It appears NASA is not yet fully ready for the launch of its space shuttle Discovery; that's why it delayed the launch for a fourth time. 

NASA delayed the launch late Friday, citing too many uncertainties about mission safety and worries about faulty fuel valves. After a full day meeting on Friday at Kennedy Space Center in Florida, the shuttle managers decided to delay the launch that was slated to be on Feb. 27. 

In webcast remarks, John Shannon, space shuttle programme manager, said, "We were really, really close to having the entire team at a point to where we could accept to go fly. There was just a sense of unease that we did not quite have the rigor that we typically expect from a question like this."

"When we go fly, we want everybody on the team to be right behind us and ready to go. We probably could have gotten there tonight, but we would have left some unease in some people's minds," Shannon added.

Originally slated to launch on Feb. 12, Discovery mission was delayed because of the fuel control valve concerns. The shuttle engineers wanted to conduct more tests. 

Recently, it was scheduled to launch on Feb. 27, which was delayed late Friday. Now, NASA officials did not set a new date for the launch, however they signaled that that April 7 could be a target date.