A. Revelation of the nature of comets
B. A perfect representative of the comets
C. Hoping for the best
D. Right time and right place for the Impact
E. What to expect of this Deep Impact?
F. Mystery in the heavens
On Monday at 1:52 a.m. ET, a probe deployed by a NASA spacecraft 83 million miles from home will smash at 23,000 mph into an ancient comet the size of Manhattan, blasting a hole perhaps 14 stories deep.
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Launched in January, NASA's $333 million Deep Impact mission is designed to answer questions that scientists have long had about comets, the ominous icebergs of space. This is the first time any space agency has staged such a deliberate crash. Scientists hope images transmitted by the probe and its mother ship will tell them about conditions in the early solar system, when comets and planets, including Earth, were formed. The team hopes to release photos of the impact as soon as they are received from the craft. NASA and observatories across the nation will be releasing webcasts.
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At the very least, NASA says, knowing how deep the probe dives into the comet could settle the debate over whether comets are compact ice cubes or porous snow cones. "We need to dig as deep a hole as possible," says mission science chief Michael A'Hearn of the University of Maryland. Until now, the closest scientists have come to a comet was when NASA's Stardust mission passed within 167 miles of the comet Wild 2 last year, collecting comet dust that is bound for a return to Earth in January. The most famous date with a comet occurred when an international spacecraft flotilla greeted Halley's comet in 1986. But these quick looks examined only the comets' dust and Surface;
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To the ancients, comets were harbingers of doom, celestial intruders on the perfection of the heavens that presaged disaster. Modern astronomers have looked on them more favorably, at least since Edmond Halley's celebrated 1705 prediction of the return of Halley's comet in 1758 and every 75 years thereafter. Today, scientists believe Tempel 1 (named for Ernst Wilhelm Leberecht Tempel, who first spotted it in 1867 while searching for comets in the sky over Marseilles, France) and other comets are windows to the earliest days of the solar system, 4. 6 billion years ago, when planets formed from the dust disk surrounding the infant sun.
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Deep Impact's copper-plated "impactor"--a 39-inch long, 820-pound beer-barrel-shaped probe--will be "run over like a penny on a train track" when it crashes, A'Hearn says. The impactor is equipped with a navigation system to make sure it smacks into the comet in the right location for the flyby craft's cameras. On Sunday, the flyby spacecraft will release the probe. Twelve minutes later, it will beat a hasty retreat with a maneuver aimed at allowing a close flyby, from 5,348 miles away, with cameras pointed. Fourteen minutes after the impact, the flyby spacecraft will scoot to within a mere 310 miles for a close-up of the damage.
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Ideally, everything will line up, and the flyby spacecraft will take images of the crater caused by the impact. It will go into a "shielded" mode as ice and dust batter the craft, then emerge to take more pictures. "The realistic worst case is hitting (the comet) but not having the flyby in the right place," A'Hearn says. "Basically, we have a bullet trying to hit a second bullet with a third bullet in the right place at the right time to watch. I'd love to have a joystick(操纵杆) to control the impactor."
Planetary scientists have "no idea" what sort of crater will result, McFadden says. Predictions range from a deep but skinny shaft driven into a porous snow cone to a football stadium-sized excavation in a hard-packed ice ball.
But astronomers should have their answer shortly after impact, which should settle some questions about the comet's crust and interior. Analysis of the chemistry of that interior, based on the light spectra given off in the impact's aftermath, could take much longer.