Today's mission: Launch; and tomorrow's . . . ?

NASA has 2.5 million parts and seven human beings to worry about today for the scheduled launch at Cape Canaveral of Discovery that will put the shuttle back in space and help move the nation beyond the Columbia tragedy.

At 21, Discovery is the oldest ship in the fleet and the last in a generation of exploration vehicles. The shuttles, built to last 10 years or 100 missions, will be retired in 2010, long past what the engineers who designed them expected. Were it not for the International Space Station, it would be difficult to justify keeping the three remaining orbiters flying. But the United States has made commitments to 15 other nations to complete work on the $100 billion station, and only the shuttle can transport the cargo to do it. NASA believes that it will take at least 18 shuttle missions to finish the job, which means a busy schedule at the Kennedy Space Center the next five years, beginning with the liftoff this afternoon or later in the week if thunderstorms pose a problem.

And after that? The nation faces decisions about how best to carry on exploration. A week ago, NASA scored a major victory when its Deep Impact probe exploded into the Tempel 1 comet, a perfect bull's-eye collision after a 268 million-mile, 173-day journey. In the 2 1/2 years since the shuttle has been grounded, two Mars Explorer robots have sent back volumes of information about the planet. The success of unmanned missions restarts the debate over whether human beings are essential to exploration or just essential to the public relations needed to sell Americans on supporting extremely expensive and risky ventures. There is an emotional attachment to manned missions that robots can't duplicate. But there is also a deadly price to pay when things go wrong.

NASA has spent more than $1 billion to improve the shuttle's equipment and procedures. A redesigned fuel tank and dozens of new sensors and cameras make another wing catastrophe less likely. Engineers have looked, with mixed success, for ways to allow astronauts to make repairs. One of the sobering lessons from Columbia was that even if the crew had detected the fatal damage that occurred during launch, they could not have fixed it. The group appointed to study the Feb. 1, 2003, breakup of Columbia as it returned to Earth made 15 recommendations for fixing the agency's "broken safety culture"; NASA has satisfied 12. The other three are strengthening the wing panels, eliminating potential debris and developing proven repair techniques. Astronauts still have limited capability to determine their fate.

The space shuttle is the most complicated and expensive dinosaur the world has seen. The next generation, the Crew Exploration Vehicle, is scheduled to be ready by 2014 and take humans back to the moon -- perhaps even onward to Mars. NASA builds its future one mission at a time, however, and nothing matters more to the exploration of space than Discovery's safe departure and return.