Three days after space shuttle Columbia disintegrated in the skies over Texas, as many as 15,000 mourners stared into the heavens Tuesday in solemn tribute to the seven astronauts who died.

President Bush led the country in mourning the heroes who died aboard the ill-fated shuttle.

"Our nation shares in your sorrow and your pride," Bush told a gathering of thousands at a memorial service at NASA's Johnson Space Center. The astronauts' grieving families, friends, colleagues and political leaders were among those in attendance.

Earlier, Bush passed his handkerchief to fallen astronaut Rick Husband's son, who was crying. The child wiped his nose and gave the handkerchief back to Bush.

The other astronauts being mourned were: William McCool, Michael Anderson, Dr. Kalpana Chawla, David Brown, Dr. Laurel Clark and Israeli Col. Ilan Ramon.

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, just 16 minutes before it was expected to land at Cape Canaveral, Fla.

"Each of these astronauts had the daring and the discipline required of their calling," Bush said. "Each of them knew great endeavors are inseparable with great risk, and each of them accepted those risks willingly, even joyfully, in the cause of discovery."

"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.

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 to a series of engineering buildings and the headquarters here.

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

The memorial service opened with an invocation by a Navy rabbi and the singing of the hymn, "God of Our Fathers."

Sean O'Keefe, NASA's administrator, said the bond between those who go into space and those on the ground "is incredibly strong. Today, our grief is overwhelming."

"We also have a tremendous duty to honor the legacy of these fallen heroes by finding out what caused the loss of the Columbia and its crew, to correct the problems we find and to make sure that this never happens again," O'Keefe said.

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 gathering in a plaza known at the Mall here at between 10,000 and 15,000. Mourners spilled beyond the square and crowded around a pond. They stood among the trees and on the lawns -- waiting 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 memorial service had a personal dimension for Pritchard, who said she attended Texas Tech with shuttle Cmdr. Husband.

"He was just the greatest guy -- always smiling, always approachable," she said.

Laura Lucier, an employee of the Canadian space agency who is based at Johnson Space Center, said that passion for space exploration drew workers here and gave the memorial service deeper meaning.

"There's nobody who works at NASA who isn't passionate about it," Lucier said. "When workers are lost, it means a lot more. You work here because you love it, not because it's a paycheck."

The impact of the Columbia's loss was felt well outside the space center's gates. Flags flew at half-staff throughout the region. The sign at a fast-food restaurant just outside Johnson's gates read, "Our prayers to our NASA family."

The White House drew inspiration from President Reagan, who delivered one of the most eloquent speeches of his presidency after the explosion of the space shuttle Challenger.

"Sometimes, when we reach for the stars, we fall short," Reagan said on Jan. 31, 1986, to a crowd of 10,000 at Johnson. "But we must pick ourselves up again and press on despite the pain."

The Associated Press contributed to this report.