Instead of three extra-vehicular activities, as originally scheduled, mission controllers have ordered two 6.5-hour spacewalks, which will be conducted by mission specialists Mike Fossum and Piers Sellers, on the fifth and seventh days of the 13 day mission.

The first spacewalk will test the 50-foot robotic arm boom extension as a work platform. The astronauts also will begin maintenance of the station's Mobile Transporter by safing or replacing a cable-cutter unit to allow the station's mobile robotic system to support the second spacewalk.

During that EVA, Fossum and Sellers will replace the detached cable on the MT, which was inadvertently cut, and its reel assembly. They also will install a spare part for the station's thermal control system for future use.

Meanwhile, the shuttle crew will continue to test equipment and procedures intended to improve the safety of the spacecraft. They also will perform maintenance on the space station and deliver more supplies and cargo for future expansions of the orbiting facility.

Officials said they are using the mission to continue analyzing safety improvements that debuted on Discovery's first Return to Flight mission, STS-114, in July 2005.

The new mission will deliver a third crewmember to the station, ESA astronaut Thomas Reiter. His addition will mark the first time the ISS has hosted a three-person crew since Expedition 6 returned to Earth on May 4, 2003. Without the space shuttle to ferry equipment to the station after the Columbia accident in February of that year, only two people could be supported onboard until the necessary provisions were in place.

Discovery will carry the Italian-built Multi-Purpose Logistics Module Leonardo, with more than two tons of equipment and supplies aboard.

This will be the fourth trip to the station for Leonardo, the first of three Italian-built MPLMs. The shuttle crew will move all equipment and supplies no longer needed on the station to Leonardo, before it is unberthed on Flight Day 10 and put back into Discovery's cargo bay for return to Earth.

Steve Lindsey, a U.S. Air Force colonel, will command Discovery on his fourth spaceflight and his second as commander. U.S. Navy Cmdr. Mark Kelly will pilot the spacecraft, on his second flight.

Others aboard include mission specialists Stephanie Wilson and Lisa Nowak, who is a Navy commander. This will be the first spaceflight for Fossum, Wilson and Nowak. Sellers also will be making his second spaceflight.

Reiter will remain on the station, working with the station crew under a contract between ESA and the Russian Federal Space Agency.

Although they canceled the third spacewalk, mission managers said if an extra day becomes available, they might reschedule the activity. That EVA would include tasks to test techniques for inspecting and repairing the Reinforced Carbon-Carbon segments that protect the orbiter's nose cone and wing leading edges.

Inspections of the orbiter's heat shield with a 50-foot-long Orbiter Boom Sensor System are scheduled for before Discovery arrives at the station and again near the end of the mission, on the day before and the day of undocking from the space station.

Undocking is set for Flight Day 11. Discovery's crew will make final preparations for the return home on Flight Day 12, with landing at the Kennedy Space Center's Shuttle Landing Facility on Flight Day 13.

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