The space shuttle Discovery is still on the ground this morning, after NASA postponed last night’s scheduled launch because of a hydrogen leak. Liftoff is now tentatively scheduled for Sunday night.

The mission was to have dropped off the final pieces of the International Space Station's (ISS) solar arrays that capture energy from the sun, and parts for its urine recycling system that would expand the ISS’s capacity from three to six crew members. It was sidelined after engineers discovered a leak in the gaseous hydrogen venting system outside of the external fuel tank. That system carries excess hydrogen away from the launch pad, and an accumulation of gas could have caused an explosion at launch.

NASA techs will examine the leak today and meet at 4 p.m. EDT to discuss its status.  

Discovery's launch date was previously pushed back by nearly a month to evaluate possible safety concerns surrounding valves that control the flow of hydrogen gas from the main engines to the external fuel tank. Those worries aren’t related to the latest mishap, NASA spokesperson William Johnson told Bloomberg News.

“It’s completely different,” he told the newswire.

Hydrogen vent line connects to the external tank for space shuttle Discovery/NASA TV