"We are still focused on our July launch window," agency spokeman Joe Pally told SpaceDaily.com.

Pally echoed remarks by his boss, NASA Administrator Michael Griffin, to the New York Times on Tuesday that the shuttle schedule remains on target for a launch in about 10 weeks. That mission, and at least one more this year, are deemed essential by NASA to stay on schedule to complete the station's assembly before the shuttle fleet is retired – an event currently planned for 2010.

This position might reflect optimism that three critical potential problems – concerning the shuttle's main fuel tank gauge, the tank's insulating foam, and the spacecraft's roll program – will be fixed to everyone's satisfaction within that time.

Wind-tunnel tests have continued to show potential problems with detachments and aerodynamic distortions caused by the external tank's insulating material. The fuel gauge problem caused a scrub of Discovery's original May launch, and engineers have uncovered evidence that a solid rocket booster structural component may have exceeded its design load limit during Columbia's launch in 2003.

All three areas of concern must be resolved before Discovery takes off again.

Assuming the problems are fixed, however, and STS-121 flies as planned, Discovery will carry six crew members and one passenger to the ISS. Along with crewmembers Steve Lindsey, Mark Kelly, Mike Fossum, Lisa Nowak, Stephanie Wilson and Piers Sellers, the shuttle will transport ESA astronaut Thomas Reiter to the station.

Reiter will return the ISS crew complement to three for the first time since the Expedition 6 crew returned to Earth on May 4, 2003, following the Columbia disaster.

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