners 1 and 2； the first American spacecraft designed to visit Venus； were being prepared。 There were those； like me； who thought the ships should carry video cameras so they could radio pictures back to Earth。 The same technology would be used a few years later when Rangers 7； 8； and 9 would photograph the Moon on the way to their crash landings—the last making a bull's…eye in the crater Alphonsus。 But time was short for the Venus mission； and cameras were heavy。 There were those who maintained that cameras weren't really scientific instruments； but rather catch…as…catch…can； razzle…dazzle； pandering to the public； and unable to answer a single straightforward； well…posed scientific question。 I thought myself that whether there are breaks in the clouds was one such question。 I argued that cameras could also answer questions that we were too dumb even to pose。 I argued that pictures were the only way to show the public—who were； after all； footing the bill—the excitement of robotic missions。 At any rate； no camera was flown； and subsequent missions have； for this particular world； at least partly vindicated that judgment： Even at high resolution from close flybys； in visible light it turns out there are no breaks in the clouds of Venus； any more than in the clouds of Titan。* These worlds are permanently overcast。
* For Titan； imaging revealed a succession of detached hazes above the main layer of aerosols。 So Venus works out to be the only world in the Solar System for which spacecraft cameras working in ordinary visible light haven't discovered something important。 Happily； we've now returned pictures from almost every world we've visited。 （NASA's International etary Explorer； which raced through the tail of et Giacobini…Zimmer in 1985； flew blind； be devoted to charged particles and magnetic fields。）
In the ultraviolet there is detail； but due to transient patches of high…altitude overcast； far above the main cloud deck。 The high clouds race around the planet much faster than the planet itself turns： super…rotation。 We have an even smaller chance of seeing the surface in the ultraviolet。
When it became clear that the atmosphere of Venus was much thicker than the air on Earth—as we now know； the pressure at the surface is ninety times what it is here—it immediately followed that in ordinary visible light we could not possibly see the surface； even if there were breaks in the clouds。 What little sunlight is able to make its tortuous way through the dense atmosphere to the surface would be reflected back； all right； but the photons would be so jumbled by repeated scattering off molecules in the lower air that no image of surface features could be retained。 It would be like a 〃whiteout〃 in polar snowstorm。 However； this effect； intense Rayleigh scattering； declines rapidly with increasing wavelength； in the near…infrared； it was easy to calculate； you could see the surface if there were breaks in the clouds or if the clouds were transparent there。
So in 1970 Jim Pollack； Dave Morrison； and I went to the McDonald Observatory of the University of Texas to try to observe Venus in the near…infrared。 We 〃hypersensitized〃 our emulsions； the good old…fashioned* glass photographic plates were treated with ammonia； and sometimes heated or briefly illuminated； before being exposed at the telescope to light from Venus。 For a time the cellars of McDonald Observatory reeked of ammonia。 We took many pictures。 None showed any detail。 We concluded that either we hadn't gone far enough into the infrared or the clouds of Venus were opaque and unbroken in the near infrared。
* Today many telescopic images are obtained with such electronic contrivances as charge…coupled devices and diode arrays； and processed by pute
