WASHINGTON, June 24 — NASA is preparing to launch a spacecraft whose mission is to make the closest observations ever of the frozen cores of comets and to stand ready to be diverted if a newly discovered comet passes near the Earth.

The robot spacecraft, called Contour for Comet Nucleus Tour, is part of an armada of international probes being sent forth to solve some of the mysteries of these icy bodies that traverse the solar system and may have played a major role in the evolution of Earth.

If successful, Contour will make the most detailed observations ever of the nuclei of two very different types of comets as they make their closest approaches to the Sun and are most active.

On Nov. 12, 2003, the spacecraft is to fly by an older, less active comet called Encke, which was first spotted in 1786. Then on June 19, 2006, Contour will visit the more active and younger Schwassman-Wachmann 3, a comet discovered in 1930 that broke into several pieces in 1995, perhaps exposing pristine inner material.

Contour is set to begin its journey on July 1 atop a Boeing Delta II rocket launched from Cape Canaveral Air Force Station in Florida. The one-ton spacecraft will go into a high, looping orbit of Earth for several weeks before firing its main engine on Aug. 15 to send it on a comet-chasing orbit around the Sun that periodically brings it back to Earth.

Comets are chunks of ice, rock and dust that are some of the oldest objects in the solar system. Scientists believe they contain materials and chemicals left over from the formation of the planets and other bodies more than 4.5 billion years ago. Recently, many scientists have come to believe that cometary bombardment of the early Earth may have contributed some of the water in the oceans, different gases in the atmosphere and even life-generating organic molecules.

Dr. Joseph Veverka, the Cornell University astronomer who is the Contour mission's principal investigator, says comets are the most numerous sizable bodies in the solar system — with estimates placing their number at a million million — but are among the least known.

"Comets are the solar system's smallest bodies, but among its biggest mysteries," he said. "We believe they hold the most primitive materials in the solar system and that they played a role in shaping some of the planets, but we really have more ideas about comets than facts."

Dr. Donald K. Yeomans, a comet expert at NASA's Jet Propulsion Laboratory and member of Contour's science team, said there were some very practical reasons to learn more about comets. Comets periodically strike Earth; one is suspected of exploding over Siberia in 1908 and leveling 800 square miles of forest. Mounting any defense against them requires more knowledge about their makeup, Dr. Yeomans said. He noted that neither comet set to be visited by Contour posed a threat of striking Earth.

Another reason for learning more about comets, he said, is that they could be a source of raw materials for future space exploration. The water in comets could be broken down into hydrogen and oxygen for rocket fuel, and the mineral content might be used for construction materials. "Comets may one day be the fueling stations for interplanetary exploration," Dr. Yeomans said.

The new push to understand comets includes NASA's Stardust, a spacecraft launched in 1999 that will fly past comet Wild 2 in 2004 to collect dust samples that will be returned to Earth in 2006. NASA plans to launch a craft called Deep Impact in 2004, which will send a half-ton projectile crashing into comet Tempel 1 the following year, blasting a deep crater to see what is inside. Next year the European Space Agency will launch a spacecraft called Rosetta, which will go into orbit around the comet Wirtanen in 2011 for two years of studies.

For centuries astronomers have studied comets from afar as their orbits have brought them close to the Sun, which heats the nucleus and causes ice to evaporate and release bursts of gas and dust. These jets of materials create a thin atmosphere called a coma, which is illuminated by the Sun, and the pressure of sunlight can sweep this material into a tail stretching millions of miles.

But comet nuclei, typically only a few miles across, are hidden from Earth observations by their size and the surrounding coma. Using spacecraft, scientists have only twice viewed close-up images of comet nuclei. In 1986, the European Space Agency's Giotto spacecraft flew within 373 miles of the core of Halley's comet, and in September 2001, NASA's Deep Space 1 passed within 1,349 miles of comet Borrelly to produce the best pictures of a comet nucleus thus far.

Contour scientists said their spacecraft was to fly as close as 60 miles to each target nucleus, delivering images 10 times as specific as those from Deep Space 1 and 25 times as good as those from Giotto. Contour's four scientific instruments will not only take pictures, but also measure and analyze surrounding gases and dust; determine the size, shape and rotation of the nucleus; and ascertain its chemical composition.

To protect the solar-powered spacecraft, which looks like an eight-sided barrel, from comet dust particles as it speeds by, engineers have outfitted the front end with a 10-inch-thick layered shield of Nextel and Kevlar fabric, the material used in bulletproof vests.

The $159 million Contour mission is part of NASA's Discovery series of low-cost space projects. The spacecraft, constructed of stock parts and instruments developed for earlier missions, was built by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which also designed the mission plan and will control the flight.

To reduce staff time and keep costs low, Contour is designed to shut down most of its systems and "hibernate" for nearly 65 percent of its journey. Controllers are to awaken the craft 60 days before an encounter.

To reach multiple cometary targets without huge amounts of rocket fuel, engineers designed Contour to use Earth's gravity for propulsion. The spacecraft's orbit brings it back to Earth each year, and by targeting its approach, engineers use the planet's gravity to swing the craft in a desired direction.

"We use several swingbys of the Earth to target comets with a minimum of fuel," said Dr. Robert W. Farquhar of Johns Hopkins, the mission director. "It's very efficient and we can keep doing it until we run out of fuel."

Astronomers said they had procedures in place to use Earth swing-bys to redirect Contour toward a new target if it appeared scientifically interesting and came close enough. Dr. Veverka said that in the last five years, two major new comets appeared close to Earth's orbit, including the spectacular Hale-Bopp in 1997. But there was no time to mount a spacecraft mission to study them.

"Now we will have an option in case another Hale-Bopp appears," he said.

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