Space Shuttle Endeavour has been grounded yet again by the same gremlin that kept it sitting on the launch pad last week: a leaking gaseous hydrogen venting system on the external fuel tank.

That system is a line carrying vented hydrogen away from the shuttle to a flare where the hydrogen can be burned off safely.

When the problem initially occurred during tank-filling operations June 13, NASA tried using a fix similar to the one that solved a hydrogen line leak on a different shuttle, Discovery, on the STS-119 Mission in March. With Discovery, changing some seals worked. (Please see Space & Missile Defense Report, Monday, June 15, 2009.)

But when NASA technicians again tried to fill the external fuel tank on Endeavour for a scheduled liftoff June 17, the leak occurred again.

Because of a thermal problem that lasts from today to July 10, occurring if the shuttle were docked then with the space station, Endeavour is grounded at least until Saturday, July 11. That thermal problem is caused by the angle of the sun, called a beta angle cut-out.

When Endeavour eventually heads to space, its STS-127 mission will feature a daunting 16 days of hard work, including installation of the exposed facility to the Japanese Kibo laboratory module already on the station. It will permit experiments to be exposed to space.

The mission will feature five spacewalks.

While there have been many lengthy delays in getting space shuttle flights off the ground, this repeated glitch comes at an especially inopportune time for NASA.

There are eight remaining shuttle flights on the manifest, including the Endeavour trip aloft, with 15 months left before the Sept. 30, 2010, date that then-President Bush mandated for retiring the space shuttle fleet.

That works out to an average clip of roughly one two-week shuttle mission every two months.