After a string of glitches and delays, Nasa was set to make a last-ditch attempt to launch the Atlantis space shuttle on Saturday.
The crew is keen to get on with the work at the space station
Overnight Nasa began fuelling the craft, and early Saturday the six astronauts were getting strapped in.
The planned 1115 (1515 GMT) launch time is Nasa's last opportunity until at least late September.
It is hoping to keep to a tight schedule of construction work on the international space station (ISS).
Atlantis was initially set to launch on 27 August, but a lightning strike, tropical storm, and technical glitches have postponed the mission four times.
On Thursday, a recurring problem with a sensor on Atlantis's external fuel tank prompted agonised directors to put off the launch again.
Saturday only emerged as an option after last-minute talks with Russian space officials, who are launching a Soyuz capsule to the space station on 18 September.
Atlantis has to leave the station before the Soyuz arrives to avoid interfering with the Russian craft.
If Atlantis cannot launch on Saturday, the mission will have to be set back until late September or October.
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
Nasa managers are under pressure to launch as there is a tight schedule of some 15 further shuttle missions to complete construction work on the ISS by 2010, when the current shuttle fleet is due to be retired.
The shuttle's crew will deliver and fit the P3/P4 truss, a 17-tonne segment of the space station's "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.
Once fitted, they will effectively double the station's current ability to generate power from sunlight.