Space shuttle Discovery’s orange external tank has been loaded with the liquid hydrogen fuel it will need for launch, so the pumps have slowed to what’s called a “stable replenish” mode. More of the fuel will trickle into the tank just fast enough to replace the amount that evaporates during the countdown. The evaporating portion of the fuel is vented out of the tank and away from the launch pad via the gaseous hydrogen vent arm. That seal is tight and controllers have seen no signs of the leakage that scrubbed countdowns for the previous mission, STS-127, last month.
