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After 10 years and a journey of 4 billion miles, the European Space Agency’s Rosetta spacecraft arrived at its destination Wednesday for the first extended, close examination of a comet.

A six-minute thruster fired early Wednesday, the last in a series of 10 in the past few months, slowed Rosetta to the pace of a person walking, about 2 mph relative to the speed of its target, Comet 67P/Churyumov-Gerasimenko.

“It is like driving a car or a bus on a motorway for 10 years,” said Andrea Accomazzo, the flight director, at a post-rendezvous news conference. “Now we’ve entered downtown. We’re downtown and we have to start orienting ourselves. We don’t know the town yet, so we have to discover it first.”

In coming months, Rosetta and its comet, C-G for short, will plunge together toward the sun. In November, a small 220-pound lander is to leave the spacecraft, set down on the comet and harpoon itself to the surface; if successful, it would be the first time a spacecraft will have gently landed on a comet.

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At this point, the comet and its shadowing spacecraft are more than 330 million miles from the sun, traveling at 35,000 mph. With the final firing of the thruster, Rosetta was a mere 60 miles from the comet’s surface.

“This morning, we hit a milestone, an important milestone of this mission,” said Laurence O’Rourke, a member of its science team. “But this mission isn’t just about arriving at a comet,” he went on. “It’s about studying the comet. It’s about placing a lander on a comet, but again the mission does not end there. The science continues. We’re trying to follow this comet all around its orbit.”

Rosetta is not close enough to be captured by the comet’s gravity, but instead will be flying a triangular path in front of the comet as it maps its surface. It will eventually move within 6.2 miles of the surface and enter orbit around the comet.

Comets, made of ice, dust and rock, are frozen leftovers from the formation of the solar system. Rosetta is named after the Rosetta Stone, the engraved block that was crucial in deciphering Egyptian hieroglyphics, and scientists hope the spacecraft’s observations will offer important clues to how the solar system came together 4.5 billion years ago. (Rosetta’s lander, Philae, is named for an island in the Nile River.)

Photographs have revealed a surprisingly irregular shape for the 2-mile by 2.5-mile-comet, possibly an amalgamation of two icy bodies or a result of uneven weathering during previous trips to the inner solar system.

At the news conference, Holger Sierks, principal investigator for Rosetta’s high-resolution camera, revealed the latest images, pointing to cliffs, deep shadows and also flat areas with boulders sitting on the surface. “We’ll learn in the coming months what this is telling us,” he said.

The spacecraft had earlier measured the flow of water vapor streaming off the comet at a rate of about two cups a second, which would fill an Olympic-size swimming pool in about 100 days. As the comet accelerates toward the sun, its surface will warm and the trickle will grow to a torrent a hundred or a thousand times that size, contributing to the long tail that is characteristic of comets.

An unsolved mystery of Earth is where the water in the oceans came from; some suggest it came from comets. The water in comets from the distant Oort cloud, far beyond Pluto, does not match the water on Earth, but the water in nearer comets may.

Much of the work in the next three months will be to find a safe place for Philae to land. Once released from Rosetta, the lander will be pulled down by the comet’s gravity and will strike its surface at a couple of miles per hour, like someone walking into a wall. “It’s hurting, but it doesn’t kill you,” said Stephan Ulamec, head of the consortium that built the lander.

Landing will pose an unprecedented challenge because there will be no second shot. Recent pictures show the comet’s surface is porous, with steep cliffs and house-sized boulders. Indeed, one person involved with Rosetta from the start said the landing was “mission impossible,” with only a slim chance of success. He spoke on condition of anonymity.

Even if the landing fails, Rosetta will remain in the comet’s orbit until at least the end of 2015, gathering data with its 11 onboard sensors. As the comet gets closer to the sun, it will begin to fizz and release the cloud of dust and ice that most people associate with comets.

“We’re going to have a ringside seat to see, for the first time, a comet turn into a comet, to develop its tail and explain what for centuries mankind has been puzzled by,” said David Southwood, a former president of the Royal Astronomical Society who was involved with the Rosetta mission from the start.

Overall, scientists hope the $1.74 billion mission will help them learn more about the origins of comets, stars, planets and maybe even life on Earth, he said.

Material from The Associated Press is included in this report.

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