Running a month behind scheduled because of hydrogen leaks, launch pad lightning strikes and stormy weather, the shuttle Endeavour was refueled for a fifth launch try Monday to kick off a 16-day space station construction mission. Forecasters predicted a 60 percent chance of another delay due to expected afternoon storms.
Hoping for the best, engineers began pumping liquid oxygen and hydrogen rocket fuel to the shuttle's external tank at 9:33 a.m. EDT. The fueling procedure was completed at 12:37 p.m. EDT when engineers transitioned to "stable replenish" mode. The hydrogen vent line that triggered two launch delays last month is working normally, with no signs of any leakage.
Commander Mark Polansky, pilot Douglas Hurley, Canadian flight engineer Julie Payette, David Wolf, Christopher Cassidy, Thomas Marshburn, and space station flight engineer Timothy Kopra began strapping in around 3:30 p.m. EDT for a launch attempt at 6:51:24 p.m. EDT.
"While we all hope that today's the day, we have a saying that you never know for sure until the solid rockets light off," Polansky said in a Twitter posting early today.
The only technical issue under discussion overnight centered on a partially detached Tyvek cover over one of Endeavour's forward left rocket thrusters. The covers, used to keep rain water and debris out of the shuttle's thruster nozzles on the launch pad, pull away during the early moments of flight.
The only way to re-attach or replace the debonded cover is to move a protective gantry back in place around the shuttle. Engineers decided not to do that before today's launch attempt, but any significant rain at the pad likely would trigger a delay. Water trapped in the nozzle could turn to ice in space and possibly damage the thruster.
As it now stands, NASA must get Endeavour off the pad by Tuesday to carry out a full-duration five-spacewalk mission. A Russian Progress resupply ship is scheduled for launch July 24 and while it can "loiter" in orbit for five days, it must dock by July 29. That means Endeavour must take off by Tuesday to complete a full-duration mission in time to undock by July 28, making way for the Progress.
Depending on how today's launch attempt plays out, NASA could opt to make a launch attempt Wednesday, but the mission likely would have to be shortened by one day. The forecast for Tuesday is 60 percent no-go, improving to 60 percent go on Wednesday.
If the shuttle doesn't get off by Tuesday or Wednesday at the latest, the flight likely will slip to July 26 or 27.