(AP) -- Six astronauts have said their goodbyes to their families and are ready to take space shuttle Endeavour on its final flight Friday as hundreds of thousands gather along Florida's Space Coast to cheer the spectacle.
Liftoff was set for almost 10 hours after another spectacle was beginning in Britain - the wedding of Prince William and Kate Middleton. The shuttle's huge fuel tank was scheduled for a fill-up at about the same hour they were to exchange vows.
Endeavour weathered a strong thunderstorm Thursday night that dropped hail on a neighboring town and delayed late launch preparations. But that was not expected to interfere with the Friday launch set for 3:47 p.m. Meteorologists predicted a 70 percent chance of good enough weather at liftoff.
VIPs watching Endeavour include President Barack Obama and his family - only the third time a president has witnessed a space launch and the first time a first family has attended one - and so many members of Congress that it's practically a quorum.
But the unseen star is one member of Congress: Arizona Rep. Gabrielle Giffords, the wife of Endeavour's commander, Mark Kelly. Giffords was shot in the head three months ago in an assassination attempt in her hometown of Tucson. A 22-year-old suspect is in custody. Giffords' condition has improved enough that she was able to leave her Houston rehabilitation center to attend her husband's launch - the fourth time she's traveled to Kennedy Space Center to watch a shuttle flight.
The large crowds are expected to start hitting the roads soon after the fuel fills into Endeavour's tank. Officials expect between 500,000 and 750,000 people to crowd around the coastal communities. Delays of several hours are expected on the roads.
This is the last flight of Endeavour and the next to last flight for the 30-year-old space shuttle fleet, after more than 530 million miles of circling the Earth. NASA started the long retirement process for the shuttle fleet in 2004 as part of cost-cutting in order to spend money on new space missions and ships.
Endeavour's launch has one thing William and Kate don't have: a $2 billion international science project. Somewhat overlooked amid the attention on Giffords and Kelly and the president's visit is Endeavour's main mission: It will place a 15,000-pound particle physics detector on the International Space Station. The experiment, which will look for elusive antimatter and the origins of mysterious dark matter, could change man's understanding of the cosmos.
Explore further: Computer simulation suggests early Earth bombarded by asteroids and comets
More information: NASA site for Endeavour mission: http://www.nasa.gov/mission-pages/shuttle/shuttlemissions/sts134/index.html