President Bush has paid tribute to the seven astronauts killed on the Space Shuttle Columbia, saying their loss will not halt the exploration of space.

Addressing the nation, Mr. Bush said the astronauts had a "high and noble purpose in life" and he told their families they had earned the respect and gratitude of the entire nation.

He said the seven people who died on Columbia knew the great risk they were assuming, but went willingly into space on a mission of exploration in service of all humanity.

President Bush said the news had brought great sadness to the United States. But he vowed that the astronauts' "cause will continue," and added, "our journey into space will go on."

The president was speaking from the White House, five hours after the Columbia broke up as it approached Earth for a landing. He said "The Columbia is lost, there are no survivors."

President Bush quoted a scripture passage from Isaiah in urging people to lift their eyes to the heavens for comfort. And he said, quote, "The same creator who names the stars also knows the names of the seven souls we mourn today."