Columbia broke up on Saturday as it began to reenter the atmosphere over Texas, shortly before it was to land.
White House spokesman Ari Fleischer said Bush had not seen the letter written to him on Aug. 25, 2002, by former NASA safety engineer Don Nelson expressing concerns over shuttle safety. "Your intervention is required to prevent another catastrophic space shuttle accident," he said.
He cited specific safety incidents including a 2000 inspection of Columbia which found 3,500 wiring defects, as well as charges such as a July 2002 inspector general's report that the shuttle safety program was not properly managed.
"The lives of our astronauts and the future of our space program must not be ignored," he said. He urged Bush to limit the size of the shuttle's crew to four until an escape pod was built.
Such a pod would not have saved Columbia's astronauts, experts have said.
White House science adviser John Marburger wrote back to Nelson on Dec. 4, saying his office had discussed the concerns with NASA officials. "NASA places a high priority on safety and has instituted a program of developing and implementing safety upgrades to reduce the risk to space shuttle crews," he said.
"Based on these discussions I do not think that it is appropriate for the president to issue a moratorium on space shuttle launches at this time." Fleischer said on Sunday that an escape pod would not have saved Columbia's astronauts.
Bush, who is to attend a memorial for the astronauts at the Johnson Space Flight Center in Houston on Tuesday, received a briefing on Monday from NASA administrator Sean O'Keefe on the investigation into the Columbia disaster and the two spoke of their determination to return to space.
"While we grieve for these astronauts, the cause for which they died will continue. America's journey into space will go on," Bush said in a speech to employees at the National Institutes of Health.
As he briefed the president O'Keefe said he intended "to get back into space as soon as possible with all safety issues having been fully, fully explored," Fleischer said.
Bush, who is to attend a memorial service in Houston on Tuesday for the shuttle crew, asked about the well-being of the astronauts' families and NASA morale, and looked ahead to an eventual resumption of flights, Fleischer said.
"The president talked about the status of the next crews and the morale of the next crew and how they are ready to go as soon as they are able to go back into space." He said, however, Bush did not suggest a timetable for the investigation. Shuttle flights have been grounded following Saturday's accident.
Bush proposed a 22 percent increase on Monday for the space shuttle program in his fiscal 2004 federal budget request to Congress, which was prepared before the Columbia disaster. He requested $3.9 billion for the program, compared with $3.2 billion in 2003.
Administration officials left the door open to further increases if sought by Congress. They said it was too early to consider whether to replace the lost shuttle -- one of four in the program, or to tackle other issues such as developing a successor spacecraft.
Columbia's loss has prompted calls for more spending to upgrade the aging shuttle fleet and develop a new space plane.
In addition to the space shuttle funding increase, the proposed total budget for NASA was slated for a smaller increase, rising $469 million to $15.47 billion, reflecting one-time expenses in 2003 that would not be repeated, Fleischer said.