NASA managers ruled out a launch attempt on Thursday but said Friday still was a possibility. The decision followed a lengthy meeting over what to do with a problem in the shuttle's electrical power system that forced a scrub on Wednesday.
Friday is the last day the U.S. space agency can get Atlantis off the ground before interfering with the planned launch of a Russian Soyuz vehicle on a trip to the space station later in the month.
A problem with a fuel cell turned up early Wednesday, forcing NASA to postpone Atlantis' launch again.
Tuesday, as the crew studied up for the shuttle mission, Altanis' mission commander, Brent Jett, and pilot, Chris Ferguson, used a Gulfstream jet to simulate landing the shuttle, while other crew members reviewed the complicated construction tasks ahead of them.
The astronauts are to resume building the space station for the first time since the Columbia disaster 3 1/2 years ago, adding a 17 1/2-ton truss to the orbiting outpost.
Crew members, who have been training for four years, will deliver a 35,000-pound, $372 million addition to the half-built space station. Their 11-day mission includes three spacewalks.
CBS News correspondent Drew Levinson notes that to avoid a schedule conflict with a Russian spacecraft also headed for the space station, Atlantis has until Friday to get off the ground. If that deadline is missed, Atlantis would then have to wait until late October to try again.
The Russians plan on Sept. 18 to launch a Soyuz capsule ferrying two new station crew members and the space station's first female tourist, Dallas-area entrepreneur Anousheh Ansari.
On the eve of Atlantis' planned launch, no major technical problems were reported. Technicians said a slightly hazardous reading of gaseous oxygen in the rear of the shuttle is not a worry, as it is considered to be the result of a bad connection during fueling of the spacecraft's power cells.
Pre-launch anticipation has filled the air.
"The first couple of inches is the hardest part of the whole flight," says mission management team chairman LeRoy Cain. "Anxious anticipation is how I would describe it."
NASA had started to move Atlantis back to the hangar because of Tropical Storm Ernesto but changed course last week. By sending the shuttle back to the launch pad, NASA gained enough time to prepare for a launch this week.
Atlantis was originally scheduled for liftoff Aug. 27 but was delayed, first by a lightning strike at the launch pad, then by Tropical Storm Ernesto.
The lightning didn't hit the spacecraft, but by the time the shuttle was cleared for launch, Tropical Storm Ernesto was approaching Florida.
If the shuttle doesn't lift off this week, NASA still is looking at the possibility of waiving a rule that Atlantis be launched in daylight in order to open up launch opportunities in late September or early October. Otherwise, the next daylight attempt could not be made until late October, said Wayne Hale, space shuttle program manager
NASA wants one more daylight launch so the space agency can have clear photos of Atlantis' external fuel tank during lift off. Foam dropping from Columbia's external tank in 2003 doomed the spacecraft and its crew of seven astronauts.