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神经网络昆斯林的新闻组分类2006

        Venus by Balloon

        For years the thick atmosphere of Venus had been a tempting 
    target to scientists who wished to explore the planet's mantle of 
    air with balloon-borne instruments.  Professor Jacques Blamont of 
    the French space agency Centre National d'Etudes Spatiales (CNES)
    had proposed such an idea as far back as 1967, only to have a joint 
    French-Soviet balloon mission canceled in 1982.  Nevertheless, 
    late in the year 1984, such dreams would eventually come true.

        When two PROTON rockets were sent skyward on December 15 and 21, 
    the Soviet Union provided Western observers with the first clear, full 
    views of the booster which had been launching every Soviet Venus probe 
    since 1975.  This was but one of many firsts for the complex mission.

        The unmanned probes launched into space that December were named
    VEGA 1 and 2, a contraction of the words VENERA and GALLEI - Gallei
    being the Russian word for Halley.  Not only did the spacecraft 
    have more than one mission to perform, they also had more than one 
    celestial objective to explore, namely the comet Halley.  

        This famous periodic traveler was making its latest return to 
    the inner regions of the solar system since its last visit in 1910.
    Since it was widely believed that comets are the icy remains from
    the formation of the solar system five billion years ago, scientists 
    around the world gave high priority to exploring one of the few such 
    bodies which actually come close to Earth.  

        Most comets linger in the cold and dark outer fringes of the solar 
    system.  Some, like Halley, are perturbed by various forces and fall 
    in towards the Sun, where they circle for millennia spewing out ice 
    and debris for millions of kilometers from the warmth of each solar 
    encounter.

        The Soviet Union, along with the European Space Agency (ESA) and
    Japan's Institute of Space and Astronautical Science (ISAS), did not
    wish to miss out on this first opportunity in human history to make a
    close examination of Halley.  The ESA would be using the cylindrical
    GIOTTO probe to make a dangerously close photographic flyby of the 
    comet, while Japan's first deep space craft - SAKIGAKE (Pioneer) and 
    SUISEI (Comet) - would view Halley from a much safer distance. 

        Scientists in the United States also desired to study the comet
    from the vantage of a space probe, at one time envisioning a vessel
    powered by solar sails or ion engines.  However, government budget
    cuts to NASA canceled the American efforts.  The U.S. would have to
    make do primarily with Earth-based observations and the sharing of 
    data from other nations, though an instrument named the Dust Counter 
    and Mass Analyzer (DUCMA), designed by Chicago University Professor 
    John Simpson, was added on the Soviet mission in May of 1984.

        The Soviets' answer to Halley were the VEGAs.  Instead of building
    an entirely new craft for the mission, the Soviets decided to modify
    their VENERA bus design to encounter the comet while performing an 
    advanced Venus mission along the way.  As VEGA 1 and 2 reached Venus,
    the buses would drop off one lander/balloon each and use the mass of 
    the shrouded planet to swing them towards comet Halley, much as the 
    U.S. probe MARINER 10 used Venus to flyby Mercury eleven years earlier. 
    The Soviet craft would then head on to Halley, helping to pinpoint the 
    location of the comet's erupting nucleus for the GIOTTO probe to dive 
    in only 605 kilometers (363 miles) away in March of 1986.

        As planned, the two VEGAs arrived at Venus in June of 1985.  VEGA
    1 released its payload first on the ninth day of the month, the lander
    making a two-day descent towards the planet.  The craft touched the
    upper atmosphere on the morning of June 11.  Sixty-one kilometers
    (36.6 miles) above the Venerean surface a small container was released
    by the lander, which produced a parachute at 55 kilometers (33 miles)
    altitude.  Thus the first balloon probe ever to explore Venus had
    successfully arrived. 

        One kilometer after the opening of the parachute, helium gas was
    pumped into the Teflon-coated plastic balloon, inflating it to a
    diameter of 3.54 meters (11.68 feet).  Dangling on a tether thirteen
    meters (42.9 feet) below was the instrument package, properly known as
    an aerostat.  The top part of the 6.9-kilogram (15.18-pound) aerostat
    consisted of a cone which served as an antenna and tether attachment
    point to the balloon.  Beneath it was the transmitter, electronics,
    and instruments.  Connected at the bottom was a nephelometer for
    measuring cloud particles.  The aerostat was painted with a special
    white finish to keep at bay the corroding mist of sulfuric acid which
    permeated the planet's atmosphere. 

