President Bush told mourners at the Johnson Space Center Tuesday that the dream of space exploration pursued by the crew of the shuttle Columbia will continue. The seven crewmembers died Saturday when their shuttle broke as it re-entered earth's atmosphere. Mr. Bush offered encouragement to the friends and families of the astronauts at an outdoor memorial service.

Under a clear blue Texas sky, the president said the astronauts lived lives of great achievement as they pursued the ancient dream of exploring space.

"For these seven, it was a dream fulfilled," said President Bush. "Each of these astronauts had the daring and discipline required of their calling. Each of them knew that great endeavors are inseparable from great risks."

Family members, dignitaries and fellow astronauts gathered on the grounds of the space center, which is the nerve center of NASA and home to the U.S. manned space program. NASA officials say more than 10,000 people attended the service.

Mr. Bush told the mourners the exploration of space will continue despite the loss. "This cause of exploration and discovery is not an option we choose," he said. "It is a desire written in the human heart. We are that part of creation which seeks to understand all creation. We find the best among us, send them forth into unmapped darkness, and pray they will return."

The astronauts died as their shuttle disintegrated on reentry after a 16-day mission. Investigators are trying to determine the cause. NASA administrator Sean O'Keefe said the space agency will honor the crew's legacy. "We also have the tremendous duty to honor the legacy of these fallen heroes," he said, "by finding out what caused the loss of the Columbia and its crew, correct what problems we find and make sure this never happens again."

President Bush spoke of each of the seven astronauts and the different paths that brought them to the space program.

Navy Captain Kent Rominger, the head of the astronaut corps, spoke of them as friends, from the India-born scientist Kalpana Chawla, whom he called a perfectionist with "a terrific sense of humor;" to Colonel Ilan Ramon, the first Israeli in space, remembered as "the perfectly poised fighter pilot with the sparkle in his eyes."

"Rick, Willie, Mike, K.C. (Kalpana Chawla), Laurel, Dave and Ilan, I know you're listening," said Captain Rominger. "Please know you are in our hearts, and we will always smile when we think of you."

Astronaut chief Rominger said all seven shared a passion for space and had become a tight knit family over years of training.

President Bush said all will be remembered for advancing space exploration. "The final days of their own lives were spent looking down upon this earth," he said, "and known on every continent, in every land they could see, the names of these astronauts are known and remembered."

Mr. Bush said the world owes a debt to the astronauts and that they will always have the gratitude of the United States.