Endeavour and its seven-member crew were supposed to launch Wednesday to the International Space Station to deliver and install a final component to Japan's laboratory module, Kibo. Other objectives include five spacewalks to perform maintenance tasks, and a one-for-one swap of astronauts serving on the space station.
This is the second mission in the last three in which a leak has developed on a venting fixture on the outside of the shuttle's external fuel tank. The leaks allowed explosive hydrogen gas to build up to unacceptable levels.
"We're going to step back and figure out what the problem is and go fix it, and then we'll fly as soon as we're ready to safely go do that," said LeRoy Cain, deputy manager of the space-shuttle program, at a briefing after managers scrubbed the launch.
In the past, controllers were able to fix the leaks by repeatedly opening and closing a valve within the fixture several times. In March, however, that approached failed to eliminate the leak from Discovery's external tank. Technicians replaced the valve and seals in the fixture, and the leak vanished.
This time around, a leak cropped up during fueling last Friday and didn't stop even after controllers repeatedly opened and closed the fixture's valve. Technicians replaced the assembly, and launch was set for Wednesday. But the leak reappeared Tuesday night during fueling. Controllers couldn't staunch it.
The leak's cause is unclear. The fixtures passed leak checks at room temperatures before the shuttles rolled to the launch pad. But shuttle handlers say they have no system to check for leaks at super-cold liquid-hydrogen temperatures until the tank is actually being fueled.
The effect on the schedule for the final seven shuttle missions is also unclear. With fresh fixtures displaying leaks in three of the last five fueling attempts, many of the remaining tanks could host fixtures that are leaks – and delays – waiting to happen.
Controllers noticed one potential clue: a difference between this leak and the two previous leaks, according to Peter Nickolenko, the mission's launch director. In the past, the leaks only appeared as technicians topped off the tank. Tuesday night's leak first appeared about 25 minutes before top-off began, when the tank is filling quickly. Then it vanished, only to reappear during the slower top-off process.
The space agency has a variety of hardware glitches that await a final fix. But "this item is coming pretty close to being our top priority," Mr. Cain says.
Engineers will be looking at everything from approaches used to install the fixtures to changes in the makeup of components.
One thing Endeavour's and Discovery's tanks have in common: Both orbiter-tank combinations had been moved to the pad, then moved again. This required technicians to connect, disconnect, and reconnect a hose to the fixture. The hose carries the unwanted hydrogen gas off to a stack some distance away where the excess hydrogen is burned off.
Troubleshooters are also likely to be looking at the potential for sabotage. Mission managers insist that the likelihood is extremely small, given a workforce motivated to launch rockets, not ground them. But the shuttle program is slated to end in September 2010 and it's not beyond the realm of possibility that a disgruntled worker could register his or her displeasure by introducing a launch-stopping flaw.
"We have talked a great deal about those kinds of potential issues," Mr. Cain said, adding that the agency remains vigilant in monitoring vehicle processing and launch preparations.