This story was updated at 3:21 p.m. EDT.

CAPE CANAVERAL, Fla. - NASA celebrated the Fourth of July with some rockets of its own Tuesday as the space shuttle Discovery launched its seven-astronaut crew into orbit.

After two scrubbed attempts, Discovery shot spaceward at 2:37:55 p.m. EDT (1837:55 GMT) from Pad 39B here at Kennedy Space Center (KSC). The launch marked NASA's first-ever shuttle flight to leave Earth on Independence Day.

"Discovery's ready, the weather's beautiful, America is ready to return the space shuttle to flight.,'' NASA launch director Michael Leinbach told Discovery's astronaut crew just before liftoff. "So good luck and Godspeed, Discovery."

"I can't think of a better place to be here on the Fourth of July," said Steven Lindsey, commander of Discovery's STS-121 mission. "We're hoping to very soon get you an up close and personal look at the rocket's red glare."

Discovery's STS-121 shuttle mission marks NASA's second orbiter test flight since the 2003 Columbia accident. The spaceflight's seven-astronaut crew will deliver more than 2.5 tons of cargo to the International Space Station (ISS), make vital repairs to the orbital laboratory and test out a robotic arm extension's stability as a work platform.

Riding into orbit aboard Discovery with Lindsey were: shuttle pilot Mark Kelly, mission specialists Michael Fossum, Lisa Nowak, Stephanie Wilson, Piers Sellers and European Space Agency (ESA) astronaut Thomas Reiter.

The crew waved small American flags - Reiter waved the banner of his native country Germany - as they headed out to the launch pad.

"He was psyched," said NASA astronaut Scott Kelly, identical twin brother of STS-121 pilot Mark Kelly, adding that today's launch brings him closer to his own command - STS-118 to launch next year. "An hour ago I was fifth in line, now I'm fourth."

Scott Kelly watched his brother rocket spaceward with their parents at nearby Banana River, he said.

Poor weather thwarted two launch attempts in as many days beginning on July 1, though neither day had as good a weather outlook as today.

"You've got to go on the Fourth of July," NASA astronaut James Reilly told SPACE.com.

NASA has never before launched a space shuttle on July 4^th but did land the Columbia orbiter and its two-man crew during the STS-4 mission in 1982.  

"I have to say the very first time I experienced a launch I thought, 'Well this is pretty cool,'" Shana Dale, NASA's deputy administrator, told SPACE.com before today's launch. "But I never expected the sense of patriotism I felt when the space shuttle went off...it's just one of those proud moments that you have. You think, 'Wow, I'm proud to be an American and this is the United States space program.'"

Reilly said he was especially excited for Fossum, Nowak and Wilson, the first-time spaceflyers of STS-121. He plans to send them an e-mail later today to welcome them to space.

Lt. Kaleb Nordgren, of the 45^th Weather Squadron at Cape Canaveral Air Force Station, said Monday Discovery had a 60 percent chance of favorable conditions for today's planned launch, but that outlook improved to 80 percent positive early this morning. The previous attempts carried 40 percent or lower chances of good weather.

A thruster heater thermostat glitch, since resolved, occurred during Discovery's initial liftoff attempt, and launch controllers set aside a circuit breaker glitch that popped up today since a redundant system was working properly.

Shuttle officials also concluded Monday that a small piece of foam insulation which fell from the shuttle's external tank was not a major concern for today's space shot. The three-inch piece of foam, weighing less than one-tenth of an ounce, fell during scrub turnaround operations to ready Discovery for today's launch.

Mission managers were primarily concerned that ice could form inside the small divot left from the foam loss, but a launch day inspection team found no traces of ice build-up in the area once Discovery's 526,000-gallon tank was fueled with its super-cold liquid hydrogen and liquid oxygen propellant.

Return to flight redux

Discovery's launch today came almost one year after its last spaceflight - NASA's STS-114 return to flight mission - and will allow its STS-121 crew to make crucial fixes to the ISS, as well as test the stability of a 50-foot (15-meter) boom as a work platform for orbiter repair.

