Greeting them will be the Expedition Five crew of Valery Korzun, Peggy Whitson and Sergei Treschev, who have been in space since June.

The six humans will spend about a week together unpacking the Soyuz-TMA, performing a suite of science experiments and preparing the Soyuz-TM capsule that has been docked to the station since April for its return to Earth on Nov. 9.

A three-person Soyuz spacecraft remains docked to the multinational complex at all times to serve as an emergency rescue vehicle for the full-time expedition crews. Russian flight rules, however, dictate the lifeboat must be replaced every six months as a safety measure.

"It's a critical piece to us," said NASA space station chief Bill Gerstenmaier. "Without the Russian Soyuz being there we'd be in a posture where we couldn't keep our crews permanently onboard station."

Space station flight controllers at the Johnson Space Center in Houston were closely monitoring the launch of the Soyuz-FG rocket, an event made more dramatic than usual because of the Oct. 15 failure of a similarly-designed Soyuz-U rocket that exploded seconds after launch from the Plesetsk Cosmodrome in northern Russia.

The accident that killed one soldier on the ground was probably caused by contamination in the rocket's hydrogen peroxide system, said Russian Space Agency chief Yuri Koptev.

As a result, additional inspections and checks during the countdown at Baikonur were ordered on the Soyuz-FG rocket, which experts say has a stellar record of reliability.

"During the past quarter century it's shown itself to be the safest human spaceflight system ever built by anyone, anywhere," said Jim Oberg, a Houston-based aerospace consultant considered one of the nation's top experts on the Russian space program.

Since 1975 the booster has successfully launched more than 150 piloted Soyuz spacecraft and robot Progress freighters into Earth orbit. The only blemish on the record: A 1983 incident in which a fire at the pad forced the cosmonauts to fire their emergency escape rocket and blast free of the area before the Soyuz exploded.

Costly manufacturing practices and tests is the reason why the Soyuz rockets that launch Soyuz capsules and Progress cargo ship always work, Oberg said.

"A lot of Russian rockets look alike and even carry similar names, but the human-related Soyuz booster comes from a special production run that sees extra care at every step," Oberg said. "Given the chance I wouldn't hesitate to stuff myself into a Soyuz seat to fly. I'd let my sons fly on a Soyuz."

Tuesday's launch featured the inaugural flight of the Soyuz-TMA ship. It is essentially the same on the outside as the Soyuz-TM, but its interior has been rearranged to allow taller crewmembers and includes some upgraded systems that will ease the training burden on future space tourists.

This was to have been the launch that included 'NSYNC pop singer Lance Bass, but the estimated $20 million fare couldn't be paid to the Russians in time. Bass is reportedly considering another run at flying in the future. Neither he nor his representatives could be immediately reached for comment following the launch.