The two American female astronauts hugged after Melroy and her six Discovery crew members floated into the space station for a two-week mission.

Earlier, Melroy performed two tricky maneuvers.

She steered the shuttle into a backflip near the station to allow the ISS crew to take images of its underbelly to check for potential damage to its heat shield. She then guided Discovery in a super-slow final approach, at three centimeters (1.2 inches) per second.

"Docking confirmed," Melroy said in a live broadcast on NASA television.

Melroy, a retired US Air Force colonel, is the second woman in the shuttle program's 26-year history to command the spacecraft, while Whitson, a scientist, is the first woman in charge of the ISS.

The shuttle, whose crew includes an Italian of the European Space Agency, Paulo Nespoli, linked up with the orbiting space lab after chasing the ISS around Earth for 48 hours.

Discovery launched from Kennedy Space Center in Cape Canaveral, Florida, on Tuesday in a mission that includes attaching the Italian-made Harmony module to the space station.

At least six pieces of foam detached from the shuttle's external fuel tank during takeoff, but NASA experts said they did not present any threat to the orbiter's heat shield.

NASA has closely watched thermal tiles on shuttles since the 2003 Columbia catastrophe when one of them broke off on takeoff and hit a wing. The damage caused the shuttle to break up on re-entry, killing all seven crew members.

"At this point nothing is worth a targeted inspection but there are still a lot of data to analyze," Rick Labrode, the shuttle flight director, told reporters.

Discovery astronauts on Wednesday checked the orbiter's wings and nose for damage using lasers and a camera atop its robotic arm.

At first glance, the shuttle does not appear to have suffered any major damage to its heat shield, which protects its from scorching temperatures during its return to Earth, said Labrode.

A committee of NASA engineers last week recommended replacing three of 44 thermal protection tiles on the orbiter's wings. But the US space agency decided the risk was not high enough to delay the launch for some two months to replace the tiles.

During the two-week mission, astronauts will perform a record five spacewalks and will install the Harmony module, move a 16-tonne truss segment and deploy a third set of power-generating solar panels.

"The pure choreography of moving (the truss segment) is daunting," said Kirk Shireman, deputy manager of the ISS program.

The new module will allow two future Japanese and European scientific laboratories to be installed on the ISS, an outpost considered a key part of US ambitions to send a manned mission to Mars.

The US space agency plans to launch at least another 11 missions to complete the ISS by 2010, when the shuttle fleet is scheduled to be taken out of service.

The ISS is a 100-billion-dollar (70.3-billion-euro) project involving 16 countries.