The astronauts will also return the station to its three-astronaut capacity and deliver more than two tons of food, equipment, new tools and other supplies for use aboard the ISS. NASA hopes the flight will clear the way to resume ISS construction.

"This flight's like the plug in the bottle," said Reilly, who will fly aboard NASA's STS-117 shuttle flight to install new solar wings aboard the ISS next year. "To get them off the planet, it starts the assembly sequence for the rest of us."

Among Discovery's cargo is a U.S.-built oxygen generator that will ultimately allow the ISS to support six-person crews, a freezer to store biological samples, and a set of centrifuges to test how plants grow in under different gravity conditions. The latter experiment, NASA officials said, could aid in crop plans for eventual long-duration stays on the Moon or Mars.

"We have a pretty exciting mission in front of us," William Gerstenmaier, NASA's associate administrator of space operations, has said. "We're looking forward to all that...the teams have done a great job." 

But it's been a long road to orbit for the STS-121 crew.

Discovery's launch was watched by myriads of onlookers and more than 100 NASA cameras and radar tools, which scanned the shuttle's ascent for signs of foam insulation debris falling from the orbiter's 15-story fuel tank.

Heat shield damage from such foam debris could seriously jeopardize the spacecraft's ability to return to Earth, as proven during the shuttle Columbia's STS-107 launch. A foam strike in that 2003 shuttle launch breached the orbiter's heat shield along its wing leading edge, critically wounding the spacecraft and leading to its destruction and the loss of seven astronauts during atmospheric reentry.

NASA redesigned portions of shuttle fuel tanks to reduce the risk of catastrophic foam shedding events, but were surprised when a large piece of insulation was seen falling from a wind deflecting ramp during Discovery's STS-114 mission. That ramp has since been removed, a change that will be scrutinized along with a series of ice frost ramps in the imagery received by NASA observers.

"I'm very pleased they've taken what seems to be the right amount of time and the appropriate amount of effort to get the answers that they need," said NASA astronaut Stephen Robinson, who conducted three spacewalks during the STS-114

NASA shuttle engineers and managers will evaluate imagery from ground-based cameras, as well as from orbital inspections by the STS-121 and ISS crews, to determine whether Discovery is safe to return to Earth with its astronaut crew. That decision is expected around Flight Day 6, or July 9, NASA has said.

"There are many, many issues, the foam is just one of them - there inspections issues and so forth - that we're eager to find out the data," said Pamela Melroy, commander of NASA's STS-120 mission to deliver a connecting node that will allow the expansion of the ISS, in an interview.  

Busy day ahead

The work day doesn't stop after launch for the STS-121 astronaut crew.

"These guys are going to be working from the moment the engines shut down to wheel stop," said NASA astronaut Andy Thomas, who served as a mission specialist on the STS-114 return to flight mission and helped brief the STS-121. "They're just going to be just go, go, go."

Just after reaching orbit, the astronauts are expected to unstrap themselves from their seats and conduct a still and video photographic survey of their discarded external tank. The images will then be sent down to flight controllers for analysis to determine how the PAL ramp removal performed and pinpoint any other trouble areas that may have shed foam insulation during launch.

Discovery's payload bay doors have to be opened - an activity currently slated for 4:03 p.m. EDT (2003 GMT) - in order to deploy the spacecraft's radiators and allow the shuttle to dock at the ISS on Monday. The crew won't get to sleep today until about 8:38 p.m. EDT (0038 July 5 GMT) in order to be fresh for the first of several crucial inspections of Discovery's heat shield using the orbiter's sensor-laden boom, NASA has said.

"The space shuttle is an amazing, awe-inspiring expression of human capability," Thomas said. "It's breathtaking."

Discovery's Sunday STS-121 launch marked the 32^nd flight for the orbiter and the 115^th shuttle launch for NASA. The space shuttle is expected to return to Earth on July 16.

The Associated Press contributed to this report.

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