Jennie
06-27-2005, 08:23 PM
Spacecraft Is on a Collision Course With a Comet, Intentionally
By WARREN E. LEARY
WASHINGTON, June 27 - A two-stage spacecraft called Deep Impact is about to make an ambitious attempt to dissect a comet by slamming into it and blowing some of its innards into space for all to see.
Launched from Florida on Jan. 12, NASA's Deep Impact is nearing the end of a finely calibrated 268-million-mile journey that puts comet Tempel 1 within its sights.
An 820-pound copper-core "impactor" is to smash into the comet's nucleus at 23,000 miles an hour in the early hours of July 4, an unprecedented event that will, if all goes well, be witnessed by its companion craft and numerous observatories in space and on Earth. Because of the distance and timing of the encounter, experts said that only Earth observers in the Pacific area using telescopes were likely to see the comet and any evidence of the impact.
Rick Grammier, the mission's project manager at the Jet Propulsion Laboratory in Pasadena, Calif., said the final part of the encounter 83 million miles away was so intricate and so fast that the twin ships would have to handle these maneuvers on their own without help from human controllers. "It's a bullet trying to hit a second bullet with a third bullet, in the right place at the right time," he said.
Scientists say the impact, which should occur at about 1:52 a.m. Eastern time, could excavate a crater as large as a sports stadium and send plumes of material from the comet's core far into space, allowing the first view of the pristine inner material that makes up these icy bodies.
Comets are believed to be remnants of the materials that formed the solar system some 4.5 billion years ago. Astronomers believe comets' interiors have undergone little change since then and contain the pristine ice, gases, dust and other materials from which the rest of the solar system formed. Understanding comets is a way of understanding how the solar system was born.
Another reason to study comets is that they, along with rocky asteroids, pose a threat of hitting the Earth and causing cataclysmic damage. Defending against such possibilities requires knowing more about these objects in hopes of deflecting or destroying dangerous ones, experts say.
Dr. Michael A'Hearn of the University of Maryland, principal investigator of the $333 million project, said comets were probably one of the best sources of the primal material of the solar system. While their outer layers have been subjected to radiation, interplanetary dust and the effects of sunlight as they swing close to the Sun during their orbits, he said, the insides should be pure.
"They are old and very cold, and they stay cold most of their lives," he said. "The material at the center should be well preserved."
Little is known about the nature and makeup of comets. The target of the mission, Tempel 1, discovered in 1867, is a dark-colored comet that moves about the Sun in an elliptic orbit between Mars and Jupiter every 5.5 years. The latest observations indicate it is an elongated object about 9 miles long and 3.7 miles across, about half the size of Manhattan.
On Monday, NASA released two pictures of Tempel 1 taken by the Hubble Space Telescope on June 14. The Hubble, looking from 75 million miles away, saw a bright eruption from the comet shooting a jet 1,400 miles into space.
This activity shows one of the hazards facing the already difficult mission. The solid nucleus of the comet is surrounded by a halo of particles called a coma that taper back into a long tail in the direction opposite the sun. These particles could damage the impactor craft or its flyby companion before they finish their tasks and relay the results back to Earth, researchers said.
About 24 hours before intercepting Tempel 1, the spacecraft will divide, leaving the impactor in the comet's path while the flyby craft fires jets to veer away. The battery-powered impactor, carrying a medium-resolution telescope instrument that is to transmit pictures until seconds before it hits, is to make three course maneuvers to pick a well-lit spot on the sunny side of the comet.
At the moment of impact, the 1,300-pound flyby craft, carrying high-resolution and wide-angle telescopes, should be 5,300 miles away relaying data from both craft. Some 14 minutes later, it will make its closest approach to the comet nucleus, passing 310 miles below it. Shortly before this close encounter, the flyby craft will have to stop taking pictures and turn so that special debris-shielding panels protect it from particles streaming from the comet's coma. Clearing the coma, the spacecraft is to swivel around to take more images of the receding comet.
