By Andy Gallacher
BBC News, Kennedy Space Center
The most dangerous part of any shuttle mission is the descent home to Earth.
Steve Lindsey: A great mission
It was this phase of Columbia's journey that ended in tragedy three years ago.
As the six crew members began to enter the atmosphere, Columbia disintegrated, leaving many doubting the future of the entire orbiter fleet.
We now know that a piece of insulating foam that fell off during Columbia's launch was to blame.
The briefcase-sized chunk of material damaged the ship's wing, allowing superheated gasses to penetrate that vital heat shield.
So there was a collective sigh of relief at the Kennedy Space Center when two sonic booms rang out across Florida.
That meant that Discovery had successfully completed the trickiest part of the re-entry process.
But even with that hurdle overcome, the shuttle's journey back to Florida was an incredible one.
Like all orbiters, the Discovery basically plunged toward the ground, acting more like a glider than an advanced spacecraft.
Banking sharply and performing something akin to an aeronautical show, the craft slowly but surely reduced its speed from a velocity that topped Mach 30.
DISCOVERY SHUTTLE FLIGHT
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Landing: 0914 EDT (1314 GMT)
Landing location: Kennedy Space Centre, Florida
Returning crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers
About half an hour before touchdown, Commander Steve Lindsey took the controls, performing a textbook landing.
As it approached the runway, the shuttle's nose was pointed down at an angle of 19 degrees.
Just 2,000ft (610m) from the ground, Cmdr Lindsey steadily pulled the nose up; and with the help of a parachute, the Discovery finally came to a gentle rest at the Kennedy Space Center.
"Welcome back Discovery, and congratulations on a great mission," mission control told him.
"It was a great mission. A really great mission," Cmdr Lindsey replied.
The collective sense of achievement and relief that could be felt across the space centre was a big one.
For Nasa and the entire shuttle programme this has been a vital mission.
Ever since the Columbia disaster, and those persistent problems with insulating foam, many have called into question the safety and reliability of the 25-year-old orbiters.
Nasa feels it can now move forward with the space shuttle programme
But officials at Nasa now hope that those doubts have been silenced.
During its 13-day mission, Discovery was photographed from every conceivable angle.
It became the most inspected shuttle in Nasa history, with no-one prepared to take any chances.
And as well as taking shots of the craft's underbelly, the astronauts proved that if necessary they can carry out repairs in orbit.
Work before retiring
That will be vital to the fleet's remaining shelf life.
The US space agency expects to retire the shuttle in 2010, but it has a busy schedule running up to that time.
Nasa plans another 16 missions, mostly aimed at completing the International Space Station; and Atlantis is due to blast off at the end of August.
But for the six crew members of Discovery, it is now time to rest and spend with their families.
As they got off the orbiter, the four men and two women could be seen walking around the craft, pointing and smiling at the shuttle that was home for two weeks.
Between shaking hands with Nasa officials, British-born astronaut Piers Sellers could be seen mouthing something to his colleagues about the workload they had undertaken.
It did not look like something that could be printed, but it certainly summed up what this crew has been through.