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(CNN) -- Space shuttle Discovery launched just before midnight Friday on a mission to the international space station.

The crew of seven astronauts includes one from Mexico and another from Sweden. One of those seven, Nicole Stott, will remain on the station as a flight engineer, while astronaut Timothy Kopra is to return home aboard the shuttle.

Also on board: The Leonardo logistics module, science experiments and the Combined Operational Load Bearing External Resistance Treadmill (COLBERT), named for fake newsman Stephen Colbert of Comedy Central's "The Colbert Report."

Colbert won an online poll conducted by NASA to name the newest space station compartment, but Colbert and the space agency compromised to give the moniker to the treadmill. The new compartment was given the name Tranquility.

NASA astronaut Cady Coleman said the treadmill is an essential addition to the space station.

"We have the treadmill now to keep them healthy, which is really part of being able to come home in one piece. So it is an essential part," Coleman said.

Discovery's liftoff, originally set for Tuesday, had been postponed three times -- first for bad weather, and twice more while mission managers checked out indications of a faulty valve.