"The whole mission is riding on what happens in about 800 seconds," Dr. A'Hearn said. "We've only got one shot at it."
By WARREN E. LEARY
WASHINGTON, June 27 - A two-stage spacecraft called Deep Impact is about to make an ambitious attempt to dissect a comet by slamming into it and blowing some of its innards into space for all to see.
Launched from Florida on Jan. 12, NASA's Deep Impact is nearing the end of a finely calibrated 268-million-mile journey that puts comet Tempel 1 within its sights.
An 820-pound copper-core "impactor" is to smash into the comet's nucleus at 23,000 miles an hour in the early hours of July 4, an unprecedented event that will, if all goes well, be witnessed by its companion craft and numerous observatories in space and on Earth. Because of the distance and timing of the encounter, experts said that only Earth observers in the Pacific area using telescopes were likely to see the comet and any evidence of the impact.
Rick Grammier, the mission's project manager at the Jet Propulsion Laboratory in Pasadena, Calif., said the final part of the encounter 83 million miles away was so intricate and so fast that the twin ships would have to handle these maneuvers on their own without help from human controllers. "It's a bullet trying to hit a second bullet with a third bullet, in the right place at the right time," he said.
Scientists say the impact, which should occur at about 1:52 a.m. Eastern time, could excavate a crater as large as a sports stadium and send plumes of material from the comet's core far into space, allowing the first view of the pristine inner material that makes up these icy bodies.
Comets are believed to be remnants of the materials that formed the solar system some 4.5 billion years ago. Astronomers believe comets' interiors have undergone little change since then and contain the pristine ice, gases, dust and other materials from which the rest of the solar system formed. Understanding comets is a way of understanding how the solar system was born.
Another reason to study comets is that they, along with rocky asteroids, pose a threat of hitting the Earth and causing cataclysmic damage. Defending against such possibilities requires knowing more about these objects in hopes of deflecting or destroying dangerous ones, experts say.
Dr. Michael A'Hearn of the University of Maryland, principal investigator of the $333 million project, said comets were probably one of the best sources of the primal material of the solar system. While their outer layers have been subjected to radiation, interplanetary dust and the effects of sunlight as they swing close to the Sun during their orbits, he said, the insides should be pure.
"They are old and very cold, and they stay cold most of their lives," he said. "The material at the center should be well preserved."
Little is known about the nature and makeup of comets. The target of the mission, Tempel 1, discovered in 1867, is a dark-colored comet that moves about the Sun in an elliptic orbit between Mars and Jupiter every 5.5 years. The latest observations indicate it is an elongated object about 9 miles long and 3.7 miles across, about half the size of Manhattan.
On Monday, NASA released two pictures of Tempel 1 taken by the Hubble Space Telescope on June 14. The Hubble, looking from 75 million miles away, saw a bright eruption from the comet shooting a jet 1,400 miles into space.
This activity shows one of the hazards facing the already difficult mission. The solid nucleus of the comet is surrounded by a halo of particles called a coma that taper back into a long tail in the direction opposite the sun. These particles could damage the impactor craft or its flyby companion before they finish their tasks and relay the results back to Earth, researchers said.
About 24 hours before intercepting Tempel 1, the spacecraft will divide, leaving the impactor in the comet's path while the flyby craft fires jets to veer away. The battery-powered impactor, carrying a medium-resolution telescope instrument that is to transmit pictures until seconds before it hits, is to make three course maneuvers to pick a well-lit spot on the sunny side of the comet.
At the moment of impact, the 1,300-pound flyby craft, carrying high-resolution and wide-angle telescopes, should be 5,300 miles away relaying data from both craft. Some 14 minutes later, it will make its closest approach to the comet nucleus, passing 310 miles below it. Shortly before this close encounter, the flyby craft will have to stop taking pictures and turn so that special debris-shielding panels protect it from particles streaming from the comet's coma. Clearing the coma, the spacecraft is to swivel around to take more images of the receding comet.
"The whole mission is riding on what happens in about 800 seconds," Dr. A'Hearn said. "We've only got one shot at it."