The shuttle's trip to the space station should take two days. Once there, Discovery's crew will unload and install the $1 billion lab and hand-deliver a specially made pump for the outpost's finicky toilet.
The school-bus-size lab, named Kibo, Japanese for hope, will be the biggest room by far at the space station and bring the orbiting outpost to three-quarters of completion.
"It's a gorgeous day to launch," NASA's launch director, Mike Leinbach, told the astronauts just before liftoff, wishing them good luck and Godspeed. Commander Mark Kelly noted that Kibo was the "hope for the space station," then radioed: "Now stand by for the greatest show on Earth!"
Nearly 400 Japanese journalists, space program officials and other guests jammed NASA's launch site, their excitement growing as the hours, then minutes counted down.
Their enthusiasm was catchy.
"This is a real milestone," NASA Administrator Michael Griffin said.
The Japanese lab is 37 feet long and more than 32,000 pounds, and fills Discovery's entire payload bay. The first part of the lab flew up in March, and the third and final section will be launched next year.
The entire lab, with all its pieces, cost more than $2 billion.
A large political contingent was also on hand led by Rep. Gabrielle Giffords, D-Ariz., who's newly married to Kelly, Discovery's commander. They invited numerous bigwigs from Arizona and Washington.
Giffords acknowledged being nervous, far more so than the day she was elected to Congress in 2006. She gripped her mother-in-law with her right arm and held her own mother's hand in her left as she watched Discovery soar.
"It was pretty exciting, pretty exciting," Giffords told The Associated Press. Although it was a smooth launch - the only problem was the failure of a backup set of electronics for swiveling engines - she said she wouldn't relax until the shuttle is back from its two-week mission.
Debris - likely ice or foam insulation - could be seen falling from the fuel tank during liftoff, but it did not appear to occur during the crucial first two minutes. This was the first tank to have all safety changes prompted by the 2003 Columbia disaster built in from the start.
Kelly's brother, Scott, didn't need an invitation to the launch - he's also a space shuttle commander. They're identical twins.
Scott Kelly said it was more nerve-racking to watch his brother launch than to be strapped in himself. Their parents and 91-year-old grandmother are always anxious on launch day, he said.
"I know my grandmother, she would rather I work at Wal-Mart," Scott Kelly said, chuckling, before liftoff.
Everyone - observers and professionals alike - was relieved once Discovery safely reached orbit. Griffin noted that NASA has enjoyed "a number of good events" in recent days: The Phoenix Mars Lander survived its trip to the red planet last weekend and already has sent back pictures of what could well be ice.
"You make it look easy. I know it's not easy," Griffin told launch controllers.
Three spacewalks are planned during Discovery's 14-day flight, to install Kibo, replace an empty nitrogen-gas tank and try out various cleaning methods on a clogged solar-wing rotating joint. The shuttle crew is made up of six Americans and one Japanese.
The space station's two Russian residents, meanwhile, will put in the new toilet pump. For more than a week, the three occupants have had to manually flush the toilet with extra water several times a day, a time-consuming, water-wasting job.
NASA and Russian space officials are hoping that the pump - which was rushed to Kennedy Space Center from Moscow just this past week - gets the toilet back in normal working order.
One of Discovery's astronauts, Gregory Chamitoff, will move into the space station for a six-month stay. He'll replace Garrett Reisman, who will return to Earth aboard the shuttle.
Also hitching a ride to the space station is Buzz Lightyear, who has long been yearning to soar "to infinity and beyond." The 12-inch action figure - made famous in the 1995 Disney film "Toy Story" - is part of NASA's "toys in space" educational program for elementary students and their teachers.