But so are thunderstorms. The Air Force and NASA said today there is a 70 percent chance that clouds and showers will get in the way of a Friday launch. The launch is currently scheduled for Friday at 11:26 a.m. EDT, if the weather allows. The forecast gradually improves over the weekend; on Saturday there's a 60 percent chance the weather will interfere, and by Sunday the threat drops to 40 percent.
The shuttle has only one chance to launch each day -- a 10-minute window during which its orbiting target, the International Space Station, is passing overhead. A Saturday launch would happen at 11:02 a.m.
"What has been a little difficult is the goodbyes," said astronaut Christopher Ferguson, Atlantis' commander, in a preflight interview with ABC News. "In terms of round numbers the shuttle workforce is 6,000 right now. By the end of July it will be somewhere around 2,000.
"A lot of people to say goodbye to in a short period of time."
This will be the 135th and final mission of NASA's 30-year shuttle program. Ferguson and three crewmates will ride Atlantis one last time on a supply run to the space station.
It is a quiet ending to a program that, in many eyes, never could live up to the promises made when it was conceived in the early 1970s. It was supposed to make spaceflight affordable, safe and routine. But 14 astronauts died in the Challenger and Columbia disasters, and flights have been estimated to cost about half a billion dollars each.
"But there is no embarrassment in setting the bar impossibly high and then failing to clear it," said former shuttle astronaut Duane Carey in an interview with The Associated Press. "What matters is that we strived mightily to do so -- and we did strive mightily. The main legacy left by the shuttle program is that of a magnificent failure."
NASA Administrator Charles Bolden, himself a former astronaut, came to the shuttle's defense. And he promised that America's human spaceflight effort would continue, even as shuttle flights come to an end.
"I spent 14 years at NASA," he said at a speech in Washington, D.C., last week. "Some of the people I respect most in the world are my fellow astronauts. Some of my best friends died flying on the shuttle, and I am not about to let human spaceflight go away on my watch."
He became emotional when he said, "So when that final shuttle landing occurs and the cheers and tears subside, we will keep on moving toward where we want to go next. Your kids and my grandkids, they're going to do things that today we can barely dream of."