        The VEGA 1 balloon was dropped into the night side of Venus just
    north of the equator.  Scientists were concerned that the gas bag 
    would burst in the heat of daylight, so they placed it in the darkened 
    hemisphere to give the craft as much time as possible to return data.  
    This action necessitated that the landers come down in the dark as 
    well, effectively removing the camera systems used on previous missions.  
    The author wonders, though, if they could have used floodlights similar 
    to the ones attached to VENERA 9 and 10 in 1975, when Soviet scientists 
    had thought the planet's surface was enshrouded in a perpetual twilight 
    due to the permanently thick cloud cover.

        The first balloon transmitted for 46.5 hours right into the day
    hemisphere before its lithium batteries failed, covering 11,600
    kilometers (6,960 miles).  The threat of bursting in the day heat did
    not materialize.  The VEGA 1 balloon was stationed at a 54-kilometer
    (32.4-mile) altitude after dropping ballast at fifty kilometers
    (thirty miles), for this was considered the most active of the three
    main cloud layers reported by PIONEER VENUS in 1978.  Indeed the
    balloon was pushed across the planet at speeds up to 250 kilometers
    (150 miles) per hour.  Strong vertical winds bobbed the craft up and
    down two to three hundred meters (660 to 990 feet) through most of the
    journey.  The layer's air temperature averaged forty degrees Celsius
    (104 degrees Fahrenheit) and pressure was a mere 0.5 Earth atmosphere.
    The nephelometer could find no clear regions in the surrounding clouds. 

        Early in the first balloon's flight, the VEGA 1 lander was already
    headed towards the Venerean surface.  Both landers were equipped with
    a soil drill and analyzer similar to the ones carried on VENERA 13 
    and 14 in 1982.  However, VEGA 1 would become unable to report the
    composition of the ground at its landing site in Rusalka Planitia, the
    Mermaid Plain north of Aphrodite Terra.  While still ten to fifteen
    minutes away from landing, a timer malfunction caused the drill to 
    accidentally begin its programmed activity sixteen kilometers (9.6 
    miles) above the surface. 

        There was neither any way to shut off the instrument before
    touchdown nor reactivate it after landing.  This was unfortunate not
    only for the general loss of data but also for the fact that most of
    Venus was covered with such smooth low-level lava plains and had never 
    before been directly examined.  Nevertheless, the surface temperature 
    and pressure was calculated at 468 degrees Celsius (874.4 degrees 
    Fahrenheit) and 95 Earth atmospheres respectively during the lander's 
    56 minutes of ground transmissions.  A large amount of background 
    infrared radiation was also recorded at the site.

        As had been done when the drills and cameras on VENERA 11 and 
    12 had failed in December of 1978, the Soviets focused on the data
    returned during the lander's plunge through the atmosphere.  The
    French-Soviet Malachite mass spectrometer detected sulfur, chlorine, 
    and possibly phosphorus.  It is the sulfur - possibly from active 
    volcanoes - which gives the Venerean clouds their yellowish color.
    The Sigma 3 gas chromatograph found that every cubic meter of air
    between an altitude of 48 and 63 kilometers (28.8 and 37.8 miles)
    contained one milligram (0.015 grain) of sulfuric acid.

        The VEGA 1 data on the overall structure of the cloud decks 
    appeared to be at odds with the information from PIONEER VENUS.
    The case was made even stronger by the fact that VEGA 2's results
    nearly matched its twin.  The VEGAs found only two main cloud layers 
    instead of the three reported by the U.S. probes.  The layers were 
    three to five kilometers (1.8 to 3 miles) thick at altitudes of 50 
    and 58 kilometers (30 and 34.8 miles).  The clouds persisted like a
    thin fog until clearing at an altitude of 35 kilometers (21 miles), 
    much lower than the PV readings.  One possibility for the discrepan-
    cies may have been radical structural changes in the Venerean air 
    over the last seven years.

