HOUSTON, United States (AFP) -- The International Space Station was an orbiting full house after the shuttle Discovery's seven astronauts joined the two ISS occupants on a pivotal mission for space exploration.

Discovery's astronauts were warmly greeted by the two ISS crew members as they floated into the space station after a two-day journey around Earth, bringing much-needed supplies and equipment for the orbiting outpost.

"It's a full house," ISS flight engineer Jeffrey Williams, an American, told mission control at the Johnson Space Center in Houston, Texas.

Officials said the vessel looked "clean" so far but that the shuttle will have to wait for the weekend to get the green light to return to Earth at the end of the mission in 10 to 11 days.

Before docking to the ISS, Discovery Commander Steven Lindsey steered the shuttle into a perfect back flip about 180 meters (600 feet) below the space station to allow the space station's crew to take photos of the heat shield protecting the spacecraft's underbelly.

The astronauts went right to work after entering the ISS at 1630 GMT, preparing the station's robotic arm to start transferring on Friday critical cargo from the shuttle to the space station.

The cargo includes a new oxygen generator for the space station that would accommodate crews of six people -- double the current number -- on the station starting in 2009.

The astronauts also brought two spacesuits for two or three spacewalks. The ISS gained a third resident with the addition of European Space Agency astronaut Thomas Reiter of Germany, who will remain in the orbiting outpost for six months. "He's settling in rapidly," Williams told mission control.

The six astronauts who will return to Earth will have to wait until this weekend to learn if the shuttle is safe to fly back to the Kennedy Space Center in Cape Canaveral, Florida, officials said.

Preliminary data have shown that the vehicle has suffered no harm, but analysts were still reviewing a mountain of pictures taken by the ISS crew and the Discovery astronauts, said mission management team chairman John Shannon.

But things were looking good so far, Shannon told reporters. "It is so clean. The vehicle is doing so well," he said.

NASA has said the astronauts could take refuge inside the ISS while waiting for a rescue mission if the shuttle suffered irreparable damage. The mission comes three years after Columbia disintegrated during its return to Earth. Its heat shield, pierced on liftoff by debris from its external fuel tank, allowed superheated gases to weaken its structure.

NASA has since striven to reduce the amount and size of foam insulation shed by the tank during liftoff.

More modifications were made after foam shed from the tank during the first post-tragedy flight last July. The debris missed the Discovery shuttle at the time, but NASA grounded the fleet until now.

NASA officials have touted the tank's performance in Tuesday's launch after it shed small debris that peeled off too late during ascension to be of any concern. The current mission, whose major goal is to show whether safety improvements have succeeded, is crucial for the future of the shuttle program and the completion of the ISS. Any major incident would likely spell an early end to the 25-year-old fleet.

NASA chief Michael Griffin wants to make space flight safer in order to resume regular shuttle missions -- about four a year -- to complete the ISS before Discovery, Atlantis and Endeavour are retired in 2010.

The space station is an essential part of U.S. ambitions to send humans to Mars, as it would serve as a stepping stone for space exploration, Griffin said.