Deep Impact, NASA's comet kamikaze spacecraft, was moving into position yesterday to set off a Fourth of July fireworks show of celestial proportions 86 million miles from tomorrow's traditional holiday parades, barbecues, and flapping flags.
If all goes well during incredibly complex maneuvers, Deep Impact will release a washing machine-sized "impactor" into the path of Comet Tempel 1. The impactor will smash into the comet at a speed of 6 miles per second, fast enough to go from New York to Los Angeles in seven minutes. With a wallop equal to 10,000 pounds of dynamite, the impact could carve out a crater the size of the Glass Bowl.
As Deep Impact flies past the crater and plume of ejected debris, it will capture the first-ever images of the mysterious material inside comets. Scientists believe it is the stuff that formed the solar system 4.6 billion years ago, later delivered water to the Earth, and perhaps planted the seeds of life. The impactor also has a digital camera to chronicle the suicide flight until two seconds before the crash.
"The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science," said Dr. Michael A'Hearn, a University of Maryland astronomer who heads the Deep Impact science team. "With the information we receive after the impact, it will be a whole new ball game."
It promises to be an eye-popper for non-scientists when replayed on television from data beamed back to Earth. Some of the fireworks may be visible to amateur astronomers watching from the Pacific coast.
Professional astronomers at 60 observatories in 20 countries will watch explosion, and orbiting telescopes such as Hubble and Spitzer will capture other images.
Comets are "dirty icebergs," huge chunks of ice, gas, dust, and rock. Debris spewed out from the impact will glow in the harsh sunlight of outer space, making Comet Tempel appear 15-40 times brighter than usual for a few days, according to NASA calculations. The flare could be brighter and more spectacular if the impactor happens to auger into a subsurface pocket of gas, which would burst out like soda pop from a shaken bottle.
NASA has tried to reassure doomsayers that the $333-million mission will not kick Comet Tempel out of its orbit, and onto a collision course with the Earth. "Deep Impact" also was a 1998 movie with a doomsday script in which a giant comet threatened to collide with Earth.
"In the world of science, this is the astronomical equivalent of a 767 airliner running into a mosquito," said Dr. Don Yeomans, a senior scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. Comet Tempel is about six miles long, half the size of Manhattan Island in New York. About a yard long and a yard in diameter, the impactor weighs only 800 pounds.
"The impact simply will not appreciably modify the comet's orbital path," Dr. Yeomans added. "Comet Tempel poses no threat to the Earth now or in the foreseeable future."
The impact is scheduled for 1:52 a.m. EDT tomorrow, after an intricate celestial ballet involving Deep Impact, the comet, and the impactor probe.
"In our quest of a great scientific payoff, we are attempting something never done before at speeds and distances that are truly out of this world," said Rick Grammier, project manager for the mission.
NASA has described the mission as the technological equivalent of hitting a bullet with a bullet while taking a picture from a third bullet lying by.
Comet Tempel 1, discovered in 1867 by German astronomer Ernst Tempel, is orbiting the sun at 66,880 mph. Deep Impact, which is about the size of a sport utility vehicle, is in a different orbit traveling at 48,990 mph. It has to play the role of a wide receiver going out for a pass in a football game, arriving in the right spot at the same time as the faster-moving football.
Launch on Jan. 12 put Deep Impact on a rough course toward tomorrow's encounter that NASA later fined-tuned by firing on-board rocket thrusters. During the rendezvous, Deep Impact will release the impactor probe into the comet's path and then fire its thrusters to dodge to the side. As it closes in on the comet, the impactor will maneuver for a bull's-eye collision.
Nobody knows exactly how big a crater will result.
NASA says it depends on the nature of Temple's surface. A very hard surface, for instance, would mean a small crater perhaps a few stories deep that reveals little of the comet's mysterious inner material. A softer surface would mean a crater 14 stories deep and as big as a football stadium. One scenario says the impact will blow Comet Tempel to smithereens.
"The idea is that the best way to find what's inside a comet is to blast a hole in it," said H. Jay Melosh, a University of Arizona scientist working on the project. "Other comet rendezvous missions have proposed sampling less than a foot into the upper surface. But that doesn't get at the ices in the interior, which are believed to be early solar system materials that have been kept in the deep freeze for the past 4.5 billion years."
Scientists think that comets are leftovers from an immense cloud of dust and gas that solidified 4.6 billion years ago to form the sun, Earth, and other planets. Billions of them orbit the sun way beyond the planets on the outer fringes of the solar system. Gravity from an outer planet or another comet occasionally pulls one out of orbit, and it zooms past the sun and becomes visible from Earth.
In addition to storing material that made the solar system, comets may have played a key role in Earth's early history.
Geologists have evidence that Earth was so hot 3.9 billion years ago that all its oceans and other surface water vaporized. Bombardments of comets, scientists think, may have helped replenish Earth's oceans. Some scientists also believe that comets provided the carbon compounds - the organic molecules - needed for life to begin. Comets are about 50 percent water and 20 percent carbon by weight.
As Deep Impact flies past the devastation, its cameras and sensors will gather data about the material heaved up from deep below Tempel's surface. It also will observe the formation and shape of the crater, which will provide scientists with other information about cometary material.
Deep Impact will pass within 310 miles of Tempel during the flyby, and faces a treacherous passage to escape with its precious load of data. The spacecraft will deploy protective dust shields for a few minutes to seal itself off from impacts.
Plans call for the flyby ship to start downloading data to NASA antennas on Earth one day after the impact, and continue for 30 days.
Astronomers believe that a blizzard of dust and rocks surrounds Tempel and travels with the comet in its orbit. That material could pepper Deep Impact during the flyby, causing damage that prevents transmission of images and other data.
About 625,000 space enthusiasts have a personal stake in the impact. NASA invited people around the world to submit their names via the Internet or a "Send Your Name to a Comet" campaign. The names are on a mini compact-disc inside the impactor, which will vaporize along with everything else onboard the probe.
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