Shuttle launch schedules face challenges

NASA is plunging ahead with preparations for the upcoming launches of both Discovery and Atlantis, but the agency is facing significant challenges that ultimately could delay both flights.

As it stands, Discovery is tentatively scheduled to launch July 1 on NASA's second post-Columbia test flight. Assembly of the International Space Station would resume with the Aug. 28 launch of Atlantis.

Discovery is scheduled to move from its processing hangar to the Kennedy Space Center Vehicle Assembly Building on May 12. A move to the launch pad would follow May 19. The STS-121 payload would be transported to the pad June 6 and Discovery's crew would take part in a launch-day dress rehearsal on June 16.

Sistership Atlantis is slated to be moved from its hangar to the assembly building on July 24. A move to the pad would follow on July 31 and its astronaut crew would climb aboard Atlantis for a practice countdown on Aug. 8.

Orbiter processing in either case is on schedule. However, several technical issues still must be resolved.

NASA still must complete wind tunnel tests and subsequent engineering analyses aimed at proving recent modifications to shuttle external tanks will not jeopardize flight safety.

Specifically, the analyses must show that fuel pressurization lines and electrical cabling will not be ripped off the outside of the tank now that NASA has removed a wind shield known as a Protuberance Air Load, or PAL, ramp. The ramp shed potentially deadly debris during Discovery's flight last July.

The change-out of suspect fuel-depletion sensors on Discovery's tank was completed last weekend. But technicians still face another week-and-a-half of work primarily aimed at reapplying foam insulation to the aft dome of the tank.

Engineers are wrestling with another technical issue, too.

Tiny metal crystalline extrusions colloquially known as "whiskers" have been detected on electronic components within main engine thrust vector control boxes and aerosurface servoamplifiers.

The concern is that the whiskers could be conductive and short-circuit mechanisms within the critical components. Engineers are analyzing the situation to determine whether any components on Discovery might have to be replaced.

Launch opportunities for Discovery and Atlantis are limited by NASA-imposed daylight flight restrictions. Launching during daylight enhances the agency's ability to prove that post-Columbia safety modifications will prevent large pieces of foam debris from popping off external tanks.

The window for Discovery's mission extends from July 1 through July 19. The next opportunity would be a window from August 28 through Sept 14. There are two other short windows this year: Oct. 26-Oct. 30 and Dec. 23-Dec. 25.