SAN FRANCISCO (Reuters) - Human increases in carbon dioxide emissions are thinning the Earth's outer atmosphere, making it easier to keep the space station aloft but prolonging the life of dangerous space debris, scientists said on Monday.

"It's a bit of a two-edge sword," said Stanley Solomon, a scientist at the National Center for Atmospheric Research in Boulder, Colorado. "In the future, it will be a little bit easier to keep the space station, for instance, in orbit. It will need a little bit less fuel."

"On the other hand, it will give space junk a much longer lifetime," he told the fall meeting of the American Geophysical Union in San Francisco.

Solomon is the co-author of a study presented on Monday that found man's burning of fossil fuels and increase of carbon dioxide emissions will make the Earth's outer atmosphere above 62 miles 3 percent less dense by 2017. The study found a decrease of about 5 percent between 1970 and 2000.

Although scientists say that carbon dioxide contributes to global warming closer to Earth's surface, in the thinner outer atmosphere where space craft orbit, a cooling effect takes place. Solar activity also impacts the outer atmosphere.

As this outermost region becomes less dense, it produces less drag on satellites, space craft and tens of thousands of pieces of discarded space debris from previous missions orbiting at about 250 miles from Earth's surface.

"These objects are now experiencing less drag proportionally than they did 30 years ago," Solomon said.

The Soviet Union kicked off the space age in 1957 by launching the Sputnik satellite into orbit. Some of the early satellites such NASA's Explorer 8 launched in 1960 are still spinning around the Earth.

A steady stream of space launches since Sputnik has left about 10,000 objects bigger than the size of a grapefruit, and 100,000 larger than a centimeter, said Kent Tobiska, president and chief scientist of Space Environment Technologies in Pacific Palisades, California.

"It's a more complex, more difficult and a more dangerous area for them," Tobiska said of modern-day spacecraft.

The International Space Station now in orbit must readjust its path several times a year to avoid colliding with such debris; a chance hit with a spacewalking astronaut could prove fatal.

A Russian cosmonaut increased by one the number of man-made objects in space by hitting a golf ball into the final frontier during a space walk last month.

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