2006年VOA标准英语-US Launches Decade-Long Mission to Pluto（在线收听）

By David McAlary
Washington
19 January 2006

The United States has launched the first spacecraft to the distant, icy planet Pluto.  The distance from Earth is so great the journey will take nine years for a mere nine-hour fly-by inspection.  Astronomers say the results will be worth the wait for what Pluto and its moon Charon can tell us about the origins of the solar system.

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An Atlas V rocket carries the New Horizons spacecraft   
  

Pluto is the last major planet in our solar system to be visited during four decades of space exploration.  The reason for the lag is not simply because it is more than 6.5 billion kilometers away, 50 times farther than from the Sun to the Earth.

Solar system research director at the U.S. space agency NASA, Andrew Dantzler, says only in the past decade have scientists realized that this icy dwarf, smaller than our moon, might harbor important secrets about the solar system.

"Pluto is a treasure trove of scientific discoveries just waiting to be uncovered," he said.  "It is different from the inner rocky planets.  It is different from the outer gaseous planets.  As such, it holds many clues as to how the solar system was formed."

Scientists once thought Pluto orbited in distant solitude.  But last year, astronomers increased their count of its moons to three.  It is also part of a wide swath of perhaps 500,000 small icy worlds circling the sun beyond Neptune called the Kuiper Belt, the first of which was detected in 1992.

The principal investigator for the Pluto mission, Alan Stern of the Southwest Research Institute in Colorado, says this belt is the largest structure in the solar system and has opened astronomers' eyes to the diversity of planets.

"Pluto looked like a misfit because it was the only one we saw, and just as a chihuahua is still a dog, these ice dwarfs are still planetary bodies," he explained.  "So the Pluto-like objects are more typical in our solar system than the nearby planets we first knew, and the opportunity is to go now and have a chance to study this most common type of planetary body in the solar system."

Because of Pluto's extreme distance, NASA is employing a lot of power to reduce travel time.  It used its biggest launch rocket, the Atlas Five, to get the spacecraft to a speed approaching 50,000 kilometers per hour within a few minutes, allowing it to pass the moon in only nine hours. 

Alan Stern says the spacecraft will swing by Jupiter in 13 months to take advantage of the planet's huge gravitational pull to increase its speed by 50 percent.  This cuts up to three years off the trip to Pluto.

"We are going farther to reach our target and we are traveling faster than any spacecraft ever has," he said.  "This is a little bit about rewriting the textbooks about the outer planets."

The spacecraft, called New Horizons, is a small probe, the size of a piano, with instruments to study Pluto's atmosphere and icy geology, and its largest moon Charon.  After it passes Pluto, it is to continue into the Kuiper Belt to investigate one or two other bodies.

Its fuel is radioactive plutonium pellets, drawing protests by anti-nuclear activists who feared a leak during launch could contaminate the environment.  The plutonium will be used mostly for aiming and course corrections.  The spacecraft will not need a lot of power.  Its electronic components use less energy than two 100-watt light bulbs.  For most of its long journey, it will be hibernating, sending back only an occasional beacon to relay data on its status. 

The man in charge of the payload is William Gibson, also of the Southwest Research Institute.

"The New Horizons payload, in summary, is the most compact, low power, high performance payload yet to fly on a U.S. planetary mission for a first-reconnaissance fly-by," he said.

In the years before the spacecraft's arrival at Pluto in 2015, the mission team will study the planet with the Hubble Space Telescope to determine if it has more moons and possible rings of dust and rock around which they must navigate.  During the fly-by of Jupiter early next year, they will test its navigation and scientific instruments to conduct further inspection of that giant planet.