        When the lander and balloon finally went silent, the last 
    functioning part of the VEGA 1 mission, the flyby bus, sailed on
    for a 708 million-kilometer (424.8 million-mile) journey around
    the Sun to become the first probe to meet comet Halley.  On March
    6, 1986, the bus made a 8,890-kilometer (5,334-mile) pass at the 
    dark and icy visitor before traveling on in interplanetary space.
    The Soviets had accomplished their first mission to two celestial
    bodies with one space vessel.

        On June 13, VEGA 2 released its lander/balloon payload for
    a two-day fall towards Venus.  Like its duplicate, the VEGA 2
    balloon radioed information back to the twenty antennae tracking
    it on Earth for 46.5 hours before battery failure on the morning
    side of the planet.  During its 11,100-kilometer (6,660-mile)
    flight over Venus, the second balloon entered in a rather still
    environment which became less so twenty hours into the mission.
    After 33 hours mission time the air became even more turbulent 
    for a further eight hours.  When the balloon passed over a five-
    kilometer (three-mile) mountain on the "continent" of Aphrodite 
    Terra, a powerful downdraft pulled the craft 2.5 kilometers (1.5 
    miles) towards the surface.

        Temperature sensors on the VEGA 2 balloon reported that the air 
    layer it was moving through was consistently 6.5 degrees Celsius 
    (43.7 degrees Fahrenheit) cooler than the area explored by the VEGA 1
    balloon.  This was corroborated by the VEGA 2 lander as it passed 
    through the balloon's level.  No positive indications of lightning
    were made by either balloon, and the second aerostat's nephelometer 
    failed to function.

        The VEGA 2 lander touched down on the northern edge of Aphrodite
    Terra's western arm on the fifteenth of June, 1,500 kilometers (900
    miles) southeast of VEGA 1.  The lander's resting place was smoother
    than thought, indicating either a very ancient and worn surface or a
    relatively young one covered in fresh lava.  The soil drill was in
    working order and reported a rock type known as anorthosite-troctolite, 
    rare on Earth but present in Luna's highlands.  This rock is rich in 
    aluminum and silicon but lacking in iron and magnesium.  A high degree 
    of sulfur was also present in the soil.  The air around VEGA 2 measured 
    463 degrees Celsius (865.4 degrees Fahrenheit) and 91 Earth atmospheres,
    essentially a typical day (or night) on Venus.

        Far above the VEGA 2 lander, its carrier bus sped past Venus at
    a distance of 24,500 kilometers (14,700 miles) and followed its twin 
    to comet Halley, making a closer flyby on March 9, 1986 at just 8,030 
    kilometers (4,818 miles).  Both probes helped to reveal that the comet 
    is a very dark and irregular-shaped mass about fourteen kilometers 
    (8.4 miles) across, rotating once every 53 hours, give or take three 
    hours.

        Since both VEGA craft were still functioning after their Halley
    encounters, Soviet scientists considered an option to send the 
    probes to other celestial objects.  One prime target was the near-
    Earth planetoid 2101 Adonis, which VEGA 2 could pass at a distance 
    of six million kilometers (3.6 million miles).  Sadly, the Soviets 
    had to back out on the opportunity to become the first nation to fly 
    a spacecraft past a planetoid when it was discovered that there was 
    not enough maneuvering fuel in the probe to reach Adonis as planned.  
    VEGA 1 and 2 were quietly shut down in early 1987.

        Future Plans Diverted

        The impressive VEGA mission had given some scientists numerous 
    ideas and hope for even more ambitious expeditions to the second 
    world from the Sun.  One example was the VESTA mission, planned for
    the early 1990s.

        This Soviet-French collaboration called for the launch of multiple 
    probes on a single PROTON rocket in either 1991 or 1992.  The craft 
    would first swing by Venus and drop off several landers and balloon 
    probes.  The aerostats would be designed to survive in the planet's 
    corrosive atmosphere for up to one month, a large improvement over 
    the VEGA balloons' two days.  The mission would then head out to 
    investigate several planetoids and comets, including a possible 
    landing on Vesta (thus the mission name), the most reflective Main 
    Belt planetoid as seen from Earth.