The launch of Space Shuttle Endeavour on its STS-118 mission to the International Space Station (ISS) has been delayed a day to 6:36 p.m. ET Wednesday, so crews will have more time to complete routine work, NASA announced.

Delay of the launch came after workers searched for the source of an air leak from the crew cabin. They found it: a faulty pressure relief valve.

Workers fixed that by swapping out the bad valve for a good one scavenged from Space Shuttle Atlantis, which won’t fly until December. The valves prevent astronauts in the cabin from being subjected to excessive air pressure.

Thanks to the launch delay, workers will have time to complete processing of pre-flight items.

Chances are good the launch will occur Wednesday, because there is but a 30 percent chance of prohibitive weather at the launch site then or Thursday, according to the space agency.

NASA is carefully checking items on Endeavour, which hasn’t flown since 2002, before the early 2003 Space Shuttle Columbia disaster.

A chunk of foam insulation on the Columbia external fuel tank broke free and smashed an undetected hole in the leading edge of a wing on the orbiter vehicle, and that hole on reentry led to loss of the ship and crew.

In response to that tragedy, NASA instituted many new safety procedures and hardware changes, such as foam insulation reforms, which first were instituted on Space Shuttle Discovery, and on Atlantis as well.

All of those upgrades have been incorporated on Endeavour, which as well has been given a thorough renovation, nose to tail.

The Crew on Endeavour will include Barbara Morgan, a teacher-astronaut who has waited two decades for her chance to ride into space.

She trained in the 1980s with teacher Christa McAuliffe at NASA astronaut facilities. Later, McAuliffe and fellow crew members perished when Space Shuttle Challenger was torn apart shortly after liftoff on Jan. 28, 1986, because of O-ring problems in the solid booster rockets

Coincidentally, some problems recently were found in the O-rings on the Endeavour solid rocket boosters. But these aren’t a threat. Rather, X-rays showed these O-rings contain some scattered spots of rubber that, because it isn’t fully mixed, isn’t flexible. But what counts is the overall flexibility and performance of the O-rings, and they easily meet specifications.

Aside from Morgan, crew members include Commander Scott Kelly, Pilot Charlie Hobaugh and mission specialists Morgan, Tracy Caldwell, Rick Mastracchio, Alvin Drew and Canadian Space Agency astronaut Dave Williams.

Morgan will operate the shuttle robotic arm to provide video views of work on the space station, as part of continuing construction and upgrading work on the space station.

That includes maneuvering a structural component, the S5 truss, into position and attaching it to the ISS. The component weighs 5,000 pounds, or two and a half tons, and it will provide clearance between sets of solar arrays on the truss structure.

Hobaugh and Anderson will operate the station robotic arm that moves the segment into place, while spacewalkers Williams and Mastracchio provide guidance from the outside and finish the installation. Kelly, Caldwell and Drew will help out inside.

“It’s less than two inches from some critical electronic components that we want to make sure we don’t come in contact with,” Kelly said. “So that’s a very tight clearance.”

Endeavour also will be a truck, hauling cargo from Earth to orbit that will sustain ISS crews.

This will be the last dedicated shuttle mission providing cargo to the station for 12 to 15 months. Russian Progress vehicles and the European Space Agency Automated Transfer Vehicle (ATV) will bring cargo to the station in the interim. So Endeavour will carry enough supplies to last the station residents for awhile.

“I think right now the manifest has us bringing up about 5,000 pounds and then bringing down about 5,000 pounds,” Kelly said. “So it’s a lot of spare parts, food, clothing, scientific experiments. We’ll unload that and then reload it with stuff that needs to come home – garbage, spare parts that are no longer needed on the station.”

Then the astronauts will become mechanics, doing some repair work.

One of the station’s control moment gyroscopes – a spinning wheel used to control the space station’s orientation – experienced problems and was shut down in October. The problems continued through a weekend, involving CMGs, computers, electrical lines and more.

Program managers determined that gyro needed to be replaced during STS-118. Kelly’s crew had less than a year to train for the task.

“The other stuff is a challenge, but we’ve known it was coming,” Montalbano said. “We’ve developed procedures, we’ve trained the crew – it’s all known. The gyroscope, it’s a little bit new to us. We’re putting a major task in when we’re well into training.”