Although safe, Discovery's hourlong descent from orbit was marked by some unusual events, reminders that spaceflight always involves a multitude of risks big and small:

- One of three power units needed to power the hydraulic landing systems was leaking, but operated properly. NASA still does not know whether harmless nitrogen gas or flammable hydrazine was escaping.

- A probe for monitoring air flow around the re-entering shuttle was sticky and took longer than usual to be released.

- Storm clouds popped up at the last minute, and Discovery had to switch direction for landing on the Kennedy Space Center runway.

- The sound of gunshots and blaring car horns heralded Discovery's return; NASA made the racket to keep birds out of the shuttle's path. As it turns out, the shuttle was decorated with bird droppings before liftoff, and the residue made it all the back from orbit, albeit a bit charred.

"It was a great mission, a really great mission," Lindsey declared at touchdown, at 9:14 a.m. (1314 GMT).

The journey spanned 13 days and 5.3 million miles (8.53 million kilometers). The mission saw the astronauts repeatedly inspect the entire thermal skin of their ship and conduct three spacewalks to do patching tests, try out a 100-foot (30-meter) boom as a possible work platform, and fix a rail car outside the station. They also dropped off several thousand pounds (kilograms) worth of station supplies and a new station resident: European Space Agency astronaut Thomas Reiter of Germany, who expanded the station crew to three for the first time since 2003.

Griffin noted that NASA faces 16 more shuttle flights to complete the space station and, hopefully, repair the popular Hubble Space Telescope. A decision is expected by fall on whether to send a shuttle to Hubble one final time, to extend the observatory's life.

NASA is up against a hard 2010 deadline for completing the space station. That's when the three remaining shuttles will be retired to make way for a new spaceship capable of carrying astronauts to the moon and eventually on to Mars.

"We don't have any slack. We have just enough shuttle flights left to do the job so we can't afford to mess up," Griffin said. "The team performed superbly systemwide (with Discovery) and what we have to do is exactly that same thing again from now until the end of 2010."

Complicating matters is that NASA faces a series of tremendously difficult assembly flights, beginning with Atlantis' upcoming mission to deliver and install a massive beam and set of solar wings. NASA is aiming for a liftoff as early as August 27.

Shuttle program manager Wayne Hale said he wants just as much intense scrutiny _ and differing opinions among engineers - for the next flight and all the ones after that.

A few weeks before Discovery's liftoff, NASA's chief engineer and top safety officer argued for putting off the mission until more design changes could be made to one area of the fuel tank. Hale was all for going ahead. Griffin cast the final, decisive vote.

After landing, Lindsey said he trusted that launch decision and noted it was an outgrowth of the management culture lessons learned from Columbia. The successful conclusion of the mission is more of a beginning to space station assembly and exploration, he said, than an end to the post-Columbia recovery era.

"I don't think we want to ever put Columbia behind us," Lindsey said.

AP science writer Seth Borenstein in Houston and AP writer Mike Schneider in Cape Canaveral contributed to this report.