SPACE CENTER, Houston (AP) _ Six Americans and one Belgian constitute the crew of space shuttle Atlantis. They will conduct the first in an 11-year series of experiments studying changes in Earth's atmosphere.

Four of the astronauts have flown in space before; the other three are rookies.

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Commander Charles Bolden Jr., 45, a Marine colonel, is commanding his first shuttle mission. He has flown in space twice before, both times as a shuttle pilot.

During the Vietnam War, Bolden flew more than 100 sorties into North and South Vietnam, Laos and Cambodia in 1972 and 1973. He later served as an ordnance test pilot for the Navy. He was picked as an astronaut in 1980.

''Growing up in Columbia, S.C., I wanted to be a lot of things. An astronaut was never one of them,'' he said. ''I knew what astronauts were, but that was not something in my brain that was in the realm of possibility for me. I kind of fell into it.''

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Shuttle pilot Brian Duffy is making his first space flight.

''I've looked forward to this for a long time,'' said Duffy, 38, an Air Force lieutenant colonel from Rockland, Mass.

Duffy, as a youngster, was intrigued by orbital mechanics and thought even then of becoming an astronaut. But, he said, ''I was just a young kid from a small town - what makes you think you can do something like that?''

NASA chose Duffy as an astronaut in 1985.

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David Leestma thrills at the view from the shuttle windows. He's flown in space twice before.

''That just takes your breath away. It brings tears to your eyes,'' said Leestma, 42, a Navy captain from Muskegon, Mich.

Leestma was selected as an astronaut in 1980. He made a spacewalk during his first space flight in 1984.

''I know my family goes through some stressful times when I get ready to fly,'' he said. ''I think they do (worry) but they try not to let on too much.''

Leestma and his wife have six children, ages 2 to 11.

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Kathryn Sullivan is the payload commander.

Sullivan, 40, became the first American woman to perform a spacewalk during her first space flight in 1984. She flew in space again in 1990.

''There are so many highlights,'' she said. ''I can't tell you a flight or a phase or an event or a sight that I would give up because something else was better.''

NASA selected Sullivan as an astronaut in 1978. President Reagan appointed her to the National Commission on Space in 1985.

She calls Los Angeles her hometown.

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Michael Foale, who holds citizenship in both the United States and Britain, is making his first space trip.

Foale, 35, said he's glad to be part of this mission ''because it involves a lot of the fun things that I think space flight is all about: stars, sun, Earth and how the sun is affecting the Earth. That's all physics, and I always liked those topics. They're big-picture topics.''

Foale grew up in England. He moved to Houston in the early 1980s to work on space shuttle navigation problems for McDonnell Douglas Aircraft Corp.

He joined NASA in 1983 and was selected as an astronaut four years later.

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Biomedical engineer Byron Lichtenberg is making his second space flight, one he considers especially important.

''We're studying our atmosphere, our environment, something that everybody can relate to,'' he said.

Lichtenberg, 44, is chief scientist for Payload Systems Inc., a company devoted to commercial exploration and development of space.

NASA chose the Stroudsburg, Pa., native in 1978 as a payload specialist, someone who occasionally flies in space, and is not a full-time astronaut. His first space trip was in 1983.

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Physicist Dirk Frimout is the first Belgian to fly in space. He is a staff member of the European Space Agency.

''The fact I can be on board, that is exceptional. I have always hoped to be, but never could believe that it would come true some day,'' he said.

Frimout, 51, said the Atlantis experiments will help scientists better understand Earth's atmosphere.

''It's a big, big puzzle, and we hope to provide another piece in that puzzle,'' he said.