It's not easy to schedule a shuttle launch. There is one ten-minute window each day when Earth's rotation puts the launch pad into the plane of the space station's orbit -- the launch site and the space station orbit must be in sync.
Complicating this, the shuttle can only visit the station when the "beta angle" -- the angle between the plane of the station's orbit and the sun -- ensures the shuttle-station stack will not get too hot, yet still be at an angle to allow the solar arrays to function.
Complicating any attempt to launch is Florida's weather. There is a long list of rules prohibiting a shuttle launch if cloud cover is too low, too dense, or if it's too windy, too hot, too cold -- or if there is a hint of rain near the launch pad or in the flight path of the shuttle and lightning in the area, forget it.
It's an understatement to say there are thousands of technical glitches that can bring a countdown to a stop.
Public affairs officer Jessica Rye of the Kennedy Space Center said scrubbing a launch in the last minutes before liftoff time costs the space agency $500,000. It happens often -- only half the shuttles launched ever lift off when first scheduled.
Launch director Mike Leinbach explained the launch process to ABC News:
"At T-zero, the shuttle is gonna fly. That's when the solids ignite and the shuttle is going to fly. Down to that point we can stop the countdown even if the main engines of the shuttle ignited.
"And we have done that three or four times in the history of the program. We terminated the program after the shuttle's main engines started up. It is very hazardous to do that, but we would rather be on the ground and safe than commit to flight with a plan that may be out of spec, so the computer either here in the control room or onboard the shuttle can stop the countdown all the way down to T-zero."