NASA said Thursday it will attempt to launch space shuttle Discovery to the international space station late Friday night, after undergoing elays caused by bad weather and fuel valve trouble.

This will be the third attempt 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.

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.