The shuttle Discovery's delayed launch on the first post-Columbia mission is off until at least Sunday, officials said today, and unless engineers find an obvious, simple-to-fix problem in the next day or so, the flight will be put on hold indefinitely.

Wayne Hale briefs reporters today. Credit: Stephen Clark/Spaceflight Now

"If we were to get extremely lucky, it is theoretically possible that we could still launch on Sunday," said Wayne Hale, deputy shuttle program manager and chairman of NASA's mission management team. "But I've got to tell you that this represents a really optimistic, good-luck scenario, which I think is not very credible. I wish I had better news.

"What we're more likely into is several days of troubleshooting and we will know tomorrow, when we get the integrated troubleshooting plan back at the mission management team, we'll be able to give you a more definitive timeline. That's the best I've got right now."

What sort of fix might be possible in time to support a Sunday launch attempt?

"I think the repair that might get us to Sunday is if we go in (the shuttle's aft compartment) and wiggle some of the wires and find a loose connection," Hale told reporters. "I mean, that's the one that comes to mind. You laugh, but that probably is the first step in any troubleshooting plan.

"Some technician is going to put his hands on the wires and connectors ... and start wiggling them. If we found a loose connection, OK, we might be back in business. I've got to tell you, the folks who put those wires together and those connections do a really good job. So the chances of that, not high."

NASA is keeping Discovery in a countdown configuration just in case engineers come up with a quick fix. But shuttle processing manager Mike Wetmore said the team can't maintain that level of readiness indefinitely and that in all likelihood, they will back out of the countdown configuration Friday. At that point, launch would be on indefinite hold pending the outcome of much more extensive troubleshooting.

Hale said 12 engineering teams had been established to troubleshoot the issue, a "vast engineering team" focused on getting the problem fixed and Discovery off the ground as soon as possible.

But the shuttle's launch window closes July 31. The next window runs from Sept. 9 through Sept. 24. If Discovery can't be quickly fixed, the flight could slip into September, pushing the second post-Columbia mission into a relatively tight November window that opens Nov. 7 and closes Nov. 10. NASA managers are already looking into what might be done to extend the November window if worse comes to worse.

But Hale said, "I'm not ready to give up on the July window at this point. We still have more than two weeks ahead of us, so that's the way we're headed."

Discovery was grounded Wednesday two-and-a-half hours before blastoff when a routine computerized test revealed problems with one of four hydrogen fuel sensors in the shuttle's external fuel tank.

The sensors are part of a backup system intended to make sure the ship's engines don't shut down too early or run too long, draining the tank dry with potentially catastrophic results. All four sensors are required for a countdown to proceed.

Discovery won't launch before Sunday, NASA says. Credit: NASA-KSC

NASA has encountered a string of problems in recent weeks with the engine cutoff - ECO - sensor system in Discovery, glitches that have proven to be surprisingly difficult to resolve. The problems began during a tanking test in April when two of the four sensors operated intermittently.

Engineers removed an electronic controller, called a point sensor box, from Discovery and replaced wiring and connectors. But the controller checked out OK and troubleshooters were unable to trace the cause of the problem A point sensor box from the shuttle Atlantis was installed and a second fueling test was conducted.

This time around, the sensors worked normally. But during additional post-test troubleshooting, the replacement sensor controller box malfunctioned. It was replaced by one taken from the shuttle Endeavour. NASA already had decided to replace Discovery's tank to address ice debris issues. With a fresh controller, replacement cabling, a new tank and solid test results, NASA managers decided to treat the sensor issue as an "unexplained anomaly" that presumably had been fixed.

But during Wednesday's countdown, the No. 2 low-level hydrogen sensor failed to switch from "wet" to "dry" during a test in which computers send signals to simulate a dry tank. When the tank was drained, the other three sensors changed from wet to dry as expected. The No. 2 sensor remained "wet" for another three hours before switching back to "dry."

"It is properly indicating that it's dry right now," Hale said. "Could we talk ourselves into going, after all of this, without doing anything? No."

Asked if shuttle engineers had to definitively resolve the problem before Discovery could be cleared for flight, Hale said "it's an unfortunate part of the business, but sometimes you live with unexplained anomalies."

"What we've got to do, I think, is to do all the troubleshooting we can to try to find the solution to the problem," he said. "In other words, you don't jump to the UA rationale first. You do all the troubleshooting you can to try to solve the problem first. If at the end of the day you've done everything that you know to do and you've got an unexplained anomaly, then you have to ask yourself, why am I safe to proceed? Where do we go to define logic that says we're safe to proceed?

"You can imagine different scenarios where you'd talk about those kinds of things. But you really have to have clear and convincing rationale that says we're safe to proceed. And that only comes after you've done all the troubleshooting you possibly can."

Hale said historically, the sensors have been reliable and until Discovery's recent problems, "we've not had any low level sensors fail."

"Now we're beginning to think there may be something else going on," he said. "The cluster here is kind of interesting because it's all been hydrogen sensors. And even though they're the same sensor going to the same electronics box as the oxygen sensors, people are beginning to ask is there something peculiar that we've done in the wiring, perhaps, that go to the hydrogen sensors. So that's one of the threads the troubleshooting team is looking into."

Finally, Hale was asked what he would tell the public about NASA's efforts to resolve the problems with Discovery.

"I would tell you that going into space is right at the limits of human technology," he said. "We're doing something that's extremely difficult. This is not like going to the airport and getting in a commercial airliner. This is much more complicated and much more difficult. There are only a few nations and the history of the world that have this capability and they've all had to expend quite a bit of national treasure and quite a bit of effort.

"In the future, one hopes it will become more commonplace and perhaps it will become more like commercial airline travel, but we are not there today. ... If you think this is routine, you surely don't understand what it is we're trying to do here."

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