After a string of glitches and delays, Nasa was set to make a last-ditch attempt to launch the Atlantis space shuttle on Saturday.
Overnight Nasa began fuelling the craft, and early Saturday the six astronauts were getting strapped in.
The planned 1115 (1515 GMT) launch time is the last Nasa's 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.