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.
If a heater must be installed, Discovery would have to be hauled back to the Vehicle Assembly Building, which would rule out a launch during the May-June window.
Other problems facing the launch team:
- One of four hydrogen depletion sensors inside the external tank did not operate properly during a tanking test April 14. Engineers initially a wiring problem was to blame, but so far no such problems have been found. The sensor is critical and flight rules require all four to be operational for launch. To replace the sensor, if that is required, engineers would need to remove insulation, open an access hatch and enter the hydrogen tank.
- A pressurization relief valve operated, or cycled, more often than expected during the tanking test. Engineers are not yet sure if this is an actual problem or not, but they are looking into it.
- Hydraulic fluid was blown onto one of the shuttle's aft rocket pods during high winds, contaminating nearly 20 insulation blankets. Engineers want to replace the blankets because of concerns about normal ascent heating and the possibility some of the residual fluid could actually ignite.
A launch delay to July would allow NASA to roll Discovery back to the Vehicle Assembly Building where engineers would have much better access and a controlled environment for any repairs or upgrades, including installation of a new external tank heater and replacement of the insulation blankets.
NASA managers have said for months they are not being driving by the launch schedule and a delay to July would appear to be in keeping with that post-Columbia philosophy. But some senior managers, sources say, argue against giving up the May-June launch window unless it's absolutely necessary.
CBS News Space Consultant William Harwood has covered America's space program full time for nearly 20 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.