Vice President Dick Cheney mourned the Columbia astronauts on Thursday as seven lost explorers of different faiths and backgrounds who were "bound together in the great cause of discovery."

"They were soldiers and scientists and doctors and pilots, but above all they were explorers," Cheney told a memorial service at Washington National Cathedral, where one of the stained glass windows holds a piece of moon rock.

The astronauts were "united not by faith or heritage, but by the calling they answered and shared," he said. "They were bound together in the great cause of discovery. They were envoys to the unknown. They advanced human understanding by showing human courage."

The cathedral was filled with mourners wearing space shuttle lapel pins, including many members of the astronauts' families. There were tears as singer Patti LaBelle performed "Way Up There," a song about space travel. An Israeli flag was displayed alongside the American flag, in honor of Ilan Ramon, the Israeli astronaut aboard.

NASA Administrator Sean O'Keefe said the Columbia crew "represented the best of the human spirit."

"They did their chosen calling proud, and they had a special grace," he said, embodying the tradition of the select few who become astronauts. "Our explorers go forward into the unknown with hope and faith."

NASA received an outpouring of support from the astronaut's families and the nation, said O'Keefe. He pledged that the space agency would find the cause of the accident and go on with space exploration and research that might yield better ways of fighting cancer or other medical breakthroughs.

"We will persevere," he said. "We will not let you down."

The Washington service followed a tribute Tuesday at the Johnson Space Center in Houston, where President Bush said of the astronauts' mission: "They go in peace for all mankind. And all mankind is in their debt."

Cheney said the greatest memorial to the seven, who died as the shuttle was returning to Earth Saturday, will be "a vibrant space program."