CAPE CANAVERAL, Fla. - It's been three long years since the loss of Columbia, but NASA's space shuttle fleet is back in action after six astronauts rode their 100-ton Discovery orbiter home safely Monday.

Discovery and its STS-121 astronaut crew touched down here at NASA's Kennedy Space Center (KSC) at 9:14 a.m. EDT (1314 GMT), ending a successful 13-day mission and the last of two test flights following the 2003 Columbia tragedy. The orbiter touched down after 202 trips around Earth.

"It was a fun entry, it was beautiful," Discovery's STS-121 commander Steven Lindsey told flight controllers after landing.

Lindsey said the superheated plasma encountered by the shuttle during reentry gave his crew quite a show.

"We could see the bright orange glow above and I could see the Earth moving below," Lindsey said.  "It was spectacular. We could actually see the Moon through the plasma."

Discovery rocketed into space on July 4, a first-ever Independence Day shuttle launch for NASA, almost one year after the agency's first post-Columbia mission returned its aging orbiter fleet to flight. Lindsey and his crew spent eight days resupplying the International Space Station (ISS), making crucial repairs to the orbital laboratory and testing new heat shield inspection and repair techniques.

"We feel like it's gone well," Lindsey said Sunday. "Everything we've hoped would happen has happened."

Returning to Earth with Lindsey were shuttle pilot Mark Kelly and mission specialists Lisa Nowak, Stephanie Wilson, Piers Sellers and Michael Fossum. The astronauts left a seventh STS-121 crewmember - European Space Agency (ESA) astronaut Thomas Reiter - aboard the ISS, where he joined the station's Expedition 13 crew.

NASA has estimated its Columbia investigation and return to flight efforts have cost about $2.3 billion through 2006.

"I think we're back to space station assembly, to shuttle flights, but we're still going to watch and we're still going to pay attention," Lindsey said earlier. "We're never ever going to let our guard down."

Spotless mission

By all accounts from NASA mission managers, engineers and flight controllers, Discovery's STS-121 mission has been the cleanest ever seen in the agency's 25-year history of shuttle flight.

"We've inspected [the heat shield] more on STS-121 than on any other mission," said Steve Stitch, NASA's reentry flight director, adding that analysts found no sign of troubling damage to the orbiter's heat-resistant tiles and carbon composite panels.

That clean bill of health comes after an intense engineering job to pull as much unneeded foam insulation as possible from the wrapping of Discovery's external tank, including a large ramp similar to one that shed a one-pound chunk of foam during the shuttle's STS-114 launch in July 2005.

A slightly larger piece of foam about the size of a briefcase doomed Columbia and its crew when it fell from the orbiter's external tank and gouged a hole in the shuttle's left wing leading edge during a Jan. 16, 2003 launch. The damaged heat shield allowed searing hot atmospheric gases into Columbia's wing, leading to its destruction.

But after a major overhaul of shuttle external tanks - NASA shuttle chief Wayne Hale has called the new rampless design the largest aerodynamic change since the first shuttle launch in 1981 - the space agency can claim an undeniable success.

The largest area of foam shed during launch was about the same as that of a legal-size sheet of paper, weighed less than one ounce and fell off in six separate pieces, NASA has said.

New cameras mounted to the orbiter's solid rocket boosters gave unprecedented - and stunning - views of Discovery's wing edges and tile-lined belly during launch to track any foam debris loss, and an effort to keep pesky vultures away from the shuttle's launch site after one struck the orbiter's STS-114 external tank last year appears to have been successful.

"I hope our legacy was that we closed out our goals of the post-Columbia flights," Lindsey told reporters Sunday.

Discovery's STS-121 mission ferried about 7,400 pounds (3,356 kilograms) of cargo to the ISS. Sellers and Fossum, the spacewalkers of STS-121, stepped outside their spacecraft three times to restore the station's railcar-like Mobile Transporter to full operations, test a 100-foot (30-meter) robotic appendage as a heat shield repair platform and demonstrate wing leading edge crack repairs.

"There was a lot to get done and we worked hard to make it all happen," Fossum said.

Among the critical deliveries was German astronaut Thomas Reiter, who returned the ISS to its full three-person capacity for the first time since the Columbia accident.

Meanwhile, the entire crew worked together for meticulously choreographed inspections of Discovery's entire heat shield using a sensor-laden boom attached to the end of the orbiter's robotic arm.

"To me, the most amazing thing was watching the footage from launch, looking at the early inspection," said NASA astronaut Mark Polansky, commander of Discovery's next flight - STS-116 to launch Dec. 14 - in an interview.

An ISS gate opens

Discovery's successful mission is the starting pistol for a marathon of ISS-bound shuttle flights to first build up the station's power and support systems, and then install new modules and laboratories that have been waiting for NASA's shuttle fleet to resume construction.

Up next is NASA's STS-115 shuttle mission, slated to launch between Aug. 28 and Sept. 5, to deliver a 17.5-ton solar array to the ISS and truss segment. A third shuttle orbiter flight slated for 2006, Polansky's STS-116 command, will install the new solar array along with another truss segment.

"Every mission is the critical one," Polansky said. "Every mission depends upon the successful completion of the previous one...I find it exciting."

With Discovery safely on Earth, NASA plans to host a post-landing news conference at about 11:00 a.m. EDT (1500 GMT), followed by comments from entry flight director Steve Stitch at 12:00 p.m. EDT (1600 GMT).

Shuttle commander Steven Lindsey and possibly other members of the STS-121 crew are tentatively scheduled to comment on their flight on NASA TV no earlier than 3:00 p.m. EDT (1900 GMT).

You are invited to follow the progress of Discovery's STS-121 crew via SPACE.com's NASA TV feed, which is available by click here.

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