2006年VOA标准英语-US Spacecraft Bringing Clues to Solar Syst（在线收听）

By David McAlary
Washington
10 January 2006

 
Courtesy of NASA
Artist's conception of Stardust spacecraft collecting dust from comet Wild-2 
  
A U.S. spacecraft is returning to Earth with a precious cargo of stardust that was gathered during a 4.5 billion kilometer roundtrip journey to a distant comet.  The spacecraft is scheduled to deposit its treasure on Sunday in the Utah desert.  Scientists around the world will examine the material to better understand the origins of our solar system.

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The notion of stardust is romantic, the stuff of poetry and song.

But astronomers like Donald Brownlee of the University of Washington have a more pragmatic outlook.  They see stardust as the source of key information about the makeup of the early solar system.  They want to study some of it under a microscope, so they designed a space mission launched seven years ago that captured particles streaking from comet Wild-2 during an encounter in 2003.

"Rays of material coming out are, in fact, the exact same small particles that went in to form the comet four-and-a-half billion years ago," he said.  "These are literally the building blocks of our planetary system.  The fact is all the atoms in our bodies were in stardust grains like are coming out of that comet now before the solar system formed."

Courtesy of NASA
Artist's impression of Stardust near comet Wild-2  
  
To get some of these grains, the Stardust spacecraft caught up with Wild-2 400 million kilometers from Earth, passing the icy body just 240 kilometers away.  As it moved through the gas and dust surrounding the comet, it caught more than 1,000 of its particles in a tray containing a wispy fiberglass filter.

"It looks like a large ice cube tray filled full of this magic material called aerogel," he explained.  "It is an ultra-low-density silica glass.  It is only a few times denser than air.  Our gift from the edge of the solar system will be contained in this grid.  A couple of days after it lands, investigators from all over the world will be diligently digging into this, trying to reveal the secrets of our origin."

Scientists say they hope the stardust will confirm the long held suspicion that carbon grains from comets played a major role in the origin of life on Earth.  They point out that comets are the most carbon-rich bodies in the solar system, full of organic compounds that fall to Earth all the time.

But rather than collecting these grains from the ground and saving the mission cost of $168 million, Donald Brownlee says it was necessary to go to the comet to get them, because they are uncontaminated by earthly material.

"The particles coming out of the comet are exactly the particles that went in," he noted.  "That is the magic of this mission.  We are getting the very best primitive samples that there are in the solar system."

The payoff comes Sunday when the Stardust spacecraft releases its sample return capsule 110,000 kilometers from Earth.  Project manager Tom Duxbury of the Jet Propulsion Laboratory in California says the capsule will enter the atmosphere at a record 45,000 kilometers per hour.

"We are coming in faster than any man-made spacecraft has ever come in before," he explained.  "We are going to come in under parachute control and this will take us down to a very soft landing onto the desert floor of Utah."

Helicopter crews are to retrieve the capsule for its trip to a special clean room at NASA's Johnson Space Center in Houston, Texas.

If the parachutes fail to open, Duxbury says that the container is durable enough to survive a crash landing.

"Our return capsule is quite small, but extremely rugged," he added.  "Also, we have our collector and aerogel inside a canister.  The aerogel is in this very, very strong, rigid aluminum grid.  So we think we have a very, very robust return capsule.  In the event that a parachute or so does not open or we might land hard, we know we can finish the rest of science opportunities."

NASA expects most of the collected stardust particles to be smaller than the width of a human hair.  As tiny as they are, Donald Brownlee says scientists will slice these particles into even smaller pieces for study, since even tiny pieces are thought to contain hundreds of thousands of microscopic grains.

"People all over the world will be analyzing these things down to the single atomic scale," said Mr. Brownlee.