Shuttle Discovery docks at the Space Station for its final mission

Resupply activity aboard the International Space Station moves into full swing today as the combined crews from the space shuttle Discovery and the space station begin offloading cargo and preparing for the first of two space walks Monday.

The orbiter and its six-member crew arrived Saturday at 3:14 p.m. Easter Standard Time in what controllers called a "textbook docking."

Discovery's arrival marks the first time in the history of the space-station program that craft from each of the four major partners in the $100 billion project – Europe, Japan, Russia, and the US – are docked at the station at the same time.

Although Discovery and its crew reached the station on schedule, it would take another 40 minutes for the shuttle and station to grip each other firmly enough to allow the station crew members to welcome their newly-arrived colleagues.

As Discovery inched into its docking port at roughly half a mile an hour, the first step in the docking process – a so-called soft dock – took place as planned. At this stage, the two craft were mated but not yet firmly locked to one another.

But "it took a little bit longer than we could have liked for the hard mate to complete," said the mission's lead flight director, Bryan Lunney, referring to the second step that locks the craft together.

In essence, the shuttle's docking ring didn't sit squarely against the station side of the docking port, but had a tiny tilt to it. It took 40 minutes for the orientation of the station and shuttle to change with respect to Earth to give gravity a chance to counteract the tilt and bring the two halves of the docking system squarely together for the hard capture.

After the crews greeted each other inside the space station, station commander Mark Kelly gave the newcomers a safety briefing, and work began to offload one of the key pieces of hardware the shuttle delivered – a platform that bolts to the station's exterior and holds spare components for the station.

The platform came packed with one item – a spare radiator assembly designed to help the station shed excess heat that can build up inside the modules. Other spares are scheduled to arrive during the shuttle program's final two missions, currently targeted for April and June.

The presence at the station of spacecraft from all of the major partners – an unmanned Russian Progress resupply rocket plus the manned Soyuz craft, unmanned cargo carriers from Japan and Europe, and the shuttle – has prompted discussions between NASA and its Russian counterpart, Roscosmos, about the possibility of taking pictures of the station with all the visiting craft.

As currently envisioned, the mini-mission would involve three members of the station crew leaving the station briefly in a Soyuz craft. The craft would back away roughly 650 yards from its docking port, then take high-resolution images of the station as the orbiting outpost slowly rotates to allow the Soyuz crew to take images from several angles.

It doesn't take three astronauts to take pictures. But crew rotations with the three-seat Soyuz are designed around three astronauts up and three back. If for some reason the capsule can't re-dock with the station, it would return to Earth. With fewer than three astronauts aboard, such a return could throw a wrench in the well-oiled gears of the crew rotation process.

Mission managers say they anticipate a final decision on the photo op Tuesday. If the Russians say "da," the day-long effort would take place Saturday, and NASA would add a day to Discovery's mission to allow the crew to complete its allotted tasks.