The launch of the American space shuttle Endeavour has been called off after another potentially dangerous fuel leak was detected.
The shuttle's original launch date last weekend was postponed due to a similar fault.
Endeavour was due to blast off from Florida early on Wednesday, but hydrogen gas leaked from a vent line on its external fuel tank, officials said.
For an hour after the leak appeared during fuelling, engineers tried to fix the problem through remote commands. But they were finally forced to call off the mission.
Launching before 11 July is no longer an option because of the angle of the Sun between 22 June and 10 July.
Nasa is concerned about something called the beta angle cut-off. The beta angle defines the percentage of time a spacecraft in low-Earth orbit spends in direct sunlight.
Data suggests the shuttle could overheat if it was docked at the space station during those dates.
Endeavour was set to deliver the third and last segment of Japan's Kibo laboratory aboard the International Space Station.
Once the launch goes ahead, the astronauts will carry out five spacewalks to attach a permanent platform weighing 1.9 tonnes to Kibo.