SPACE CENTER, Houston (AP) -- Led by President Bush, thousands of grieving space workers and their families, friends, neighbors and political leaders paid a solemn farewell Tuesday to the seven Columbia astronauts. ''Their mission was almost complete and we lost them so close to home,'' Bush said.

Bush bowed his head in mourning and first lady Laura Bush wiped tears as the men and women who perished in the space shuttle disaster were memorialized at the home of Mission Control. The shuttle broke up Saturday as it was returning to Earth.

''America's space program will go on,'' Bush declared in the outdoor ceremony, held beneath a clear blue sky and a few wisps of white clouds.

''Each of these astronauts had the daring and the discipline required of their calling. Each of them knew great endeavors are inseparable from great risk, and each of them accepted those risks willingly, even joyfully, in the cause of discovery,'' Bush said, speaking in a calm, steady voice. ''For these seven, it was a dream fulfilled.''

Thousands of people bunched together on a mass of green lawn stretching more than 200 yards from the white, square-shaped building that houses Mission Control.

''All mankind is in their debt,'' Bush said of the fallen astronauts as members of his audience sniffed and wiped tears from their eyes.

Bush was meeting with family members after the service, which ended with the ringing of a Navy bell -- seven times, one for each of the deceased astronauts -- and a ''missing man'' formation flyover, in which four jets roared above the crowd, with one peeling away and soaring high and out of sight in the sky.

Held at NASA's Johnson Space Center, the service opened with in invocation by a Navy rabbi and the singing of the hymns, ''God of Our Fathers'' and ''Eternal Father.''

Sean O'Keefe, NASA's administrator, his voice at times breaking with emotion, said the bond between those who go into space and those on the ground ''is incredibly strong. Today, our grief is overwhelming.'' O'Keefe said the government would honor the legacy of the fallen astronauts ''by finding out what caused the loss….to correct the problems we find and to make sure that this never happens again.''

Bush gave a brief rundown on each member of the Columbia crew.

4Air Force Col. Rick D. Husband, shuttle commander, 45, was ''a boy of four when he first thought of being an astronaut. As a man and astronaut, he found it was even more important to love his family and serve his Lord.''

4Columbia pilot William C. McCool, 41, known by friends ''as the most steady and dependable of men.''

4Ilan Ramon, 48, the much-decorated Israeli air force colonel and hero, ''was a patriot, the devoted son of a Holocaust survivor, served his country in two wars.''

4Navy flight surgeon David Brown, 46, thought of astronauts as a boy as ''movie stars'' and ''grew up to be a physician, an aviator who could land on the deck of a carrier in the night and a shuttle astronaut.''

4Navy flight surgeon Laurel Clark, 41, ''a physician and a flight surgeon who loved adventure, loved her work, loved her husband and her son. A friend who heard Laurel speaking to mission control said, 'There was a smile in her voice'.''

4Payload commander Michael P. Anderson, 43, was ''a role model, especially for his two daughters and for the many children he spoke to in schools'' who once told his minister, ''If this thing doesn't come out right, don't worry about me, I'm just going on higher.''

4Mission specialist Kalpana Chawla, 41, ''She left India as a student, but she would see the nation of her birth, all of it, from hundreds of miles above.''

The president and first lady were accompanied on Air Force One here by Neil Armstrong, the first astronaut to walk on the moon. Former senator and astronaut John Glenn and his wife, Annie, also were on the board along with O'Keefe and a delegation of congressional figures.

''It's too bad we couldn't have pushed this day back forever,'' lamented Glenn, the first U.S. astronaut to orbit the Earth.

NASA estimated the crowd at between 10,000 and 15,000. Mourners spilled beyond the square and crowded around a pond to hear the presidential eulogy.

''He's the leader of our country, and his being here wasn't necessary, but it does show we are mourning,'' said Rochelle Pritchard, a NASA contract worker who helps manufacture robotic flight control gear.

The service had a personal dimension for Pritchard, who said she attended Texas Tech with shuttle Cmdr. Rick Husband, who was among those who perished Saturday. ''He was just the greatest guy -- always smiling, always approachable,'' she said.