EDWARDS AIR FORCE BASE, Calif. – Late. Safe. But facing an uncertain future.

Signaling its arrival with two thunderous sonic booms, the space shuttle Discovery hurtled out of a black desert sky to a smooth touchdown yesterday after scrapping four landing attempts at its Florida base because of rain and lightning. The landing at a backup site was met with cheers and palpable relief after a tense two-week mission that raised fears of a Columbia-like disaster.

“Congratulations on a truly spectacular test flight,” Mission Control said after Discovery came to a stop on the concrete runway at 5:11 a.m. PDT. “Welcome home, friends.”

The mission exposed how vulnerable the shuttle fleet remains, despite a tremendous amount of money and effort invested in America’s first manned space mission in the two and a half years since the Columbia tragedy.

Shortly after liftoff July 26, a 1-pound chunk of foam insulation fell from the fuel tank – the very thing that doomed Columbia – but it missed Discovery. Still, NASA grounded all shuttle flights until engineers fix the problem.

“We’re going to try as hard as we can to get back in space this year,” NASA Administrator Michael Griffin said at a post-landing news conference. “But we’re not going to go until we’re ready to go.”

Shuttle managers freely acknowledged the foam mistake, while stressing that the inspection, photography, and other shuttle data-gathering systems put in place for this flight worked well. What’s more, NASA officials said no severe damage was detected on Discovery while it was in orbit.

“I hope this shows people that we’re coming back,” NASA spaceflight chief William Readdy said from Cape Canaveral, Fla. “We’ve got some more work to do. We know what we need to do and we’ll do it.”

Unlike previous Edwards landings, in which throngs of spectators gathered for a shuttle return, the public was barred from viewing Discovery due to increased security after the September 11, 2001, terrorist attacks.

The 17,000-mph plunge from orbit took Discovery through a passage of the same kind of intense heating that exposed the mortal wound in Columbia’s wing. The shuttle soared across the Pacific and over Southern California, passing just north of Los Angeles on its way to Edwards as it completed a journey that spanned 5.8 million miles and 219 orbits of Earth.

NASA adjusted the flight path in order to skirt Los Angeles because of new safety considerations in the wake of the Columbia disaster, which rained debris onto Texas and Louisiana.

During Discovery’s approach, Dr. Jon Clark, a NASA neurologist and husband of Columbia astronaut Laurel Clark, said he quietly remembered his wife and closely compared the two missions.

“I thought, ‘This is when the tire light went on,'” Dr. Clark said from Kennedy Space Center, referring to an initial sensor reading that Columbia was breaking up. “I was paralleling the two missions.”

Two hours after touchdown, the astronauts walked around the shuttle to inspect for possible damages.

“It looks fantastic,” Commander Eileen Collins said. Ms. Collins said America should continue launching shuttles until the scheduled completion of the international space station in 2010 – a sentiment echoed by NASA Administrator Michael Griffin.

“Some people say we should stop flying the shuttle because we had an accident – frankly we’ve had two accidents – but we are people who believe in this mission and we are going to continue it,” Ms. Collins said.

President Bush congratulated the Discovery team. “It was a great achievement,” Mr. Bush said. “It was an important step for NASA as it regains the confidence of the American people and begins to transition to the new mission we’ve set out for NASA.”

NASA said it will be a week before Discovery leaves California, riding piggyback atop a modified Boeing 747 back to Cape Canaveral. The cross-country trip is expected to cost the space agency about $1 million.

Discovery’s crew accomplished its main objectives to resupply the international space station and fix broken equipment. The first shuttle to visit the space station since 2002, Discovery spent nine days docked to the orbiting lab.

The switch to landing in California was a big disappointment for the astronauts’ families, who had been waiting two weeks to greet their loved ones in Florida. Astronauts planned to reunite with their families today, when they all plan to meet in Houston.

In Rochester, N.Y., Ms. Collins’s 79-year-old father, James, a retired postal worker, described it as “the best day of my life.”