As NASA engineers ponder how the shuttle Endeavor's gaseous hydrogen venting system started leaking and delayed the spacecraft's launch, the agency said it will try to put the shuttle in orbit on Wednesday at 5:40 a.m. ET. The shuttle's problems likewise push the launch of the moon-probing Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS) back at least one day, to Thursday, June 18.

NASA believes a seal on the external fuel tank that was misaligned when it was coupled to one of the shuttle's engines caused the leak, NASA Shuttle Test Director Stephen Payne, said today during a press conference. "Our teams have been working very hard over the last couple of days to get this piece of equipment fixed," he said, adding, however, that the space agency was unsure of what's causing the misalignment.

The venting system (pdf), located outside the shuttle's external fuel tank, is designed to carry excess hydrogen safely away from the launch pad as the Endeavor blasts off. During launch, the external fuel tank supplies the shuttle's main engines with liquid oxygen and hydrogen propellants.

The engines transform this liquid into gaseous hydrogen, which is returned to the external tank to provide pressure and keep the propellant flowing out of the tank. A valve is then supposed to regulate this flow of gaseous hydrogen from the main engines back to the fuel tank so the tank delivers propellant to the main engines at the correct pressure.

Endeavour experienced a similar problem in November, when flight controllers noticed that gaseous hydrogen was flowing from one of the shuttle's engines at a higher than normal rate. After the shuttle returned from that mission, engineers discovered that the suspect flow control valve was cracked and a small piece was missing. A similar problem delayed Discovery's March launch for four days as ground crews swapped out the suspect seal between the vent pipe and the shuttle, according to

Image of Endeavour's flight crew © NASA