For the first time in 21/2 years, astronauts are going to fly into space aboard the space shuttle. It's supposed to happen tomorrow at 3:51 p.m. unless there's a last-minute glitch.

The shuttle, called Discovery, is a monster. It looks like a passenger jet, but with all the fuel it needs to overcome Earth's gravity and get into orbit it weighs 41/2 million pounds as it stands on the launch pad at Kennedy Space Center in Florida. Discovery is scheduled to spend almost two weeks in space, most of that time unloading cargo at the International Space Station.

Flying into space aboard a shuttle is like riding a gigantic firecracker. The shuttle's three main rocket engines guzzle fuel so fast that, if fuel were water, the engines would empty a family-sized swimming pool in 25 seconds. Launch speed can reach 18 times the speed of sound.

Eileen Collins, mission commander for tomorrow's flight, says a shuttle launch "pushes you down in your seat and makes your helmet feel very heavy."

The flight is the first since the shuttle Columbia disintegrated over Texas on Feb. 1, 2003, killing all seven astronauts aboard. Columbia had a hole in the heat shield on its left wing that nobody knew about, and hot gases got inside the spacecraft and destroyed it as it was re-entering Earth's atmosphere.

The hole was caused by a piece of insulating foam that broke off from the shuttle's external fuel tank during liftoff. The fuel tank is the orange, torpedo-shaped thing in front of the orbiter. The tank holds 1.6 million pounds of liquid oxygen and hydrogen.

Engineers have spent 21/2 years strengthening the shuttle and fixing the tank so it won't shed as much foam or ice, but still it's not perfect. Flying into space is always risky; "to characterize it otherwise would be inappropriate," said deputy space shuttle manager Wayne Hale.

How risky?

Well, Discovery may look terrific when it's on the launch pad, but engineers know that it is a very dangerous experimental machine -- a 2-million-piece jigsaw puzzle capable of sudden, horrible surprises. Shuttles have flown 113 times since 1981 and two of the orbiters, Columbia and Challenger (in 1986), have been lost. That's not a good average.

To find out if bad things are happening this time, flight controllers will film, photograph and scan Discovery with 107 cameras during its launch. And when the orbiter meets up with the space station two days after launch, Collins will have Discovery do a backflip so the astronauts on the station can photograph its belly, looking for damage to the heat shield.

Discovery is carrying five repair kits and will test two of them during spacewalks. One is a caulking gun filled with goo that is spread on cracks with a spatula. The other is a liquid that can be spread on dings with a swab -- like shoe polish.

Collins in 1999 became the first woman to command a shuttle mission. She understands the risks of space travel, but says she has wanted to do it since watching "Star Trek" as a kid.

"We are doing new, never-before-done, high-risk work," Collins said. "But I always tell people that if you're going to fly in space you have to learn to enjoy the experience. Look out the windows, float around upside down, do all the things you always thought of doing."

-- Guy Gugliotta

From the International Space Station, an enlarged view of Discovery rolling toward the launch pad. The shuttle is scheduled to reach the space station this week.Above, Space Shuttle Discovery lifts off in 1999.