The resident crew on the station now numbers six individuals
Space is about to get a bit crowded.
Seven shuttle astronauts will blast off from Florida on Saturday to join up with six colleagues already on the International Space Station (ISS).
The orbiting platform has never before had so many individuals moving around it at the same time.
The Endeavour ship is scheduled to lift off at 0717 local time (1117 GMT).
The flight-time to the ISS is just three days.
The union some 350km above the planet will be a significant moment for the space station project as it nears the end of its construction phase.
The 13 spacefarers represent all the major station partners, with seven from the US, two each from Russia and Canada, and one each from Europe and Japan.
Their ages range from 37 to 55; all but one are men.
Endeavour will carry its usual complement of seven astronauts
Although 13 people have been in space at the same time once before, in 1995, they were not all in the same place.
"I don't know what it's going to be like," said Endeavour commander Mark Polansky, a veteran of two prior spaceflights. "We know it's going to be challenging with 13 people aboard."
His ship is visiting the station to deliver the final components of Japan's Kibo laboratory.
During five spacewalks, an external platform will be added to the lab which will enable those experiments to be performed that require materials to be exposed to the harsh environment of space.
Endeavour astronauts also have to fit equipment to the exterior of the platform such as batteries and a spare space-to-ground antenna.
In addition, Endeavour will deliver a new crew member (Tim Kopra) to the ISS and bring back another (Koichi Wakata) who has lived aboard the platform for more than three months.
Endeavour is making the 127th space shuttle flight, and the 29th to the station.
Seven more flights to the station remain before the shuttles retire in 2010.
Endeavour is scheduled to return to Earth on Monday, 29 June.