Six Discovery crew members were all smiles July 17 after completing a mission that should lead to the resumption of more routine shuttle missions at a rate of about four a year until the fleet is retired in 2010.

“It’s good to be back,” said Discovery’s STS-121 mission commander Steven Lindsey. “We had a long, but successful mission.”

Lindsey deftly piloted the spacecraft — which is mostly a 100-ton glider when it lands — to a graceful stop despite a thick cloud layer and some last minute runway changes. “We didn’t see anything until about 10,000 feet,” said Lindsey, a four-time shuttle flyer. “I’d never gone through weather like that on a real shuttle landing but it’s not a big deal.”

Discovery’s STS-121 spaceflight carried the astronauts around their home planet 202 times, about 8.5 million kilometers . By the numbers, the mission ran 12 days, 18 hours, 37 minutes and 54 seconds, and cost an estimated $1 billion, the space agency said.

In addition to Lindsey, STS-121 pilot Mark Kelly and mission specialists Lisa Nowak, Stephanie Wilson, Piers Sellers and Michael Fossum rode Discovery back to Earth . A seventh crew member, European Space Agency astronaut Thomas Reiter, joined the ISS Expedition 13 crew and stayed aboard after Discovery’s departure.

Nowak, Wilson and Fossum made their spaceflight debuts with the mission.

“It does feel very good to not be a rookie anymore, to be able to have a spaceflight under my belt,” Wilson said.

Kelly, who se identical twin brother Scott Kelly is also a NASA astronaut and set to command STS-118 in August 2007, said he is looking forward to spending time with his family.

“We’re going to go into town tonight and go home tomorrow,” Kelly said, adding that he looks forward to seeing his children but had lost track of the days. “I’m looking forward to the weekend. … what day is it?”

Fossum settled one remaining mystery from Discovery’s flight surrounding some “surface deposits” — also known as bird droppings — which the STS-121 crew discovered during an in-depth inspection of their spacecraft’s wing edges and nose cap.

“They made it home,” Fossum said. “They were a bit charred.”

Fossum and Sellers performed three spacewalks during their busy spaceflight, yet confessed that the mission’s most gratifying moment was not the orbital work, but its completion.

“Most satisfying, I think, is you know when we all looked at each other, we all stopped frankly, and said, ‘You know, we’re done,’” Sellers said. “For me, you know, my most satisfying moment to see everyone else looking around at each other with smiles and say, ‘We’re done, we did it.’”

The STS-121 mission marked second of the agency’s two orbiter test flights since the 2003 Columbia accident that destroyed one orbiter and killed seven astronauts.

“I don’t think we want to ever put Columbia behind us,” Lindsey said, adding that the tragic loss led to vital improvements in safety and NASA culture. “We’ve learned the cultural organizational lessons of Columbia and that’s the one thing we don’t ever want to forget.”