The U.S. space shuttle Discovery has docked at the International Space Station, marking the next phase in a complex mission to expand the international orbiter. VOA's Alex Villarreal reports from Washington.

Space station commander Peggy Whitson (left) welcomes shuttle commander Pam Melroy aboard station

Space station crew members, led by Commander Peggy Whitson, welcomed Discovery Commander Pam Melroy and her crew when they boarded the station.

Whitson and Melroy are the first women in NASA history to lead two spacecraft at the same time.

Their crews united after pressure checks ensured the hatches between the spacecraft were safe to open, just two hours after Discovery docked.

Speaking to reporters Thursday, deputy ISS program manager Kirk Shireman said the docking is exciting, but much work remains.

"Getting on orbit and docking to the ISS is the first step, but we have still a daunting task ahead of us," he said.  

Officials say this will be one of the most difficult missions in the shuttle program's history.

Discovery brought with it a new module to expand the size of the space station. The module will provide docking ports for Japanese and European scientific laboratories to be installed on the ISS.

Shireman said the module is the gateway to having all international partners on the orbiter.

Discovery's 10-day mission also includes using the station's robotic arm to move a massive solar panel section to another part of the ISS.

Shuttle Flight Director Rick LaBrode said the move will be difficult, but remains optimistic.

"We're confident that we're going to be able to do it. We've done it, we've practiced it, and we're ready. So it's going to be exciting," he said.

Before docking, Discovery did a backflip so the ISS crew could take photos of its underbelly. Specialists at mission control will analyze the photos to check for damage.

LaBrode said nothing significant has been found so far.

Before launch, engineers found possible defects in three panels. An independent review board urged NASA to replace the tiles, but agency officials decided not to delay the launch.

Discovery is scheduled to return to Earth on November 6.