NASA has canceled the launch of the Discovery STS-119 mission to the International Space Station due to concerns about the high risk of a possibly failing hydrogen flow control valve.

This is the fourth time the space agency cancels this mission.

The engineers working on this mission gathered and talked about the risks in a session that took 13 hours. They reached the conclusion that it would be too risky to launch Discovery in these conditions so they delayed the mission. Another date was not decided yet.

However, NASA has until mid-March to launch Discovery to the ISS and there are pretty good chances it will do it. If it doesn’t manage to launch the mission by then, it will have to wait in line as Russia will carry out a scheduled launch of a Soyuz. If NASA doesn’t meet the mid-March term, the next opened window to launch Discovery will be after April 6th.

The agency explained that the shuttles have three valves that control the flow of gaseous hydrogen from the main engines to the external fuel tank. One of those valves was damaged on Endeavour's STS-126 gig last November.

In order to launch the Discovery in safe conditions, NASA said it has to gather more data and carry out more tests. The space agency also wants to analyze what would happen if a valve would fail, break and strike part of the shuttle and external fuel tank.

"We need to complete more work to have a better understanding before flying. We were not driven by schedule pressure and did the right thing. When we fly, we want to do so with full confidence," said Bill Gerstenmaier, associate administrator for Space Operations at NASA.

The Discovery mission has the objective of delivering a new set of solar wings to the ISS.