The STS-120 crew members await the start of a training session in April 2007 at the Space Vehicle Mockup Facility at Johnson Space Center in Houston. From the left are astronauts Pamela Melroy, STS-120 commander; Daniel M. Tani, Expedition 16 flight engineer; George Zamka, STS-120 pilot; and Douglas Wheelock, Scott Parazynski, Stephanie Wilson and the European Space Agency's (ESA) Paolo Nespoli, all mission specialists. (NASA)

The assembly of the International Space Station has been compared to building a giant Lego toy in space, but for the arriving crew of the space shuttle Discovery, the next stage of the mission will be more akin to solving a jigsaw puzzle.

At top, a detailed view of the Harmony Node 2. At bottom, a view of Harmony's eventual location next to the International Space Station's U.S.-built Destiny laboratory. (NASA)

When NASA's STS-120 crew arrives at the station, they will deliver the Harmony Node 2, a pressurized component that will act as a junction between the station's U.S. laboratory and two future laboratories: one from Japan and another from Europe.

After years of labour on solar panels and inner workings, Harmony represents the first major update to the station's interior in six years and marks the beginning of what will be a more aggressive assembly plan as NASA tries to complete the station before it retires the space shuttle program in 2010.

The crew will also be moving a retracted solar panel that has been atop the station's structure for seven years to its rightful place on the port side.

And as usual on a space shuttle mission, there will be a crew swap, with U.S. astronaut Daniel Tani replacing countryman Clayton Anderson aboard the space station.

Women in charge

Discovery successfully lifted off at 11:38 a.m. ET on Oct. 23, 2007, from NASA's Kennedy Space Center in Florida. The 14-day mission, which includes five spacewalks, is scheduled to end with the return of the Discovery crew at 4:47 a.m. ET on Nov. 6.

Retired U.S. Air Force colonel Pamela Melroy will command Discovery's crew on their mission to the orbiting space platform, becoming the second woman to command a space shuttle. She'll also be one of two women in charge during the mission — alongside U.S. space station commander Peggy Whitson — the first time women have played both roles.

Also arriving aboard Discovery are pilot George Zamka and mission specialists Scott Parazynski, Douglas Wheelock, Stephanie Wilson and Paolo Nespoli, a European Space Agency astronaut from Italy.

Meeting them on the space station will be Whitson, Anderson and veteran Russian flight engineer Yuri Malenchenko, who has spent over 300 days in space as a crew member of both the International Space Station and Russia's Mir space station.

Delivering Harmony to space

Once the shuttle docks at the station, the first part of the mission will be to install the Harmony node to its temporary position. The Italian-built module is the first habitable component of the station to be added since a Russian Soyuz rocket brought the Pirs docking compartment in 2001.

Installing Harmony will be a three-step process, since the docking port the space shuttle arrives on is also Harmony's eventual resting place, right in front of the Destiny laboratory. To get Harmony to its rightful place, the component — which weighs about 14,000 kg when loaded for flight — will be parked in a temporary spot on the side of the space station. When the space shuttle departs, the station's crew will move the docking port onto Harmony, and then move the node and attach it to the Destiny lab.

The node is a seven-by-four-metre passageway that will connect the Destiny lab to the European Space Agency's Columbus laboratory and Japan's Kibo module.

The delivery and installation of those two components will be the focus of the next two missions. Columbus is scheduled to arrive in December, while Kibo is scheduled to arrive over two shuttle flights in February and April.

Also arriving in the February flight will be the Canadian Dextre robotics system, a mechanical manipulator that can be attached as a hand for the station's Canadarm 2 or ride independently on the same mobile base system the Canadarm 2 rides. When installed, Dextre should be able to perform many of the tasks currently performed by spacewalking astronauts.

Solar panel shuffle

The other main task of the Discovery mission is the transfer of the P6 Truss and its solar panels to their permanent spot on the left side of the station.

The truss's solar panels, first installed seven years ago, have been the station's primary source of power but were retracted earlier this year in anticipation of their transfer to their new home.

NASA knows those solar panels well — in December 2006 astronauts Robert Curbeam and Christer Fuglesang needed an extra spacewalk to retract the structure after it became stuck during the automated process. The two astronauts ultimately had to manually fold some of the panels of the 35-metre array.

Once the P6 truss is installed, there will be only one more component of the station's solar arrays left to install, the starboard S6 truss. The S6 truss is scheduled to be delivered and installed in November 2008.

The Discovery mission also has a fifth spacewalk that will allow astronauts to test a thermal tile patch, one that could be used in case repairs are needed to the shuttle. In August, the space shuttle Endeavour suffered a small gouge in its thermal tiles during launch. NASA ultimately decided not to repair it in orbit because it posed an insignificant risk.

The space agency has already had to shuffle the schedule of its shuttle flights this year because of delays in various repairs to shuttles, most notably. Prior to Discovery's launch, NASA engineers had to repair a leaking hydraulic seal, though the repair did not affect the launch date.

The accelerated pace of the missions comes in response to NASA's stated desire to wind down the space shuttle program in 2010 so it can concentrate on its Orion spaceships, which the U.S. space agency hopes will be able to bring manned space exploration back to the moon and later to Mars.