NASA is considering sending two astronauts on an unprecedented spacewalk to the belly of space shuttle Discovery to smooth out rough spots in the ship's heat shield, US space agency officials said.

NASA managers are concerned that two protruding bits of material known as "gap fillers" could overheat Discovery as it plunges through Earth's atmosphere for landing.

Heat shield damage was responsible for the loss of shuttle Columbia and its seven astronauts during NASA's last shuttle mission more than two years ago.

"My immediate knee-jerk reaction was that we could live with this," said deputy shuttle program manager Wayne Hale. "On the other hand, if the (spacewalk repair) plan is relatively simple, why worry? Why would you not just go take care of it?"

Thousands of gap fillers are tucked between the black heat-resistant ceramic tiles on the shuttle's belly, which typically experiences temperatures of about 2,500 degrees Fahrenheit as the shuttle re-enters the atmosphere.

Two gap fillers on Discovery are sticking up about an inch from the tiles, a protrusion that could trigger a 15 percent to 25 percent temperature spike, said orbiter projects manager Steve Poulos.

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