CAPE CANAVERAL, Fla. -- Gambling Sunday against a new threat from Florida's fickle weather, officials counted down toward a Monday morning blastoff for shuttle Challenger and space teacher Christa McAuliffe.
Challenger, a high-tech schoolhouse from which McAuliffe will hold class for American kids from coast to coast, stood poised for launch at 9:37 a.m. EST.
The mission, 25th in the nation's ambitious shuttle series and the 10th for Challenger, will make McAuliffe the first ordinary citizen to fly in orbit.
An unexpected Sunday afternoon forecast raised the threat of low morning cloud decks that might turn the liftoff attempt into a cliffhanger. Earlier predictions had called for fine launch weather Monday.
But space agency officials, who lost a gamble on one cold front Sunday and expected a second Arctic chill to plunge temperatures below the minimum for launch operations Tuesday, decided to bet on a Monday blastoff.
Challenger's countdown, halted Saturday night when officials abandoned plans for a Sunday morning launch, resumed at 8:17 p.m. Crews rolled back a big protective service structure so they could fuel Challenger with half a million gallons of supercold liquid hydrogen and oxygen.
McAuliffe and her six crewmates -- commander Francis 'Dick' Scobee, co-pilot Michael Smith, Judith Resnik, Ellison Onizuka, Ronald McNair and engineer Gregory Jarvis -- were so confident of a launch they elected to pass up watching the Super Bowl in favor of last-minute preparations for flight.
Besides McAuliffe's televised classes during Challenger's fourth day in orbit, their showcase mission plan called for launching a $100 million communications satellite from the shuttle's cargo bay Monday evening and deploying a boxy, reusable space probe Tuesday to study Halley's comet.
Scobee and Smith took advantage of a gap in the clouds before dinner Sunday to fly a few practice landings in a shuttle training aircraft. Bedtime for the crew was 7 p.m. in preparation for a 5:15 a.m. wakeup call Monday.
'It's going to go tomorrow,' McAuliffe, a 37-year-old New Hampshire social studies teacher, laughingly told a TV crew that spotted her during a Sunday morning bicycle ride near the launch site. 'I'm positive!'
Challenger's flight already has been postponed four times in two weeks, including two delays caused by weather. Each new postponement made it harder for NASA to achieve its plans for a record 15 shuttle missions in 1986.
The latest troublesome weather forecast called for a possibility of cloud banks at an altitude of 2,500 feet over the launch site, which could keep Challenger from making an emergency return to the spaceport in event of engine failure just after liftoff.
It also said unacceptable cloud banks might develop at 1,500 feet over an emergency landing strip in Casablanca, Morocco.
But officials said the shuttle had a three-hour 'window,' starting at launch time, during which it could wait if necessary for the clouds to break.
'It's hard to tell what willhappen,' NASA spokesman Hugh Harris said. 'The forecast is not as good as we'd like, but on the other hand we think there's an excellent chance they can get off sometime during the three-hour window.'
Space agency officials, in a Saturday night gamble they lost, canceled plans for a Sunday morning liftoff based on forecasts calling for the first cold front to blanket the Florida spaceport with rain and low clouds Sunday. The front slowed down unexpectedly, and the day dawned with sunny skies.
'It looks like we could have made it,' launch director Gene Thomas said Sunday. 'Conditions were good for launch this morning. We made a decision based on the data we were given (Saturday night).'
By midday, however, the first cold front had arrived with rain and heavy gray clouds that covered the Kennedy Space Center most of Sunday.
'What's really going to get us (if some other problem prevents a Monday liftoff) is a secondary front behind it, which will bring arctic temperatures,' NASA spokesman Dick Young said.
Challenger's orbital classroom activities are the highlight of the six-day mission.
McAuliffe plans to conduct two lessons from space Thursday that will be beamed by the Public Broadcasting System to homes and schoolrooms across the nation. One lesson will explain differences between life in space and on Earth, while the other will show why factories may someday operate in orbit.
She also will film six lessons, to be sent to schools after landing, that take advantage of weightlessness to illustrate concepts such as Newton's laws of motion.