Nasa is optimistic that Wednesday's planned space shuttle launch will go ahead, after forecasts predicted favourable weather conditions.
Nasa officials believe Atlantis will begin its journey on Wednesday
Officials added that no technical issues had been reported that could delay Atlantis' lift-off, scheduled for 1229 EDT (1629 GMT).
Two previous launch attempts had been delayed because of a lightning strike and Tropical Storm Ernesto.
The orbiter is due to deliver giant solar arrays to the space station.
"The countdown is going extremely well, we are not tracking any problems," launch director Mike Leinbach said at a briefing.
Earlier weather forecasts had warned of unsettled weather around the launch pad at the Kennedy Space Center, Florida.
However, the weather is not likely to cause problems, according to Nasa weather officer Kathy Winters.
"It is a little more moist, so we are going to bump our number from 10% to 30% chance of Kennedy Space Center weather prohibiting launch," she said.
ISS: ORBITING OUTPOST
Construction work has been on hold for four years
16 nations contribute to the ISS, including the US, Russia, Japan, Canada, Brazil and European Space Agency states
The ISS will eventually be the size of a football field
The six-strong crew's task is to deliver and fit the P3/P4 truss, a 17-tonne segment of the International Space Station's (ISS) "backbone" that includes a huge set of solar arrays and a giant rotary joint to allow them to track the Sun.
The arrays will be the second of four sets, and will span 73m (240ft) when fully extended.
The wings will provide power for three science laboratories, two living chambers and other systems onboard the ISS. They effectively double the station's current ability to generate power from sunlight.
The half-built $100bn space station must be completed before 2010, when the shuttle fleet is due to be retired.
A further 16 shuttle flights are needed to finish the ISS, and Nasa hopes one orbiter flight can also be found to service the Hubble Space Telescope.
The mission marks only the third shuttle flight since the loss of Columbia and its seven astronauts in 2003.