The countdown for shuttle Endeavour's launch, attended by U.S. President Barack Obama and U.S. Rep. Gabrielle Giffords, was hitch free early Friday, NASA said.
If all goes according to plan, Giffords' husband, Capt. Mark Kelly, the flight's commander, and five other astronauts are to lift off from Cape Canaveral, Fla., at 3:47 p.m. EDT Friday for a 14-day mission to the International Space Station.
Giffords, who spent the past three months in a Houston rehabilitation hospital recovering from a gunshot wound to the head in a Jan. 8 assassination attempt in Tucson, plans to watch the liftoff from a private viewing area, officials said.
Obama, first lady Michelle Obama and their two daughters intend to view the launch with NASA Director Charles Bolden Jr. and astronaut Janet Lynn Kavandi, a veteran of three shuttle missions and one of NASA's most senior astronauts.
They plan to view the liftoff from NASA's Launch Control Center, used for the supervision of all U.S. manned space flights.
The last sitting president to attend a shuttle launch was Bill Clinton, who watched astronaut and U.S. Sen. John Glenn, D-Ohio, lift off in Discovery Oct. 29, 1998 -- at age 78 the oldest person to fly in space and the only one to fly in both the Mercury and shuttle programs.
Project Mercury was the first U.S. human spaceflight program, from 1959 to 1963. Glenn was the first American to orbit Earth Feb. 20, 1962.
The only possible Endeavour hang-up might be the weather, chief NASA weather officer Kathy Winters said. The storm system that killed some 300 people in six Southern states, including more than 200 in Alabama, brought thunderstorms to Cape Canaveral Thursday night and early Friday morning.
The sun was forecast to return by late morning, but if low clouds were still present at launch time -- or if winds kicked up -- the liftoff would be delayed at least 24 hours, NASA said.
Jeffrey G. Spaulding, shuttle test director at the Kennedy Space Center, said NASA had at least three opportunities for liftoff next week if the launch is delayed.
The launch will begin Endeavour's final space outing before it heads for retirement at the California Science Center in Los Angeles' Exposition Park. It will also be the second-to-last shuttle launch ever, ahead of Atlantis' scheduled June 28 launch, closing out the 30-year shuttle program.
Endeavour is to carry a $2 billion Alpha Magnetic Spectrometer, one of the biggest physics experiments ever launched, into orbit. The spectrometer is designed to search for clues on what the universe is made of and how it began, the origin of so-called dark matter, anti-matter and questions of extragalactic astronomy, NASA said.
The astronauts are scheduled to conduct four spacewalks to perform space-station maintenance. The mission could be extended two days to perform more upkeep, NASA said.
The Endeavour mission will be the 134th in the program and the 25th for the spacecraft and the 36th shuttle mission to the space station.
The Endeavour was built to replace the Challenger, which broke apart 73 seconds after liftoff Jan. 28, 1986, killing all seven astronauts aboard.