NASA Administrator Michael D. Griffin said yesterday the space shuttle is ready for a July 13 launch, capping nearly 21/2 years of safety modifications undertaken after the Columbia disaster grounded the three remaining orbiters.

The space shuttle Discovery is on launch pad 39B at the Kennedy Space Center, undergoing final preparations and loading. Barring last-minute glitches or a midsummer Florida rainstorm, the orbiter should lift off for its 13-day mission at 3:51 p.m.

Griffin, speaking at a televised conference from the space center, said the shuttle team has completed a "a very thorough and very successful Flight Readiness Review," the rigorous, two-day checkout preceding every shuttle flight. "We're currently go for launch on July 13," he said.

Bill Parsons, space shuttle program manager, said engineers still had several reports and equipment tests to complete, "but it looks like we'll be able to close it out with no problems." Finishing the readiness review was a "big step," he added. "I had a lump in my throat."

The space shuttle fleet has not flown since Feb. 1, 2003, when Columbia disintegrated over Texas during reentry because a chunk of foam insulation from the external tank had punched a hole in the shuttle's heat shielding during launch.

Since then, engineers have redesigned the external tank, built a new network of ground-based telescopes and other devices for detecting damage to the heat shielding, and undergone a series of management changes to make flight coordinators more responsive to safety concerns.

"Space flight is risky, and as the generations unroll we will learn how to make it routine," Griffin said yesterday. "We have a ways to go." Still, he added, "it is my assessment that the proximate causes of the loss of Columbia have been addressed. . . . We honestly believe that this is the cleanest flight we have ever done."

Delays in the safety modifications prompted three postponements of Discovery's launch, but during a series of public appearances over the past week by Griffin, other NASA officials and members of the independent Return to Flight Task Group, it became increasingly clear that the wait was over.

Yesterday's confirmation was subdued, almost anticlimactic. Asked whether the Discovery team has planned a celebration for having reached the finish line, William Readdy, the associate administrator for space operations, demurred. "The celebration isn't about weighing anchor," he told reporters. "The [celebration] is about when Discovery's at anchor after a successful mission."

Nonetheless, Sen. Kay Bailey Hutchison (R-Tex.), chairman of the Senate Commerce subcommittee on science and space, issued a statement congratulating NASA for its "hard work and dedication" in preparing for launch -- "an important milestone in space exploration for the United States."

Mike Leinbach, the launch director, said the preflight routine is about 31/2 days ahead of schedule, so engineers will be able to spend Sunday and Independence Day with their families.

But as he spoke, rain pelted down outside, drenching the space center in a midafternoon deluge, a frequent feature of Florida summers and a potential showstopper for a shuttle launch that requires a clear day.

"The rain will let up; it always does," Leinbach said. "We hope it does on July 13." But "let's face it," he added. "Launching in the middle of the afternoon will be a real challenge."

NASA Administrator Michael D. Griffin, center, appears at a briefing with Dean Acosta, left, and William Readdy of NASA.