CAPE CANAVERAL, Fla. – NASA will try to launch space shuttle Discovery to the international space station late Friday night, after back-to-back delays caused by bad weather and fuel valve trouble.

It will be the third try for NASA, which is still struggling to understand why a critical shuttle fuel valve appeared to malfunction Tuesday midway through the fueling process.

The valve was tested Wednesday night, and all indications were that it was a problem with a sensor rather than the valve itself. The hydrogen fuel valve, a big 8-inch device, is located in Discovery's engine compartment.

After meeting Thursday, mission managers decided they needed more time to settle on a plan if the problem reappears, and aimed for a liftoff late Friday night.

"The better part of valor here is to take a day, let us go polish that (plan) off, really make sure we understand what's going on," said Mike Moses, chairman of the mission management team. "The team was 80, 90 percent of the way there already" in the analysis.

Another potential issue popped up Thursday when a mechanical failure canceled the test firing of a new moon rocket in Utah. The problem was a faulty valve in a power unit nearly identical to a system used in the shuttle. Shuttle managers said they'd take a look at what went wrong in Utah to see if there's any potential implication for the shuttle.

In an unusual lineup of events, NASA had two launch opportunities for Friday, in the early wee hours and just before midnight. Liftoff time will be 11:59 p.m.

Moses joked about bypassing Friday's first opportunity. "Just to put the right spin on it, we're here to not talk about a scrub. In fact, we're still going to launch on Friday. So that's good," he said at a news conference.

Thunderstorms ruined the first launch attempt early Tuesday morning; the valve problem canceled an early Wednesday attempt.

Forecasters put the odds of good launch weather at just 60 percent. Thunderstorms are a concern for both fueling and launch.

NASA has until Sunday to launch Discovery, otherwise the shuttle will have to get in line behind a Japanese cargo ship and a Russian spacecraft set to go to the space station in September. That would push the shuttle mission into mid-October.

Seven astronauts are assigned to the 13-day flight. They will deliver a full load of space station supplies, including a treadmill named for Comedy Central's Stephen Colbert.

Colbert won the online vote earlier this year for naming rights to a yet-to-be-launched space station room. NASA went with the name Tranquility, however, in honor of this summer's 40th anniversary of the first manned moon landing.

"Yeah, that will scare the aliens," Colbert said of the name Tranquility.