"We've had a very clean countdown so far," NASA test director Jeremy Graeber told reporters Thursday.
The shuttle and its six astronauts will deliver a Russian compartment to the International Space Station. The chamber is filled with more than 3,000 pounds of U.S. supplies, including food and laptop computers. It will be the first -- and last -- time a shuttle carries a Russian module to the orbiting lab. Only two other shuttle flights remain.
Once the shuttles are retired, NASA will leave station deliveries to commercial companies and other countries, and focus on eventual trips to asteroids and Mars.
Dozens of Russians were on hand for the launch, as were about 150 Twittering guests. It's the second time NASA has staged a tweet-up at the launch site; the first was late last year.
The tweeters gathered with their laptops, cameras and other gadgets Thursday under a large tent at the press site, just three miles from the launch pad. They threw out questions to astronauts and shuttle managers not involved with Friday's liftoff, and even got autographs.
NASA expects bigger crowds than usual for the remaining flights.