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.