There's a 60 percent chance that rain or thunderstorms will postpone the liftoff of Atlantis, shuttle weather officer Kathy Winters said Tuesday. The weather improves with each passing day, as the launch time gets slightly earlier.
Friday's launch time is 11:26 a.m.
The countdown clocks started ticking Tuesday afternoon. The four astronauts assigned to the 12-day flight arrived at Kennedy Space Center on Monday.
Atlantis is making one final supply run to the International Space Station before retiring. As many as 1 million spectators are expected to jam the Cape Canaveral area for the historic send-off.
NASA test director Jeremy Graeber said the launch team is doing its best to put off any emotions associated with the end of the 30-year shuttle era, until Atlantis flies.
"The team gets into the mode of this-is-launch-countdown, and that's really the focus that everybody has," Graeber told journalists. "The rest of the emotion that really comes with the end of the space shuttle program, I think will really kind of roll in as launch is completed."
Graeber, for one, can't wait to take his seat in Launch Control. "It's a really cool job ... and to get to do it one more time is a great feeling," he said.
Atlantis is loaded with thousands of pounds of food, clothes, experiments and other supplies for the orbiting complex. NASA wants to stockpile a year's worth of provisions in case commercially sponsored cargo ships get delayed. The first such launch is targeted for later this year.
NASA is under orders to get out of the Earth-to-orbit business and focus instead on trips to true outer space: an asteroid and Mars.
This will be the 135th flight for the shuttle program and the 33rd for Atlantis, the last shuttle to be retired. Discovery was first in March, followed by Endeavour at the beginning of June. Each shuttle will head to a museum.
NASA said it must launch Atlantis by Sunday -- choosing the best two out of three days -- otherwise it will have to wait until at least July 16. That's because of an unmanned rocket due to lift off next week.
If the crowds are as huge as anticipated, NASA said it will try Friday and then probably wait until Sunday to give launch controllers enough time to deal with the heavy traffic and get some rest.