2003年10月1日
Galileo's gone, but successor in works
By Richard Stenger
CNN
Monday, September 22, 2003 Posted: 11:57 AM EDT (1557 GMT)
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Trouble since the start
Just getting to Jupiter in working order was a significant technical achievement. Years of launch postponements forced mission engineers to detour the interplanetary trip from a straight shot to a spiral trajectory, sending Galileo on a series of gravitational slingshot boosts by Venus and Earth again before it headed into deep space.
The delays proved costly. Tightly wrapped in storage much longer than planned during the interplanetary cruise, the main antenna failed to deploy. To beam back data, NASA had to rely instead on a much smaller secondary one and do extensive technical improvisation.
"The engineers had to redesign the software, reprogram the tape recorder and had to get Galileo to do data compression," said Alexander, the seventh and final Galileo skipper at NASA's Jet Propulsion Laboratory in Pasadena, California.
"It was a complete stretch to get it to do that, which saved the mission."
The workaround allowed Galileo to perform most of its intended science but changed the focus enough to provide serendipitous rewards.
"That loss forced us to concentrate on the satellites," Alexander said. "And from my perspective, the satellites are where the action is. That's where the big surprises came."
Besides Io, Galileo closely inspected Ganymede, Callisto and Europa. The probe's camera beamed back thousands of pictures of the moons, each a study in eccentricity.
Io, the innermost of the four, located in Jupiter's powerful radiation belts, is a burning, yellowish sphere with dark and light volcanic splotches. Europa, the next one out, boasts a frozen surface dotted with city-size chunks of ice and crisscrossed by mysterious dark red bands.
Ganymede, the largest, is bigger than Mercury. Callisto, the outermost, has the oldest surface of any known planet or moon.
Hints of water and life
Despite their differences, the moons have striking similarities, Galileo found. All most likely have thin atmospheres. Three are thought to hide vast stores of liquid or slushy oceans, stoking speculation that they could harbor some hardy form of primitive life.
One in particular displays the most convincing signs of a hidden ocean -- warm and constantly replenished with material from the icy surface.
"Europa is the star of the show," Belton said. "By proving that there is indeed a liquid, briny ocean, [Galileo] transformed it from a mere moon to a prime candidate for extraterrestrial life."
To prevent the chance, however small, of any surviving terrestrial germs on Galileo from contaminating Europa or its sibling satellites, NASA decided to crash the craft.
During the final descent, Galileo again entered Jupiter's radiation belts. Earlier trips into the turbulent sea of highly charged particles subjected the craft to four times the radiation it was designed to withstand.
The trips took a toll, damaging numerous electronic components, including the onboard camera. On its most recent foray in November, when it skimmed over a tiny inner moon called Amalthea, it weathered enough radiation to kill a human 1,000 times over and nearly lost its data recorder.
Somehow mission technicians coaxed it back into service.
"It was one of the most astounding recoveries that I think the mission made," said ex-mission manager Eliene Theilig, one of hundreds of former and current Galileo scientists and engineers who took park in a de facto funeral wake for the spacecraft at JPL mission control.
posted by Biochemie on 12:26 上午
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