"It would cause what we call critical damage," Hale said.
Ken Welzyn, NASA's external tank chief engineer at Marshall Space Flight Center in Huntsville, Alabama, said there are 34 ice frost ramps on each shuttle fuel tank.
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In a worst case scenario, in which a 3.5-ounce piece of foam pulls free from a key ice frost ramp at the worst time and inflicts the most damage, it could prove a dangerous hazard to the shuttle and its crew, NASA said.
From STS-121 to retirement Commanded by shuttle veteran Steven Lindsey, NASA's STS-121 mission will complete a series of shuttle repair and safety tests required before the space agency resumes ISS construction.
The STS-121 crew will perform at least two spacewalks - and possibly a third if shuttle resources permit - as well as ferry ISS crewmember Thomas Reiter and a fresh load of supplies to the orbital laboratory.
"That didn't drive this particular discussion," Hale said.
"We're trying to make appropriate decisions in light of the schedule, and not let it drive us to overly risky or foolish decisions just to make a schedule that we know has some time in it to allow for engineering problems to be solved." Reducing tank foam debris NASA has been working to reduce the shedding of dangerously large pieces of fuel tank foam insulation since the 2003 Columbia disaster, in which seven astronauts and one orbiter were lost during reentry.