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Scientists to blast hole in a comet

The 144-kg (317-pound) projectile will charge towards comet Tempel 1 at 37,000 km (23,000 miles) per hour, taking pictures of the comet before being wiped out by its surrounding dust cloud, researchers said. The blast could leave a hole as big as a house or a stadium in the comet, they added.

Comets are basically giant, dirty snowballs circulating throughout our Solar System. They are believed to contain material left over from the formation of the Solar System, 4½ billion years ago.

Scientists want to blast a hole into the comet because only the interior is believed to contain original, undisturbed material. This is because whenever a comet passes near the Sun, its outer layers heat up and change.

The project could reveal new facts about the interior of comets, including their chemical composition. This might shed light on some questions that have long intrigued scientists. One such question is whether some of the chemicals needed for the origin of life might have come from comets, as some theorists believe.

Scientists associated with the NASA mission described the project at a briefing in Washignton, D.C. on Dec. 14. The researchers included NASA’s Tom Morgan; Rick Grammier of NASA’s Jet Propulsion Laboratory, Pasadena, Calif.; Mike A’Hearn of the University of Maryland; and Karen Meech of the Institute for Astronomy, (Hilo), Hawaii.

The camera aboard the projectile will be able to take pictures with a level of detail unprecedented for comet images. It possibly could be able to distinguish features on the comet as small as a melon, according to scientists, though this depends on how close the camera gets before being destroyed.

Cameras on board the “flyby” spacecraft that shoots out the impact projectile will also take pictures, as will other telescopes on Earth and in space, scientists said. People between Arizona and New Zealand, including amateur observers, will be in a position to view the impact on or around July 4, 2005, the researchers said. People outside that area will still be able to view the aftermath of the initial impact, which could last for hours or days, Meech said.

The cost of the project, dubbed Deep Impact, is about $330 million, scientists said. This pays, among other things, for a “smart” impact projectile that will be able to maneuver itself into the comet while taking pictures, and which contains its own propulsion devices.

Different scientists have different ideas about what will happen the copper impact projectile hits the comet. What exactly happens then will reveal facts about the comet’s interior, said A’Hearn.

Most likely, a “cone” of ejected material will come flying out of the nucleus of the comet – the most solid part of the object, which is at its head, he said. Three main components make up a comet: the coma, a fuzzy looking cloud of gas and dust surrounding a nucleus that is a mixture of ice and rocks, and a tail that also consists of gas and dust.

“Just by looking at shape of cone, we can learn about strength of the cometary nucleus,” he said. Some researchers believe entire cone may lift off the nucleus, which would be an indication that the nucleus is very strong, he added. “Others think we will fracture the nucleus into several pieces,” he added. And “others think we will just compress material downward,” without any particular signs of an explosion.

At the same time, scientists observing the events with spectrometers, instruments which analyze light, will be able to learn about the types of molecules inhabiting the comet, Meech said. 

The projectile will be far too small to significantly change the comet’s trajectory, the researchers said.

The researchers said that by January 12, a rocket containing the flyby “Deep Impact” spacecraft, with the projectile inside it, will lift off. Deep Impact will release its projectile when it and the comet are about 83 million miles (134 million km) from Earth, which is slightly less than the distance between the Sun and the Earth.

Although researchers want to learn about the comet's interior, they also want to better understand the evolutionary changes in its mantle, or upper crust. This is one reason they picked Tempel 1. Discovered in 1867 by Ernst Tempel, it has made many passages through the inner solar system, orbiting the Sun every 5.5 years. This makes Tempel 1, otherwise a fairly typical comet, a good target to study evolutionary change in the mantle, researchers said.

Astronomers believe Tempel 1 comes from the Kuiper belt, a disc-shaped region of comets and asteroids that lies beyond Neptune’s orbit and extends out for several hundred times the distance between the Earth and the Sun.

—EJL

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