CAPE CANAVERAL, Fla. (AP) — With good weather predicted for launch time, NASA started fueling space shuttle Endeavour today in preparation for its evening liftoff.

The space agency began pumping more than 500,000 gallons of supercold liquid hydrogen and liquid oxygen into the ship's tank shortly after 8 a.m. The process was expected to take about three hours, leaving plenty of time before the scheduled 6:36 p.m. launch, which will take former schoolteacher Barbara Morgan into space.

Forecasters predicted an 80 percent chance that the weather will be favorable for liftoff.

Morgan, who was Christa McAuliffe's backup on Challenger two decades ago, and her six crewmates are slated to spend two weeks at the international space station.

Their mission continues construction on the orbiting outpost. The crew is to attach a new truss segment to the space station, replacing a gyroscope that helps control the station's orientation and delivering 5,000 pounds of cargo.

If the mission is extended from 11 days to 14 days, a decision that won’t be made until the mission is well under way, the astronauts will add a fourth spacewalk to the planned three to install protective panels to stop debris from hitting the station.

Morgan went back to teaching after the Challenger disaster but continued her NASA-related speaking engagements. In 1998, she became the first teacher to join the astronaut corps, trained to conduct tasks on a mission, rather than be a guest on a flight as McAuliffe was.

While in space, Morgan plans to answer questions from schoolchildren in Idaho, and if the mission is extended, from Virginia and Massachusetts as well.

Endeavour was initially scheduled to lift off Tuesday but was delayed for a day because NASA had to replace a leaky valve in the crew cabin.

See NASA's special countdown page here.