The New York Times reported Friday that the documents by engineers and managers for the space agency show at least three changes in the statistical methods used in assessing the risks of debris like ice and insulating foam striking a shuttle during launching.
One presentation said lesser standards must be used to support accepting the risks of flight "because we cannot meet" the traditional standards, according to the newspaper.
The Times said there is debate within the agency about whether the changes are a reasonable reassessment of the hazards of flight or whether they jettison long-established rules to justify getting back to space quickly.
Debris was blamed for the disintegration of the shuttle Columbia as it was returning from space in February 2003.
Experts who have seen the documents told the Times that they do not suggest that the shuttle Discovery - scheduled to lift off from Cape Canaveral, Fla., on May 22 - is unsafe.
Shuttle systems engineering manager John Muratore, the author of one of the documents along with a colleague, told the Times he's "never jiggled a number" in his 25-year career and that the engineering challenge is enormously complex.
Earlier this month, Muratore openly acknowledged that even marshmallow-size pieces of insulating foam from the fuel tank could doom the space shuttle under the worst circumstances. He told reporters it is a risk NASA and the nation must accept for flights to resume anytime soon, and that it would take a total redesign of the tank to completely eliminate foam loss.
Paul A. Czysz, emeritus professor of aerospace engineering at St. Louis University, who read the documents at the Times' request, said they did not demonstrate that the shuttle is too dangerous to return to space or that NASA is stinting on efforts to make it safer.
To achieve a profound safety improvement, he said, NASA would need to replace the shuttle fleet, which was designed in the 1970s, with an entirely new vehicle.
But Czysz, who spent some 30 years with NASA contractor McDonnell Douglas, likened the statistical shifts to moving the goal posts at a football game. "I was amazed at how they were adjusting every test to make it come out right," he said.
Muratore told the Times the statistical process was not meant to move any goal posts.
A call by The Associated Press to Johnson Space Center, where Muratore works, was not immediately returned Friday.
NASA officials maintain that the shuttle is safer than it has ever been because of changes made after the Columbia disaster, and they have long acknowledged that not all debris risk can be eliminated.