The space shuttle Endeavour was taking on fuel Monday morning in preparation for the penultimate flight of the shuttle era.
Liftoff was slated for 8:56 a.m., and NASA was confident that the electrical problems that delayed the mission two weeks ago had been fixed.
As many as half a million people are expected to gather in the area around the Kennedy Space Center in Florida to watch Endeavor blast off, bound for the International Space Station one last time before heading to retirement at a Los Angeles museum.
Among them will be mission commander Mark Kelly's wife, Arizona Rep. Gabrielle Giffords — recovering after being gravely wounded in a shooting rampage in Tucson earlier this year.
"Who's ready for the best show on Earth?," Giffords' staff said in a Twitter update hours before liftoff.
Kelly and his five crewmates waved, gave a thumbs-up and shook their fists in the air as they headed to the launch pad in the pre-dawn hours.
"Good to be aboard," Kelly told launch controllers.
They posed at the base for a group picture Monday, before taking the elevator up and climbing into Endeavour.
"Took my last shower for a few weeks," astronaut Mike Fincke reported in a tweet. "The flight docs gave a good look-over. My only issue: too much boyish enthusiasm. (no known cure)."
Pilot Gregory Johnson said in his own Twitter update: "I am really excited and charged up for this mission! Slept great."
NASA has spent the past two weeks replacing a switch box with a blown fuse as well as a suspect thermostat, and installing new wiring.
Giffords, who flew in Sunday from Houston, where she's undergoing rehabilitation for a gunshot wound to the head, will be among 45,000 guests were expected to watch the liftoff at the launch site. An additional 500,000 spectators were likely to jam area roads and towns to send off Endeavour and her crew.
$2 Billion Alpha Magnetic Spectrometer
The shuttle's liftoff will also be watched by more than the usual number of physicists. That's because Endeavour will be carrying up a $2 billion particle physics detector known as the Alpha Magnetic Spectrometer.
The AMS will be mounted onto the International Space Station, where it will collect cosmic rays — charged particles that zoom through space — for a decade.
The equipment was designed to search for primordial antimatter created during the Big Bang, and the mysterious dark matter that makes up much of our universe.
Nobel Prize-winning physicist Samuel Ting of the Massachusetts Institute of Technology pushed for more than 16 years to make the project happen, raising money to build it with the help of hundreds of researchers in more than a dozen countries.
Ting said no one knows what the AMS might discover.
"I mean, if you find what you predicted, it's not interesting," he said in an April interview. "The interesting thing is to destroy the current idea, to find something new."
The Baby Of The Fleet
Endeavour first flew in 1992 and ended its first mission 19 years ago Monday. The shuttle has flown 116.4 million miles in 24 previous flights. It is the baby of NASA's shuttle fleet and was built to replace the Challenger, lost in a 1986 launch accident.
Sisters Anita and Andrea Johnson drove across the state from Tampa for a front-row view.
"We usually stand in our backyard to watch the launches but this time we wanted to see it up close and personal," Anita Johnson said as she pulled lawn chairs out of her car parked across the Indian River in Titusville, Fla.
To the south toward Cape Canaveral and Cocoa Beach, recreational vehicles and cars already were lined up along the Banana and Indian rivers late Sunday. And signs outside area businesses cheered Endeavour on with messages of "godspeed" and "go."
NASA is retiring its three remaining space shuttles after 30 years to concentrate on interplanetary travel. The space agency wants to hand over the business of getting crews and cargo to the space station, to private companies. At least one company, Space Exploration Technologies Corp., said it can get astronauts to the space station within three years of getting NASA approval.
One final mission remains, by Atlantis in July.
NPR's Nell Greenfieldboyce and Judith Smelser of member station WMFE contributed to this report, which contains material from The Associated Press. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.