Launch currently is targeted for May 22 in a launch period that extends to June 3. But senior managers are considering a delay to the next available launch period, which opens July 13 around 3;45 p.m. Engineers at the Kennedy Space Center have delayed loading maneuvering thruster rocket fuel pending the outcome of the discussion.
A decision was expected to be announced Friday morning.
The primary problem facing program managers is the potential threat of ice debris shaking off the external fuel tank during launch and causing impact damage to the shuttle's heat-shield tiles or wing leading edge panels.
The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the left wing's leading edge that was caused by the impact of external tank foam insulation that broke off during launch 16 days earlier.
The foam responsible for Columbia's demise was intended to prevent ice from building up around the fittings that attach struts holding the nose of the shuttle to the thank. The so-called bipod foam has been eliminated in favor of small heaters.
Foam application techniques were changed to minimize the chances for foam shedding in general. Engineers believe the largest piece of foam that can come off the tank today is less than a half ounce. The piece that hit Columbia weighed some 1.67 pounds.
But recent testing shows ice buildups in two areas of the tank still pose a threat. One of those areas is in the so-called inter-tank region between the upper oxygen tank and the lower hydrogen tank where an oxygen feedline bellows is located. The bellows allows the line to flex slightly during launch.
The testing shows ice can build up on the bellows or on a bracket holding the line in place. Another ice problem area is near the tip of the tank around a bracket that holds a repressurization line.
NASA managers held a second debris verification review, or DVR, at the Johnson Space Center in Houston earlier this week and still were unable to conclusively demonstrate ice was not a threat, according to sources who requested anonymity.