Update: New Discovery Plan Next Week

Space shuttle program managers will develop a plan by Wednesday that they hope allows them to fly Discovery - and later missions - while working to redesign valves in the shuttles' main propulsion system.

Once targeted for Feb. 12 and most recently Feb. 27, a Discovery launch may not be possible before a "cutout" for a Russian Soyuz mission to the International Space Station that runs from March 13 to April 6. The shuttle might not be able to launch during that window.

In a briefing late Friday at Kennedy Space Center, NASA managers said they wanted to do more work to understand the root cause behind the failure of a valve that maintains pressure in the shuttle's orange external tank, by allowing or restricting the flow of hydrogen gas to it.

They said they came close to a decision to launch Discovery, but some engineers questioned assumptions and data from the extensive valve tests conducted across the country in recent weeks, which produced a mountain of fresh data to digest.

"There was a just a sense of unease that we did not quite have the rigor that we typically expect for a question like this," said John Shannon, shuttle program manager.

A small piece broke off a valve during Endeavour's launch last November without doing any damage.

But managers worried about a repeat of that event, possibly involving a bigger chunk of valve.

That could result in an explosion or main engine shutdown that could have disastrous consequences for the crew of seven astronauts.

Shannon said the weeks of testing and Friday's marathon flight readiness review, which began at 9 a.m. and concluded before a 10:30 p.m. press briefing, had the goal of making sure that "we didn't just get lucky" on that STS-126 mission.

The area of most concern is within the orbiter's aft section, inches from where the three valves - one for each main engine - pop up and down to increase or decrease the flow of hydrogen gas.

Meanwhile, work to redesign the valves has already begun. The redesigns could be relatively minor tweaks to their shape or changes in materials that could take four to six months, or a more comprehensive change that could take a year or more.

But officials don't expect the issue to keep shuttles from flying.

"I think the overall philosophy is, while we're working this redesign, we'll have a strategy that allows us to continue to fly with the valves we've got, and we'll do that in parallel," said Bill Gerstenmaier, NASA's associate administrator for space operations.

Shannon said some additional tests and better explanation of their results would help present a convincing rationale for flying, showing a "statistically defensible set of data that tells the story" that probabilities of a valve failure are low and could be survived.

Gerstenmaier said the valve problem has essentially existed since the first shuttle flight in 1981, but modern imaging and computer modeling technology now enabled teams to do a more thorough inspection than ever of microscopic cracks.

And lessons learned from the loss of Columbia's crew in 2003 ensured that engineers didn't assume the problem was minor because of Endeavour's safe result.

The managers said there was no indication yet that later shuttle flights, including a Hubble Space Telescope servicing mission targeted to launch May 12, would need to be pushed back.

Nine shuttle missions remain to be flown before the fleet is retired next year, eight of them to the space station.

"This is just one of those things, in a very complicated vehicle, you're going to have issues like this," said Shannon. "We're gong to make sure we do it right."

Once it has launched, Discovery is scheduled to fly a two-week mission to install a final piece of the space station's central truss and deploy a last set of huge solar array wings.